openafs/doc/xml/AdminGuide/auagd006.xml

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<chapter id="HDRWQ5">
  <title>An Overview of OpenAFS Administration</title>

  <para>This chapter provides a broad overview of the concepts and
  organization of AFS. It is strongly recommended that anyone involved in
  administering an AFS cell read this chapter before beginning to issue
  commands.</para>

  <sect1 id="HDRWQ6">
    <title>A Broad Overview of AFS</title>

    <para>This section introduces most of the key terms and concepts
    necessary for a basic understanding of AFS. For a more detailed
    discussion, see <link linkend="HDRWQ7">More Detailed Discussions of
    Some Basic Concepts</link>.</para>

    <sect2 renderas="sect3">
      <title>AFS: A Distributed File System</title>

      <para>AFS is a distributed file system that enables users to share
      and access all of the files stored in a network of computers as
      easily as they access the files stored on their local machines. The
      file system is called distributed for this exact reason: files can
      reside on many different machines (be distributed across them), but
      are available to users on every machine.</para>
    </sect2>

    <sect2 renderas="sect3">
      <title>Servers and Clients</title>

      <para>AFS stores files on file server machines. File server machines
      provide file storage and delivery service, along with other
      specialized services, to the other subset of machines in the
      network, the client machines. These machines are called clients
      because they make use of the servers' services while doing their own
      work. In a standard AFS configuration, clients provide computational
      power, access to the files in AFS and other "general purpose" tools
      to the users seated at their consoles. There are generally many more
      client workstations than file server machines.</para>

      <para>AFS file server machines run a number of server processes, so
      called because each provides a distinct specialized service: one
      handles file requests, another tracks file location, a third manages
      security, and so on. To avoid confusion, AFS documentation always
      refers to server machines and server processes, not simply to
      servers. For a more detailed description of the server processes,
      see <link linkend="HDRWQ17">AFS Server Processes and the Cache
      Manager</link>.</para>
    </sect2>

    <sect2 renderas="sect3">
      <title>Cells</title>

      <para>A cell is an administratively independent site running AFS. As
      a cell's system administrator, you make many decisions about
      configuring and maintaining your cell in the way that best serves
      its users, without having to consult the administrators in other
      cells. For example, you determine how many clients and servers to
      have, where to put files, and how to allocate client machines to
      users.</para>
    </sect2>

    <sect2 renderas="sect3">
      <title>Transparent Access and the Uniform Namespace</title>

      <para>Although your AFS cell is administratively independent, you
      probably want to organize the local collection of files (your
      filespace or tree) so that users from other cells can also access
      the information in it. AFS enables cells to combine their local
      filespaces into a global filespace, and does so in such a way that
      file access is transparent--users do not need to know anything about
      a file's location in order to access it. All they need to know is
      the pathname of the file, which looks the same in every cell. Thus
      every user at every machine sees the collection of files in the same
      way, meaning that AFS provides a uniform namespace to its
      users.</para>
    </sect2>

    <sect2 renderas="sect3">
      <title>Volumes</title>

      <para>AFS groups files into volumes, making it possible to
      distribute files across many machines and yet maintain a uniform
      namespace. A volume is a unit of disk space that functions like a
      container for a set of related files, keeping them all together on
      one partition. Volumes can vary in size, but are (by definition)
      smaller than a partition.</para>

      <para>Volumes are important to system administrators and users for
      several reasons. Their small size makes them easy to move from one
      partition to another, or even between machines. The system
      administrator can maintain maximum efficiency by moving volumes to
      keep the load balanced evenly. In addition, volumes correspond to
      directories in the filespace--most cells store the contents of each
      user home directory in a separate volume. Thus the complete contents
      of the directory move together when the volume moves, making it easy
      for AFS to keep track of where a file is at a certain time.</para>

      <para>Volume moves are recorded automatically, so users do not have
      to keep track of file locations. Volumes can be moved from server to
      server by a cell administrator without notifying clients, even while
      the volume is in active use by a client machine. Volume moves are
      transparent to client machines apart from a brief interruption in
      file service for files in that volume.</para>
    </sect2>

    <sect2 renderas="sect3">
      <title>Efficiency Boosters: Replication and Caching</title>

      <para>AFS incorporates special features on server machines and
      client machines that help make it efficient and reliable.</para>

      <para>On server machines, AFS enables administrators to replicate
      commonly-used volumes, such as those containing binaries for popular
      programs. Replication means putting an identical read-only copy
      (sometimes called a clone) of a volume on more than one file server
      machine. The failure of one file server machine housing the volume
      does not interrupt users' work, because the volume's contents are
      still available from other machines. Replication also means that one
      machine does not become overburdened with requests for files from a
      popular volume.</para>

      <para>On client machines, AFS uses caching to improve efficiency.
      When a user on a client machine requests a file, the Cache Manager
      on the client sends a request for the data to the File Server
      process running on the proper file server machine. The user does not
      need to know which machine this is; the Cache Manager determines
      file location automatically. The Cache Manager receives the file
      from the File Server process and puts it into the cache, an area of
      the client machine's local disk or memory dedicated to temporary
      file storage. Caching improves efficiency because the client does
      not need to send a request across the network every time the user
      wants the same file. Network traffic is minimized, and subsequent
      access to the file is especially fast because the file is stored
      locally. AFS has a way of ensuring that the cached file stays
      up-to-date, called a callback.</para>
    </sect2>

    <sect2 renderas="sect3">
      <title>Security: Mutual Authentication and Access Control
      Lists</title>

      <para>Even in a cell where file sharing is especially frequent and
      widespread, it is not desirable that every user have equal access to
      every file. One way AFS provides adequate security is by requiring
      that servers and clients prove their identities to one another
      before they exchange information. This procedure, called mutual
      authentication, requires that both server and client demonstrate
      knowledge of a "shared secret" (like a password) known only to the
      two of them. Mutual authentication guarantees that servers provide
      information only to authorized clients and that clients receive
      information only from legitimate servers.</para>

      <para>Users themselves control another aspect of AFS security, by
      determining who has access to the directories they own. For any
      directory a user owns, he or she can build an access control list
      (ACL) that grants or denies access to the contents of the
      directory. An access control list pairs specific users with specific
      types of access privileges. There are seven separate permissions and
      up to twenty different people or groups of people can appear on an
      access control list.</para>

