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<H1>Administration Guide</H1>
<HR><P ALIGN="center"> <A HREF="../index.htm"><IMG SRC="../books.gif" BORDER="0" ALT="[Return to Library]"></A> <A HREF="auagd002.htm#ToC"><IMG SRC="../toc.gif" BORDER="0" ALT="[Contents]"></A> <A HREF="auagd005.htm"><IMG SRC="../prev.gif" BORDER="0" ALT="[Previous Topic]"></A> <A HREF="#Bot_Of_Page"><IMG SRC="../bot.gif" BORDER="0" ALT="[Bottom of Topic]"></A> <A HREF="auagd007.htm"><IMG SRC="../next.gif" BORDER="0" ALT="[Next Topic]"></A> <A HREF="auagd026.htm#HDRINDEX"><IMG SRC="../index.gif" BORDER="0" ALT="[Index]"></A> <P>
<HR><H1><A NAME="HDRWQ5" HREF="auagd002.htm#ToC_9">An Overview of AFS Administration</A></H1>
<P>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.
<HR><H2><A NAME="HDRWQ6" HREF="auagd002.htm#ToC_10">A Broad Overview of AFS</A></H2>
<P>This section introduces most of the key terms and concepts
necessary for a basic understanding of AFS. For a more detailed
discussion, see <A HREF="#HDRWQ7">More Detailed Discussions of Some Basic Concepts</A>.
<P><B>AFS: A Distributed File System</B>
<P>AFS is a <I>distributed file system</I> 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.
<P><B>Servers and Clients</B>
<P>In fact, AFS stores files on a subset of the machines in a network, called
<I>file server machines</I>. File server machines provide file
storage and delivery service, along with other specialized services, to the
other subset of machines in the network, the <I>client
machines</I>. 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.
<P>AFS file server machines run a number of <I>server processes</I>, 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 <I>server
machines</I> and <I>server processes</I>, not simply to
<I>servers</I>. For a more detailed description of the server
processes, see <A HREF="#HDRWQ17">AFS Server Processes and the Cache Manager</A>.
<P><B>Cells</B>
<P>A <I>cell</I> 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.
<P><B>Transparent Access and the Uniform Namespace</B>
<P>Although your AFS cell is administratively independent, you probably want
to organize the local collection of files (your <I>filespace</I> or
<I>tree</I>) so that users from other cells can also access the
information in it. AFS enables cells to combine their local filespaces
into a <I>global filespace</I>, and does so in such a way that file access
is <I>transparent</I>--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 <I>uniform namespace</I> to its users.
<P><B>Volumes</B>
<P>AFS groups files into <I>volumes</I>, 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.
<P>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.
<P><B>Efficiency Boosters: Replication and Caching</B>
<P>AFS incorporates special features on server machines and client machines
that help make it efficient and reliable.
<P>On server machines, AFS enables administrators to <I>replicate</I>
commonly-used volumes, such as those containing binaries for popular
programs. Replication means putting an identical read-only copy
(sometimes called a <I>clone</I>) 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.
<P>On client machines, AFS uses <I>caching</I> to improve
efficiency. When a user on a client workstation requests a file, the
<I>Cache Manager</I> 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 <I>cache</I>, 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
<I>callback</I>.
<P><B>Security: Mutual Authentication and Access Control Lists</B>
<P>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 <I>mutual authentication</I>, 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.
<P>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 <I>access control list</I> (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.
<P>For a more detailed description of AFS's mutual authentication
procedure, see <A HREF="auagd007.htm#HDRWQ75">A More Detailed Look at Mutual Authentication</A>. For further discussion of ACLs, see <A HREF="auagd020.htm#HDRWQ562">Managing Access Control Lists</A>.
<HR><H2><A NAME="HDRWQ7" HREF="auagd002.htm#ToC_11">More Detailed Discussions of Some Basic Concepts</A></H2>
<P>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.
<P><H3><A NAME="HDRWQ8" HREF="auagd002.htm#ToC_12">Networks</A></H3>
<A NAME="IDX5538"></A>
<P>A <I>network</I> is a collection of interconnected computers able to
communicate with each other and transfer information back and forth.
<P>A networked computing environment contrasts with two types of computing
environments: <I>mainframe</I> and <I>personal</I>.
