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>In fact, AFS stores files on a subset of the machines in a network, called 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. Volume moves are recorded
      automatically, so users do not have to keep track of file locations.</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 workstation 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 networked computing environment contrasts with two types of computing environments: <emphasis>mainframe</emphasis> and
      <emphasis>personal</emphasis>. <indexterm>
          <primary>network</primary>

          <secondary>as computing environment</secondary>
        </indexterm> <indexterm>
          <primary>environment</primary>

          <secondary>types compared</secondary>
        </indexterm> <itemizedlist>
          <listitem>
            <para>A <emphasis>mainframe</emphasis> computing environment is the most traditional. It uses a single powerful computer
            (the mainframe) to do the majority of the work in the system, both file storage and computation. It serves many users,
            who access their files and issue commands to the mainframe via terminals, which generally have only enough computing
            power to accept input from a keyboard and to display data on the screen.</para>

            <indexterm>
              <primary>mainframe</primary>

              <secondary>computing environment</secondary>
            </indexterm>
          </listitem>

          <listitem>
            <para>A <emphasis>personal</emphasis> computing environment is a single small computer that serves one (or, at the most,
            a few) users. Like a mainframe computer, the single computer stores all the files and performs all computation. Like a
            terminal, the personal computer provides access to the computer through a keyboard and screen.</para>

            <indexterm>
              <primary>personal</primary>

              <secondary>computing environment</secondary>
            </indexterm>
          </listitem>
        </itemizedlist></para>

      <para>A network can connect computers of any kind, but the typical network running AFS connects high-function personal
      workstations. Each workstation has some computing power and local disk space, usually more than a personal computer or
      terminal, but less than a mainframe. 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. In a mainframe environment, the file
      system consists of all the files on the mainframe's storage disks, whereas in a personal computing environment it consists of
      the files on the computer's local disk.</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. Clients also provide users with access to the files stored on
        the file server machines. Clients do not run any special processes per se, but do use a modified kernel that enables them to
        communicate with the AFS server processes running on the file server machines and to cache files. This collection of kernel
        modifications is referred to as the Cache Manager; see <link linkend="HDRWQ28">The Cache Manager</link>. There are usually
        many more client machines in a cell than file server machines.</para>
      </formalpara>

      <formalpara>
        <title>Client and Server Configuration</title>

        <indexterm>
          <primary>personal</primary>

          <secondary>workstation</secondary>

          <tertiary>as typical AFS machine</tertiary>
        </indexterm>

        <para>In the most typical AFS configuration, both file server machines and client machines are high-function workstations
        with disk drives. While this configuration is not required, it does have some advantages.</para>
      </formalpara>

      <para>There are several advantages to using personal workstations as file server machines. One is that it is easy to expand
      the network by adding another file server machine. It is also easy to increase storage space by adding disks to existing
      machines. Using workstations rather than more powerful mainframes makes it more economical to use multiple file server
      machines rather than one. Multiple file server machines provide an increase in system availability and reliability if popular
      files are available on more than one machine.</para>

      <para>The advantage of using workstations as clients is that caching on the local disk speeds the delivery of files to
      application programs. (For an explanation of caching, see <link linkend="HDRWQ16">Caching and Callbacks</link>.) Diskless
      machines can access AFS if they are running NFS(R) and the NFS/AFS Translator, an optional component of the AFS
      distribution.</para>
    </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 and client machines defined as belonging to the cell; a machine can only belong to one cell
      at a time. Users also belong to a cell in the sense of having an account in it, but unlike machines can belong to (have an
      account in) multiple cells. 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--whose cell membership defines the 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.</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. 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 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 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 one-line file 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.</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 third-party <emphasis>Kerberos Server</emphasis> replaces the old <emphasis>Authentication Server</emphasis> and helps ensure that communications on the network are secure. It verifies
    user identities at login and provides the facilities through which participants in transactions prove their identities to one
    another (mutually authenticate).</para>

    <para>The Protection Server helps users control who has access to their files and directories. Users can 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>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.</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>Network Time Protocol Daemon (NTPD)</emphasis> is not an AFS server process per se, 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>

    <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>

    <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 automates the process of
          adding and changing <emphasis>server encryption keys</emphasis>, which are important in mutual authentication. The BOS
          Server also provides a simple interface for modifying two files that contain information about privileged users and
          certain special file server machines. 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="HDRWQ20">
      <title>The Kerberos Server</title>

      <indexterm>
        <primary>Kerberos Server</primary>

        <secondary>description</secondary>
      </indexterm>
      <indexterm>
        <primary>Authentication Server</primary>

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


      
      <para>The <emphasis>Kerberos Server</emphasis> 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. The
            Kerberos Server 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 Server is a required service which is provided by
      a third-party Kerberos server that supports version 5 of the
      Kerberos protocol. Kerberos server 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.  (Most Kerberos commands begin with
      the letter
      <emphasis role="bold">k</emphasis>). This technology was originally developed by the Massachusetts Institute of Technology's
      Project Athena.</para>

      <para>The Kerberos Server also maintains the <emphasis>Authentication Database</emphasis>, in which it stores user
      passwords converted into encryption key form as well as the AFS server encryption key. 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>

      <note><para>The <emphasis>Authentication Server</emphasis> known as
      kaserver which uses Kerberos 4 is obsolete and has been replaced by
      the Kerberos Server. All references to the <emphasis>Kerberos
      Server</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="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 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.</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 usernames (the name typed at the login prompt) 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 tokens that
      the Authentication Server grants to authenticated users are stamped with usernames (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 tokens, 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 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="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 BOS 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. Cells running the
      international edition of AFS do not use a system control machine to distribute configuration files. 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> 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.</para>

      <para>Administrators initially configure the Backup System 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, 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="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="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 per se, 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 server, and the fileserver machines are not in
      sync.</para></important>

      <note><title>Legacy runntp</title>
      <para>It is no longer recommended to run the legacy NTPD process
      called <emphasis>runntp</emphasis> that is part of the OpenAFS
      suite. Running the NTPD software that comes with your operating
      system or from <ulink url="http://www.ntp.org/">www.ntp.org</ulink>
      is preferred.</para></note>

    </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 not technically a
      stand-alone process, but rather a set of extensions or modifications in the client machine's kernel 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>
  </sect1>
</chapter>