      <para>For a more detailed description of AFS's mutual authentication
      procedure, see <link linkend="HDRWQ75">A More Detailed Look at
      Mutual Authentication</link>. For further discussion of ACLs, see
      <link linkend="HDRWQ562">Managing Access Control
      Lists</link>.</para>
    </sect2>
  </sect1>

  <sect1 id="HDRWQ7">
    <title>More Detailed Discussions of Some Basic Concepts</title>

    <para>The previous section offered a brief overview of the many
    concepts that an AFS system administrator needs to understand.  The
    following sections examine some important concepts in more
    detail. Although not all concepts are new to an experienced
    administrator, reading this section helps ensure a common
    understanding of term and concepts.</para>

    <sect2 id="HDRWQ8">
      <title>Networks</title>

      <indexterm>
        <primary>network</primary>

        <secondary>defined</secondary>
      </indexterm>

      <para>A <emphasis>network</emphasis> is a collection of
      interconnected computers able to communicate with each other and
      transfer information back and forth.</para>

      <para>A network can connect computers of any kind, but the typical
      network running AFS connects servers or high-function personal
      workstations with AFS file server machines. For more about the
      classes of machines used in an AFS environment, see <link
      linkend="HDRWQ10">Servers and Clients</link>.</para>
    </sect2>

    <sect2 id="HDRWQ9">
      <title>Distributed File Systems</title>

      <indexterm>
        <primary>file system</primary>

        <secondary>defined</secondary>
      </indexterm>

      <indexterm>
        <primary>distributed file system</primary>
      </indexterm>

      <para>A <emphasis>file system</emphasis> is a collection of files
      and the facilities (programs and commands) that enable users to
      access the information in the files. All computing environments have
      file systems.</para>

      <para>Networked computing environments often use
      <emphasis>distributed file systems</emphasis> like AFS. A
      distributed file system takes advantage of the interconnected nature
      of the network by storing files on more than one computer in the
      network and making them accessible to all of them. In other words,
      the responsibility for file storage and delivery is "distributed"
      among multiple machines instead of relying on only one. Despite the
      distribution of responsibility, a distributed file system like AFS
      creates the illusion that there is a single filespace.</para>
    </sect2>

    <sect2 id="HDRWQ10">
      <title>Servers and Clients</title>

      <indexterm>
        <primary>server/client model</primary>
      </indexterm>

      <indexterm>
        <primary>server</primary>

        <secondary>definition</secondary>
      </indexterm>

      <indexterm>
        <primary>client</primary>

        <secondary>definition</secondary>
      </indexterm>

      <para>AFS uses a server/client model. In general, a server is a
      machine, or a process running on a machine, that provides
      specialized services to other machines. A client is a machine or
      process that makes use of a server's specialized service during the
      course of its own work, which is often of a more general nature than
      the server's. The functional distinction between clients and server
      is not always strict, however--a server can be considered the client
      of another server whose service it is using.</para>

      <para>AFS divides the machines on a network into two basic classes,
      <emphasis>file server machines</emphasis> and <emphasis>client
      machines</emphasis>, and assigns different tasks and
      responsibilities to each.</para>

      <formalpara>
        <title>File Server Machines</title>

        <indexterm>
          <primary>file server machine</primary>
        </indexterm>

        <indexterm>
          <primary>server</primary>

          <secondary>process</secondary>

          <tertiary>definition</tertiary>
        </indexterm>

        <para><emphasis>File server machines</emphasis> store the files in
        the distributed file system, and a <emphasis>server
        process</emphasis> running on the file server machine delivers and
        receives files. AFS file server machines run a number of
        <emphasis>server processes</emphasis>. Each process has a special
        function, such as maintaining databases important to AFS
        administration, managing security or handling volumes. This
        modular design enables each server process to specialize in one
        area, and thus perform more efficiently. For a description of the
        function of each AFS server process, see <link
        linkend="HDRWQ17">AFS Server Processes and the Cache
        Manager</link>.</para>
      </formalpara>

      <para>Not all AFS server machines must run all of the server
      processes. Some processes run on only a few machines because the
      demand for their services is low. Other processes run on only one
      machine in order to act as a synchronization site. See <link
      linkend="HDRWQ90">The Four Roles for File Server
      Machines</link>.</para>

      <formalpara>
        <title>Client Machines</title>

        <indexterm>
          <primary>client</primary>

          <secondary>machine</secondary>

          <tertiary>definition</tertiary>
        </indexterm>

        <para>The other class of machines are the <emphasis>client
        machines</emphasis>, which generally work directly for users,
        providing computational power and other general purpose tools but
        may also be other servers that use data stored in AFS to provide
        other services. Clients also provide users with access to the
        files stored on the file server machines. Clients run a Cache
        Manager, which is normally a combination of a kernel module and a
        running process that enables them to communicate with the AFS
        server processes running on the file server machines and to cache
        files. See <link linkend="HDRWQ28">The Cache Manager</link> for
        more information. There are usually many more client machines in a
        cell than file server machines.</para>
      </formalpara>
    </sect2>

    <sect2 id="HDRWQ11">
      <title>Cells</title>

      <indexterm>
        <primary>cell</primary>
      </indexterm>

      <para>A <emphasis>cell</emphasis> is an independently administered
      site running AFS. In terms of hardware, it consists of a collection
      of file server machines defined as belonging to the cell. To say
      that a cell is administratively independent means that its
      administrators determine many details of its configuration without
      having to consult administrators in other cells or a central
      authority. For example, a cell administrator determines how many
      machines of different types to run, where to put files in the local
      tree, how to associate volumes and directories, and how much space
      to allocate to each user.</para>

      <para>The terms <emphasis>local cell</emphasis> and <emphasis>home
      cell</emphasis> are equivalent, and refer to the cell in which a
      user has initially authenticated during a session, by logging onto a
      machine that belongs to that cell. All other cells are referred to
      as <emphasis>foreign</emphasis> from the user's perspective. In
      other words, throughout a login session, a user is accessing the
      filespace through a single Cache Manager--the one on the machine to
      which he or she initially logged in--and that Cache Manager is
      normally configured to have a default local cell. All other cells
      are considered foreign during that login session, even if the user
      authenticates in additional cells or uses the <emphasis
      role="bold">cd</emphasis> command to change directories into their
      file trees. This distinction is mostly invisible and irrelavant to
      users. For most purposes, users will see no difference between local
      and foreign cells.</para>

      <indexterm>
        <primary>local cell</primary>
      </indexterm>

      <indexterm>
        <primary>cell</primary>

        <secondary>local</secondary>
      </indexterm>

      <indexterm>
        <primary>foreign cell</primary>
      </indexterm>

      <indexterm>
        <primary>cell</primary>

        <secondary>foreign</secondary>
      </indexterm>

      <para>It is possible to maintain more than one cell at a single
      geographical location. For instance, separate departments on a
      university campus or in a corporation can choose to administer their
      own cells. It is also possible to have machines at geographically
      distant sites belong to the same cell; only limits on the speed of
      network communication determine how practical this is.</para>

      <para>Despite their independence, AFS cells generally agree to make
      their local filespace visible to other AFS cells, so that users in
      different cells can share files if they choose. If your cell is to
      participate in the "global" AFS namespace, it must comply with a few
      basic conventions governing how the local filespace is configured
      and how the addresses of certain file server machines are advertised
      to the outside world.</para>
    </sect2>

    <sect2 id="HDRWQ12">
      <title>The Uniform Namespace and Transparent Access</title>

      <indexterm>
        <primary>transparent access as AFS feature</primary>
      </indexterm>

      <indexterm>
        <primary>access</primary>

        <secondary>transparent (AFS feature)</secondary>
      </indexterm>

      <para>One of the features that makes AFS easy to use is that it
      provides transparent access to the files in a cell's
      filespace. Users do not have to know which file server machine
      stores a file in order to access it; they simply provide the file's
      pathname, which AFS automatically translates into a machine
      location.</para>

      <para>In addition to transparent access, AFS also creates a
      <emphasis>uniform namespace</emphasis>--a file's pathname is
      identical regardless of which client machine the user is working
      on. The cell's file tree looks the same when viewed from any client
      because the cell's file server machines store all the files
      centrally and present them in an identical manner to all
      clients.</para>

      <para>To enable the transparent access and the uniform namespace
      features, the system administrator must follow a few simple
      conventions in configuring client machines and file trees. For
      details, see <link linkend="HDRWQ39">Making Other Cells Visible in
      Your Cell</link>.</para>
    </sect2>

    <sect2 id="HDRWQ13">
      <title>Volumes</title>

      <indexterm>
        <primary>volume</primary>

        <secondary>definition</secondary>
      </indexterm>

      <para>A <emphasis>volume</emphasis> is a conceptual container for a
      set of related files that keeps them all together on one file server
      machine partition. Volumes can vary in size, but are (by definition)
      smaller than a partition. Volumes are the main administrative unit
      in AFS, and have several characteristics that make administrative
      tasks easier and help improve overall system
      performance. <itemizedlist>
          <listitem>
            <para>The relatively small size of volumes makes them easy to
            move from one partition to another, or even between
            machines.</para>
          </listitem>

          <listitem>
            <para>You can maintain maximum system efficiency by moving
            volumes to keep the load balanced evenly among the different
            machines. If a partition becomes full, the small size of
            individual volumes makes it easy to find enough room on other
            machines for them.</para>

            <indexterm>
              <primary>volume</primary>

              <secondary>in load balancing</secondary>
            </indexterm>
          </listitem>

          <listitem>
            <para>Each volume corresponds logically to a directory in the
            file tree and keeps together, on a single partition, all the
            data that makes up the files in the directory (including
            possible subdirectories). By maintaining (for example) a
            separate volume for each user's home directory, you keep all
            of the user's files together, but separate from those of other
            users. This is an administrative convenience that is
            impossible if the partition is the smallest unit of
            storage.</para>

            <indexterm>
              <primary>volume</primary>

              <secondary>correspondence with directory</secondary>
            </indexterm>

            <indexterm>
              <primary>directory</primary>

              <secondary>correspondence with volume</secondary>
            </indexterm>

            <indexterm>
              <primary>correspondence</primary>

              <secondary>of volumes and directories</secondary>
            </indexterm>
          </listitem>

          <listitem>
            <para>The directory/volume correspondence also makes
            transparent file access possible, because it simplifies the
            process of file location. All files in a directory reside
            together in one volume and in order to find a file, a file
            server process need only know the name of the file's parent
            directory, information which is included in the file's
            pathname.  AFS knows how to translate the directory name into
            a volume name, and automatically tracks every volume's
            location, even when a volume is moved from machine to
            machine. For more about the directory/volume correspondence,
            see <link linkend="HDRWQ14">Mount Points</link>.</para>
          </listitem>

          <listitem>
            <para>Volumes increase file availability through replication
            and backup.</para>

            <indexterm>
              <primary>volume</primary>

              <secondary>as unit of</secondary>

              <tertiary>replication</tertiary>
            </indexterm>

            <indexterm>
              <primary>volume</primary>

              <secondary>as unit of</secondary>

              <tertiary>backup</tertiary>
            </indexterm>
          </listitem>

          <listitem>
            <para>Replication (placing copies of a volume on more than one
            file server machine) makes the contents more reliably
            available; for details, see <link
            linkend="HDRWQ15">Replication</link>. Entire sets of volumes
            can be backed up as dump files (possibly to tape) and restored
            to the file system; see <link linkend="HDRWQ248">Configuring
            the AFS Backup System</link> and <link
            linkend="HDRWQ283">Backing Up and Restoring AFS
            Data</link>. In AFS, backup also refers to recording the state
            of a volume at a certain time and then storing it (either on
            tape or elsewhere in the file system) for recovery in the
            event files in it are accidentally deleted or changed. See
            <link linkend="HDRWQ201">Creating Backup
            Volumes</link>.</para>
          </listitem>

          <listitem>
            <para>Volumes are the unit of resource management. A space
            quota associated with each volume sets a limit on the maximum
            volume size. See <link linkend="HDRWQ234">Setting and
            Displaying Volume Quota and Current Size</link>.</para>

            <indexterm>
              <primary>volume</primary>

              <secondary>as unit of</secondary>

              <tertiary>resource management</tertiary>
            </indexterm>
          </listitem>
        </itemizedlist>
      </para>
    </sect2>

    <sect2 id="HDRWQ14">
      <title>Mount Points</title>

      <indexterm>
        <primary>mount point</primary>

        <secondary>definition</secondary>
      </indexterm>

      <para>The previous section discussed how each volume corresponds
      logically to a directory in the file system: the volume keeps
      together on one partition all the data in the files residing in the
      directory. The directory that corresponds to a volume is called its
      <emphasis>root directory</emphasis>, and the mechanism that
      associates the directory and volume is called a <emphasis>mount
      point</emphasis>. A mount point is similar to a symbolic link in the
      file tree that specifies which volume contains the files kept in a
      directory. A mount point is not an actual symbolic link; its
      internal structure is different.</para>

      <note>
        <para>You must not create, in AFS, a symbolic link to a file whose
        name begins with the number sign (#) or the percent sign (%),
        because the Cache Manager interprets such a link as a mount point
        to a regular or read/write volume, respectively.</para>
      </note>

      <indexterm>
        <primary>root directory</primary>
      </indexterm>

      <indexterm>
        <primary>directory</primary>

        <secondary>root</secondary>
      </indexterm>

      <indexterm>
        <primary>volume</primary>

        <secondary>root directory of</secondary>
      </indexterm>

      <indexterm>
        <primary>volume</primary>

        <secondary>mounting</secondary>
      </indexterm>

      <para>The use of mount points means that many of the elements in an
      AFS file tree that look and function just like standard UNIX file
      system directories are actually mount points. In form, a mount point
      is a symbolic link in a special format that names the volume
      containing the data for files in the directory. When the Cache
      Manager (see <link linkend="HDRWQ28">The Cache Manager</link>)
      encounters a mount point--for example, in the course of interpreting
      a pathname--it looks in the volume named in the mount point. In the
      volume the Cache Manager finds an actual UNIX-style directory
      element--the volume's root directory--that lists the files contained
      in the directory/volume. The next element in the pathname appears in
      that list.</para>

      <para>A volume is said to be <emphasis>mounted</emphasis> at the
      point in the file tree where there is a mount point pointing to the
      volume. A volume's contents are not visible or accessible unless it
      is mounted. Unlike some other file systems, AFS volumes can be
      mounted at multiple locations in the file system at the same
      time.</para>
    </sect2>

    <sect2 id="HDRWQ15">
      <title>Replication</title>

      <indexterm>
        <primary>replication</primary>

        <secondary>definition</secondary>
      </indexterm>

      <indexterm>
        <primary>clone</primary>
      </indexterm>

      <para><emphasis>Replication</emphasis> refers to making a copy, or
      <emphasis>clone</emphasis>, of a source read/write volume and then
      placing the copy on one or more additional file server machines in a
      cell. One benefit of replicating a volume is that it increases the
      availability of the contents. If one file server machine housing the
      volume fails, users can still access the volume on a different
      machine. No one machine need become overburdened with requests for a
      popular file, either, because the file is available from several
      machines.</para>

      <para>Replication is not necessarily appropriate for cells with
      limited disk space, nor are all types of volumes equally suitable
      for replication (replication is most appropriate for volumes that
      contain popular files that do not change very often). For more
      details, see <link linkend="HDRWQ50">When to Replicate
      Volumes</link>.</para>
    </sect2>

    <sect2 id="HDRWQ16">
      <title>Caching and Callbacks</title>

      <indexterm>
        <primary>caching</primary>
      </indexterm>

      <para>Just as replication increases system availability,
      <emphasis>caching</emphasis> increases the speed and efficiency of
      file access in AFS. Each AFS client machine dedicates a portion of
      its local disk or memory to a cache where it stores data
      temporarily. Whenever an application program (such as a text editor)
      running on a client machine requests data from an AFS file, the
      request passes through the Cache Manager. The Cache Manager is a
      portion of the client machine's kernel that translates file requests
      from local application programs into cross-network requests to the
      <emphasis>File Server process</emphasis> running on the file server
      machine storing the file. When the Cache Manager receives the
      requested data from the File Server, it stores it in the cache and
      then passes it on to the application program.</para>

      <para>Caching improves the speed of data delivery to application
      programs in the following ways:</para>

      <itemizedlist>
        <listitem>
          <para>When the application program repeatedly asks for data from
          the same file, it is already on the local disk. The application
          does not have to wait for the Cache Manager to request and
          receive the data from the File Server.</para>
        </listitem>

        <listitem>
          <para>Caching data eliminates the need for repeated request and
          transfer of the same data, so network traffic is reduced.  Thus,
          initial requests and other traffic can get through more
          quickly.</para>

          <indexterm>
            <primary>AFS</primary>

            <secondary>reducing traffic in</secondary>
          </indexterm>

          <indexterm>
            <primary>network</primary>

            <secondary>reducing traffic through caching</secondary>
          </indexterm>

          <indexterm>
            <primary>slowed performance</primary>

            <secondary>preventing in AFS</secondary>
          </indexterm>
        </listitem>
      </itemizedlist>

      <indexterm>
        <primary>callback</primary>
      </indexterm>

      <indexterm>
        <primary>consistency guarantees</primary>

        <secondary>cached data</secondary>
      </indexterm>

      <para>While caching provides many advantages, it also creates the
      problem of maintaining consistency among the many cached copies of a
      file and the source version of a file. This problem is solved using
      a mechanism referred to as a <emphasis>callback</emphasis>.</para>

      <para>A callback is a promise by a File Server to a Cache Manager to
      inform the latter when a change is made to any of the data delivered
      by the File Server. Callbacks are used differently based on the type
      of file delivered by the File Server:  <itemizedlist>
          <listitem>
            <para>When a File Server delivers a writable copy of a file
            (from a read/write volume) to the Cache Manager, the File
            Server sends along a callback with that file. If the source
            version of the file is changed by another user, the File
            Server breaks the callback associated with the cached version
            of that file--indicating to the Cache Manager that it needs to
            update the cached copy.</para>
          </listitem>

          <listitem>
            <para>When a File Server delivers a file from a read-only
            volume to the Cache Manager, the File Server sends along a
            callback associated with the entire volume (so it does not
            need to send any more callbacks when it delivers additional
            files from the volume). Only a single callback is required per
            accessed read-only volume because files in a read-only volume
            can change only when a new version of the complete volume is
            released. All callbacks associated with the old version of the
            volume are broken at release time.</para>
          </listitem>
        </itemizedlist>
      </para>

      <para>The callback mechanism ensures that the Cache Manager always
      requests the most up-to-date version of a file. However, it does not
      ensure that the user necessarily notices the most current version as
      soon as the Cache Manager has it. That depends on how often the
      application program requests additional data from the File System or
      how often it checks with the Cache Manager.</para>
    </sect2>
  </sect1>

  <sect1 id="HDRWQ17">
    <title>AFS Server Processes and the Cache Manager</title>

    <indexterm>
      <primary>AFS</primary>

      <secondary>server processes used in</secondary>
    </indexterm>

    <indexterm>
      <primary>server</primary>

      <secondary>process</secondary>

      <tertiary>list of AFS</tertiary>
    </indexterm>

    <para>As mentioned in <link linkend="HDRWQ10">Servers and
    Clients</link>, AFS file server machines run a number of processes,
    each with a specialized function. One of the main responsibilities of
    a system administrator is to make sure that processes are running
    correctly as much of the time as possible, using the administrative
    services that the server processes provide.</para>

    <para>The following list briefly describes the function of each server
    process and the Cache Manager; the following sections then discuss the
    important features in more detail.</para>

    <para>The <emphasis>File Server</emphasis>, the most fundamental of
    the servers, delivers data files from the file server machine to local
    workstations as requested, and stores the files again when the user
    saves any changes to the files.</para>

    <para>The <emphasis>Basic OverSeer Server (BOS Server)</emphasis>
    ensures that the other server processes on its server machine are
    running correctly as much of the time as possible, since a server is
    useful only if it is available. The BOS Server relieves system
    administrators of much of the responsibility for overseeing system
    operations.</para>

    <para>The Protection Server helps users control who has access to
    their files and directories. It is responsible for mapping Kerberos
    principals to AFS identities. Users can also grant access to several
    other users at once by putting them all in a group entry in the
    Protection Database maintained by the Protection Server.</para>

    <para>The <emphasis>Volume Server</emphasis> performs all types of
    volume manipulation. It helps the administrator move volumes from one
    server machine to another to balance the workload among the various
    machines.</para>

    <para>The <emphasis>Volume Location Server (VL Server)</emphasis>
    maintains the Volume Location Database (VLDB), in which it records the
    location of volumes as they move from file server machine to file
    server machine. This service is the key to transparent file access for
    users.</para>

    <para>The <emphasis>Salvager</emphasis> is not a server in the sense
    that others are. It runs only after the File Server or Volume Server
    fails; it repairs any inconsistencies caused by the failure. The
    system administrator can invoke it directly if necessary.</para>

    <para>The <emphasis>Update Server</emphasis> distributes new versions
    of AFS server process software and configuration information to all
    file server machines. It is crucial to stable system performance that
    all server machines run the same software.</para>

    <para>The <emphasis>Backup Server</emphasis> maintains the Backup
    Database, in which it stores information related to the Backup
    System. It enables the administrator to back up data from volumes to
    tape. The data can then be restored from tape in the event that it is
    lost from the file system. The Backup Server is optional and is only
    one of several ways that the data in an AFS cell can be backed
    up.</para>

    <para>The <emphasis>Cache Manager</emphasis> is the one component in
    this list that resides on AFS client rather than file server
    machines. It not a process per se, but rather a part of the kernel on
    AFS client machines that communicates with AFS server processes. Its
    main responsibilities are to retrieve files for application programs
    running on the client and to maintain the files in the cache.</para>

    <para>AFS also relies on two other services that are not part of AFS
    and need to be instaled separately:</para>

    <para>AFS requires a <emphasis>Kerberos KDC</emphasis> to use for user
    authentication. It verifies user identities at login and provides the
    facilities through which participants in transactions prove their
    identities to one another (mutually authenticate). AFS uses Kerberos
    for all of its authentication. The Kerberos KDC replaces the old
    <emphasis>Authentication Server</emphasis> included in OpenAFS. The
    Authentication Server is still available for sites that need it, but
    is now deprecated and should not be used for any new
    installations.</para>

    <para>The <emphasis>Network Time Protocol Daemon (NTPD)</emphasis> is
    not an AFS server process, but plays a vital role nonetheless. It
    synchronizes the internal clock on a file server machine with those on
    other machines. Synchronized clocks are particularly important for
    correct functioning of the AFS distributed database technology (known
    as Ubik); see <link linkend="HDRWQ103">Configuring the Cell for Proper
    Ubik Operation</link>. The NTPD is usually provided with the operating
    system.</para>

    <sect2 id="HDRWQ18">
      <title>The File Server</title>

      <indexterm>
        <primary>File Server</primary>

        <secondary>description</secondary>
      </indexterm>

      <para>The <emphasis>File Server</emphasis> is the most fundamental
      of the AFS server processes and runs on each file server machine. It
      provides the same services across the network that the UNIX file
      system provides on the local disk: <itemizedlist>
          <listitem>
            <para>Delivering programs and data files to client
            workstations as requested and storing them again when the
            client workstation finishes with them.</para>
          </listitem>

          <listitem>
            <para>Maintaining the hierarchical directory structure that
            users create to organize their files.</para>
          </listitem>

          <listitem>
            <para>Handling requests for copying, moving, creating, and
            deleting files and directories.</para>
          </listitem>

          <listitem>
            <para>Keeping track of status information about each file and
            directory (including its size and latest modification
            time).</para>
          </listitem>

          <listitem>
            <para>Making sure that users are authorized to perform the
            actions they request on particular files or
            directories.</para>
          </listitem>

          <listitem>
            <para>Creating symbolic and hard links between files.</para>
          </listitem>

          <listitem>
            <para>Granting advisory locks (corresponding to UNIX locks) on
            request.</para>
          </listitem>
        </itemizedlist>
      </para>
    </sect2>

    <sect2 id="HDRWQ19">
      <title>The Basic OverSeer Server</title>

      <indexterm>
        <primary>BOS Server</primary>

        <secondary>description</secondary>
      </indexterm>

      <para>The <emphasis>Basic OverSeer Server (BOS Server)</emphasis>
      reduces the demands on system administrators by constantly
      monitoring the processes running on its file server machine. It can
      restart failed processes automatically and provides a convenient
      interface for administrative tasks.</para>

      <para>The BOS Server runs on every file server machine. Its primary
      function is to minimize system outages. It also</para>

      <itemizedlist>
        <listitem>
          <para>Constantly monitors the other server processes (on the
          local machine) to make sure they are running correctly.</para>
        </listitem>

        <listitem>
          <para>Automatically restarts failed processes, without
          contacting a human operator. When restarting multiple server
          processes simultaneously, the BOS server takes interdependencies
          into account and initiates restarts in the correct order.</para>

          <indexterm>
            <primary>system outages</primary>

            <secondary>reducing</secondary>
          </indexterm>

          <indexterm>
            <primary>outages</primary>

            <secondary>BOS Server role in,</secondary>
          </indexterm>
        </listitem>

        <listitem>
          <para>Accepts requests from the system administrator. Common
          reasons to contact BOS are to verify the status of server
          processes on file server machines, install and start new
          processes, stop processes either temporarily or permanently, and
          restart dead processes manually.</para>
        </listitem>

        <listitem>
          <para>Helps system administrators to manage system configuration
          information. The BOS Server provides a simple interface for
          modifying two files that contain information about privileged
          users and certain special file server machines. It also
          automates the process of adding and changing <emphasis>server
          encryption keys</emphasis>, which are important in mutual
          authentication, if the Authentication Server is still in use,
          but this function of the BOS Server is deprecated. For more
          details about these configuration files, see <link
          linkend="HDRWQ85">Common Configuration Files in the /usr/afs/etc
          Directory</link>.</para>
        </listitem>
      </itemizedlist>
    </sect2>

    <sect2 id="HDRWQ21">
      <title>The Protection Server</title>

      <indexterm>
        <primary>protection</primary>

        <secondary>in AFS</secondary>
      </indexterm>

      <indexterm>
        <primary>Protection Server</primary>

        <secondary>description</secondary>
      </indexterm>

      <indexterm>
        <primary>protection</primary>

        <secondary>in UNIX</secondary>
      </indexterm>

      <para>The <emphasis>Protection Server</emphasis> is the key to AFS's
      refinement of the normal UNIX methods for protecting files and
      directories from unauthorized use. The refinements include the
      following: <itemizedlist>
          <listitem>
            <para>Defining associations between Kerberos principals and
            AFS identities. Normally, this is a simple mapping between
            principal names in the Kerberos realm associated with an AFS
            cell to AFS identities in that cell, but the Protection Server
            also manages mappings for users using cross-realm
            authentication from a different Kerberos realm.</para>

            <para>Defining seven access permissions rather than the
            standard UNIX file system's three. In conjunction with the
            UNIX mode bits associated with each file and directory
            element, AFS associates an <emphasis>access control list
            (ACL)</emphasis> with each directory. The ACL specifies which
            users have which of the seven specific permissions for the
            directory and all the files it contains. For a definition of
            AFS's seven access permissions and how users can set them on
            access control lists, see <link linkend="HDRWQ562">Managing
            Access Control Lists</link>.</para>

            <indexterm>
              <primary>access</primary>

              <secondary></secondary>

              <see>ACL</see>
            </indexterm>
          </listitem>

          <listitem>
            <para>Enabling users to grant permissions to numerous
            individual users--a different combination to each individual
            if desired. UNIX protection distinguishes only between three
            user or groups: the owner of the file, members of a single
            specified group, and everyone who can access the local file
            system.</para>
          </listitem>

          <listitem>
            <para>Enabling users to define their own groups of users,
            recorded in the <emphasis>Protection Database</emphasis>
            maintained by the Protection Server. The groups then appear on
            directories' access control lists as though they were
            individuals, which enables the granting of permissions to many
            users simultaneously.</para>
          </listitem>

          <listitem>
            <para>Enabling system administrators to create groups
            containing client machine IP addresses to permit access when
            it originates from the specified client machines. These types
            of groups are useful when it is necessary to adhere to
            machine-based licensing restrictions or where it is difficult
            for some reason to obtain Kerberos credentials for processes
            running on those systems that need access to AFS.</para>
          </listitem>
        </itemizedlist>
      </para>

      <indexterm>
        <primary>group</primary>

        <secondary>definition</secondary>
      </indexterm>

      <indexterm>
        <primary>Protection Database</primary>
      </indexterm>

      <para>The Protection Server's main duty is to help the File Server
      determine if a user is authorized to access a file in the requested
      manner. The Protection Server creates a list of all the groups to
      which the user belongs. The File Server then compares this list to
      the ACL associated with the file's parent directory. A user thus
      acquires access both as an individual and as a member of any
      groups.</para>

      <para>The Protection Server also maps Kerberos principals to
      <emphasis>AFS user ID</emphasis> numbers (<emphasis>AFS
      UIDs</emphasis>). These UIDs are functionally equivalent to UNIX
      UIDs, but operate in the domain of AFS rather than in the UNIX file
      system on a machine's local disk. This conversion service is
      essential because the tickets that the Kerberos KDC gives to
      authenticated users are stamped with principal names (to comply with
      Kerberos standards). The AFS server processes identify users by AFS
      UID, not by username. Before they can understand whom the token
      represents, they need the Protection Server to translate the
      username into an AFS UID. For further discussion of the
      authentication process, see <link linkend="HDRWQ75">A More Detailed
      Look at Mutual Authentication</link>.</para>
    </sect2>

    <sect2 id="HDRWQ22">
      <title>The Volume Server</title>

      <indexterm>
        <primary>Volume Server</primary>

        <secondary>description</secondary>
      </indexterm>

      <para>The <emphasis>Volume Server</emphasis> provides the interface
      through which you create, delete, move, and replicate volumes, as
      well as prepare them for archiving to disk, tape, or other media
      (backing up). <link linkend="HDRWQ13">Volumes</link> explained the
      advantages gained by storing files in volumes. Creating and deleting
      volumes are necessary when adding and removing users from the
      system; volume moves are done for load balancing; and replication
      enables volume placement on multiple file server machines (for more
      on replication, see <link
      linkend="HDRWQ15">Replication</link>).</para>
    </sect2>

    <sect2 id="HDRWQ23">
      <title>The Volume Location (VL) Server</title>

      <indexterm>
        <primary>VL Server</primary>

        <secondary>description</secondary>
      </indexterm>

      <indexterm>
        <primary>VLDB</primary>
      </indexterm>

      <para>The <emphasis>VL Server</emphasis> maintains a complete list
      of volume locations in the <emphasis>Volume Location Database
      (VLDB)</emphasis>. When the Cache Manager (see <link
      linkend="HDRWQ28">The Cache Manager</link>) begins to fill a file
      request from an application program, it first contacts the VL Server
      in order to learn which file server machine currently houses the
      volume containing the file. The Cache Manager then requests the file
      from the File Server process running on that file server
      machine.</para>

      <para>The VLDB and VL Server make it possible for AFS to take
      advantage of the increased system availability gained by using
      multiple file server machines, because the Cache Manager knows where
      to find a particular file. Indeed, in a certain sense the VL Server
      is the keystone of the entire file system--when the information in
      the VLDB is inaccessible, the Cache Manager cannot retrieve files,
      even if the File Server processes are working properly. A list of
      the information stored in the VLDB about each volume is provided in
      <link linkend="HDRWQ180">Volume Information in the
      VLDB</link>.</para>

      <indexterm>
        <primary>VL Server</primary>

        <secondary>importance to transparent access</secondary>
      </indexterm>
    </sect2>

    <sect2 id="HDRWQ26">
      <title>The Salvager</title>

      <indexterm>
        <primary>Salvager</primary>

        <secondary>description</secondary>
      </indexterm>

      <para>The <emphasis>Salvager</emphasis> differs from other AFS
      Servers in that it runs only at selected times. The BOS Server
      invokes the Salvager when the File Server, Volume Server, or both
      fail. The Salvager attempts to repair disk corruption that can
      result from a failure.</para>

      <para>As a system administrator, you can also invoke the Salvager as
      necessary, even if the File Server or Volume Server has not
      failed. See <link linkend="HDRWQ232">Salvaging
      Volumes</link>.</para>
    </sect2>

    <sect2 id="HDRWQ24">
      <title>The Update Server</title>

      <indexterm>
        <primary>Update Server</primary>

        <secondary>description</secondary>
      </indexterm>

      <para>The <emphasis>Update Server</emphasis> is an optional process
      that helps guarantee that all file server machines are running the
      same version of a server process. System performance can be
      inconsistent if some machines are running one version of the File
      Server (for example) and other machines were running another
      version.</para>

      <para>To ensure that all machines run the same version of a process,
      install new software on a single file server machine of each system
      type, called the <emphasis>binary distribution machine</emphasis>
      for that type. The binary distribution machine runs the server
      portion of the Update Server, whereas all the other machines of that
      type run the client portion of the Update Server. The client
      portions check frequently with the <emphasis>server
      portion</emphasis> to see if they are running the right version of
      every process; if not, the <emphasis>client portion</emphasis>
      retrieves the right version from the binary distribution machine and
      installs it locally. The system administrator does not need to
      remember to install new software individually on all the file server
      machines: the Update Server does it automatically. For more on
      binary distribution machines, see <link linkend="HDRWQ93">Binary
      Distribution Machines</link>.</para>

      <indexterm>
        <primary>Update Server</primary>

        <secondary>server portion</secondary>
      </indexterm>

      <indexterm>
        <primary>Update Server</primary>

        <secondary>client portion</secondary>
      </indexterm>

      <para>The Update Server also distributes configuration files that
      all file server machines need to store on their local disks (for a
      description of the contents and purpose of these files, see <link
      linkend="HDRWQ85">Common Configuration Files in the /usr/afs/etc
      Directory</link>). As with server process software, the need for
      consistent system performance demands that all the machines have the
      same version of these files. The system administrator needs to make
      changes to these files on one machine only, the cell's
      <emphasis>system control machine</emphasis>, which runs a server
      portion of the Update Server. All other machines in the cell run a
      client portion that accesses the correct versions of these
      configuration files from the system control machine. For more
      information, see <link linkend="HDRWQ94">The System Control
      Machine</link>.</para>
    </sect2>

    <sect2 id="HDRWQ25">
      <title>The Backup Server</title>

      <indexterm>
        <primary>Backup System</primary>

        <secondary>Backup Server described</secondary>
      </indexterm>

      <indexterm>
        <primary>Backup Server</primary>

        <secondary>description</secondary>
      </indexterm>

      <para>The <emphasis>Backup Server</emphasis> is an optional process
      that maintains the information in the <emphasis>Backup
      Database</emphasis>. The Backup Server and the Backup Database
      enable administrators to back up data from AFS volumes to tape and
      restore it from tape to the file system if necessary. The server and
      database together are referred to as the Backup System. This Backup
      System is only one way to back up AFS, and many AFS cells use
      different methods.</para>

      <para>Administrators who wish to use the Backup System initially
      configure it by defining sets of volumes to be dumped together and
      the schedule by which the sets are to be dumped. They also install
      the system's tape drives and define the drives' <emphasis>Tape
      Coordinators</emphasis>, which are the processes that control the
      tape drives.</para>

      <para>Once the Backup System is configured, user and system data can
      be dumped from volumes to tape or disk. In the event that data is
      ever lost from the system (for example, if a system or disk failure
      causes data to be lost), administrators can restore the data from
      tape. If tapes are periodically archived, or saved, data can also be
      restored to its state at a specific time.  Additionally, because
      Backup System data is difficult to reproduce, the Backup Database
      itself can be backed up to tape and restored if it ever becomes
      corrupted. For more information on configuring and using the Backup
      System, and on other AFS backup options, see <link
      linkend="HDRWQ248">Configuring the AFS Backup System</link> and
      <link linkend="HDRWQ283">Backing Up and Restoring AFS
      Data</link>.</para>
    </sect2>

    <sect2 id="HDRWQ28">
      <title>The Cache Manager</title>

      <indexterm>
        <primary>Cache Manager</primary>

        <secondary>functions of</secondary>
      </indexterm>

      <para>As already mentioned in <link linkend="HDRWQ16">Caching and
      Callbacks</link>, the <emphasis>Cache Manager</emphasis> is the one
      component in this section that resides on client machines rather
      than on file server machines. It is a combination of a daemon
      process and a set of extensions or modifications in the client
      machine's kernel, usually implemented as a loadable kernel module,
      that enable communication with the server processes running on
      server machines. Its main duty is to translate file requests (made
      by application programs on client machines) into <emphasis>remote
      procedure calls (RPCs)</emphasis> to the File Server. (The Cache
      Manager first contacts the VL Server to find out which File Server
      currently houses the volume that contains a requested file, as
      mentioned in <link linkend="HDRWQ23">The Volume Location (VL)
      Server</link>). When the Cache Manager receives the requested file,
      it caches it before passing data on to the application
      program.</para>

      <para>The Cache Manager also tracks the state of files in its cache
      compared to the version at the File Server by storing the callbacks
      sent by the File Server. When the File Server breaks a callback,
      indicating that a file or volume changed, the Cache Manager requests
      a copy of the new version before providing more data to application
      programs.</para>
    </sect2>

    <sect2 id="HDRWQ20">
      <title>The Kerberos KDC</title>

      <indexterm>
        <primary>Kerberos KDC</primary>
        <secondary>description</secondary>
      </indexterm>
      <indexterm>
        <primary>Authentication Server</primary>
        <secondary>description</secondary>
        <seealso>Kerberos KDC</seealso>
      </indexterm>
      <indexterm>
        <primary>Active Directory</primary>
        <secondary>Kerberos KDC</secondary>
      </indexterm>
      <indexterm>
        <primary>MIT Kerberos</primary>
        <secondary>Kerberos KDC</secondary>
      </indexterm>
      <indexterm>
        <primary>Heimdal</primary>
        <secondary>Kerberos KDC</secondary>
      </indexterm>

      <para>The <emphasis>Kerberos KDC</emphasis> (Key Distribution
      Center) performs two main functions related to network security:
        <itemizedlist>
          <listitem>
            <para>Verifying the identity of users as they log into the
            system by requiring that they provide a password or some other
            form of authentication credentials. The Kerberos KDC grants
            the user a ticket, which is converted into a token to prove to
            AFS server processes that the user has authenticated. For more
            on tokens, see <link linkend="HDRWQ76">Complex Mutual
            Authentication</link>.</para>
          </listitem>

          <listitem>
            <para>Providing the means through which server and client
            processes prove their identities to each other (mutually
            authenticate). This helps to create a secure environment in
            which to send cross-network messages.</para>
          </listitem>
        </itemizedlist>
      </para>

      <para>The Kerberos KDC is a required service, but does not come with
      OpenAFS. One Kerberos KDC may provide authentication services for
      multiple AFS cells. Each AFS cell must be associated with a Kerberos
      realm with one or more Kerberos KDCs supporting version 4 or 5 of
      the Kerberos protocol. Kerberos version 4 is not secure and is
      supported only for backwards compatibility; Kerberos 5 should be
      used for any new installation.</para>

      <para>A Kerberos KDC maintains a database in which it stores
      encryption keys for users and for services, including the AFS server
      encryption key. For users, these encryption keys are normally formed
      by converting a user password to a key, but Kerberos KDCs also
      support other authentication mechanisms. To learn more about the
      procedures AFS uses to verify user identity and during mutual
      authentication, see <link linkend="HDRWQ75">A More Detailed Look at
      Mutual Authentication</link>.</para>

      <para>Kerberos KDC software is included with some operating systems
      or may be acquired separately. MIT Kerberos, Heimdal, and Microsoft
      Active Directory are known to work with OpenAFS as a Kerberos
      Server.This technology was originally developed by the Massachusetts
      Institute of Technology's Project Athena.</para>

      <note>
        <para>The <emphasis>Authentication Server</emphasis>, or kaserver,
        was a Kerberos version 4 KDC. It is obsolete and should no longer
        be used. A third-party Kerberos version 5 KDC should be used
        instead. The Authentication Server is still provided with OpenAFS,
        but only for backward compatibility and legacy support for sites
        that have not yet migrated to a Kerberos version 5 KDC.  the
        Kerberos Server. All references to the <emphasis>Kerberos
        KDC</emphasis> in this guide refer to a Kerberos 5 server.</para>
      </note>

      <indexterm>
        <primary>AFS</primary>

        <secondary></secondary>

        <see>AFS UID</see>
      </indexterm>

      <indexterm>
        <primary>username</primary>

        <secondary>use by Kerberos</secondary>
      </indexterm>

      <indexterm>
        <primary>UNIX</primary>

        <secondary>UID</secondary>

        <tertiary>functional difference from AFS UID</tertiary>
      </indexterm>

      <indexterm>
        <primary>Kerberos</primary>

        <secondary>use of usernames</secondary>
      </indexterm>
    </sect2>

    <sect2 id="HDRWQ27">
      <title>The Network Time Protocol Daemon</title>

      <indexterm>
        <primary>ntpd</primary>

        <secondary>description</secondary>
      </indexterm>

      <para>The <emphasis>Network Time Protocol Daemon (NTPD)</emphasis>
      is not an AFS server process, but plays an important role. It helps
      guarantee that all of the file server machines and client machines
      agree on the time. The NTPD on all file server machines learns the
      correct time from a parent NTPD source, which may be located inside
      or outside the cell.</para>

      <para>Keeping clocks synchronized is particularly important to the
      correct operation of AFS's distributed database technology, which
      coordinates the copies of the Backup, Protection, and Volume
      Location Databases; see <link linkend="HDRWQ52">Replicating the
      OpenAFS Administrative Databases</link>. Client machines may also
      refer to these clocks for the correct time; therefore, it is less
      confusing if all file server machines have the same time. For more
      technical detail about the NTPD, see <ulink
      url="http://www.ntp.org/">The NTP web site</ulink> or the
      documentation for your operating system.</para>

      <important><title>Clock Skew Impact</title> <para>Client machines
      that are authenticating to an OpenAFS cell with valid credentials
      may still fail when the clocks of the client machine, Kerberos KDC,
      and the File Server machines are not in sync.</para></important>
    </sect2>
  </sect1>
</chapter>