<A NAME="IDX5539"></A>
<A NAME="IDX5540"></A>
<UL>
<P><LI>A <I>mainframe</I> 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 <I>terminals</I>, which generally have only
enough computing power to accept input from a keyboard and to display data on
the screen.
<A NAME="IDX5541"></A>
<P><LI>A <I>personal</I> 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.
<A NAME="IDX5542"></A>
</UL>
<P>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 <A HREF="#HDRWQ10">Servers and Clients</A>.
<P><H3><A NAME="HDRWQ9" HREF="auagd002.htm#ToC_13">Distributed File Systems</A></H3>
<A NAME="IDX5543"></A>
<A NAME="IDX5544"></A>
<P>A <I>file system</I> 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.
<P>Networked computing environments often use <I>distributed file
systems</I> 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.
<P><H3><A NAME="HDRWQ10" HREF="auagd002.htm#ToC_14">Servers and Clients</A></H3>
<A NAME="IDX5545"></A>
<A NAME="IDX5546"></A>
<A NAME="IDX5547"></A>
<P>AFS uses a server/client model. In general, a <I>server</I> is a
machine, or a process running on a machine, that provides specialized services
to other machines. A <I>client</I> 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.
<P>AFS divides the machines on a network into two basic classes, <I>file
server machines</I> and <I>client machines</I>, and assigns different
tasks and responsibilities to each.
<P><B>File Server Machines</B>
<A NAME="IDX5548"></A>
<A NAME="IDX5549"></A>
<P><I>File server machines</I> store the files in the distributed file
system, and a <I>server process</I> running on the file server machine
delivers and receives files. AFS file server machines run a number of
<I>server processes</I>. 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 <A HREF="#HDRWQ17">AFS Server Processes and the Cache Manager</A>.
<P>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 <A HREF="auagd008.htm#HDRWQ90">The Four Roles for File Server Machines</A>.
<P><B>Client Machines</B>
<A NAME="IDX5550"></A>
<P>The other class of machines are the <I>client machines</I>, 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 <I>Cache Manager</I>; see <A HREF="#HDRWQ28">The Cache Manager</A>. There are usually many more client machines in a
cell than file server machines.
<P><B>Client and Server Configuration</B>
<P>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.
<A NAME="IDX5551"></A>
<P>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.
<P>The advantage of using workstations as clients is that <I>caching</I>
on the local disk speeds the delivery of files to application programs.
(For an explanation of caching, see <A HREF="#HDRWQ16">Caching and Callbacks</A>.) Diskless machines can access AFS if they are
running NFS<SUP>(R)</SUP> and the NFS/AFS Translator, an optional component of the
AFS distribution.
<P><H3><A NAME="HDRWQ11" HREF="auagd002.htm#ToC_15">Cells</A></H3>
<A NAME="IDX5552"></A>
<P>A <I>cell</I> 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.
<P>The terms <I>local cell</I> and <I>home cell</I> 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 <I>foreign</I> 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 <B>cd</B> command to change directories into their file trees.
<A NAME="IDX5553"></A>
<A NAME="IDX5554"></A>
<A NAME="IDX5555"></A>
<A NAME="IDX5556"></A>
<P>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.
<P>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.
<P><H3><A NAME="HDRWQ12" HREF="auagd002.htm#ToC_16">The Uniform Namespace and Transparent Access</A></H3>
<A NAME="IDX5557"></A>
<A NAME="IDX5558"></A>
<P>One of the features that makes AFS easy to use is that it provides
<I>transparent access</I> 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.
<P>In addition to transparent access, AFS also creates a <I>uniform
namespace</I>--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.
<P>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 <A HREF="auagd007.htm#HDRWQ39">Making Other Cells Visible in Your Cell</A>.
<P><H3><A NAME="HDRWQ13" HREF="auagd002.htm#ToC_17">Volumes</A></H3>
<A NAME="IDX5559"></A>
<P>A <I>volume</I> 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.
<UL>
<P><LI>The relatively small size of volumes makes them easy to move from one
partition to another, or even between machines.
<P><LI>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.
<A NAME="IDX5560"></A>
<P><LI>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.
<A NAME="IDX5561"></A>
<P>
<A NAME="IDX5562"></A>
<P>
<A NAME="IDX5563"></A>
<P><LI>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 <A HREF="#HDRWQ14">Mount Points</A>.
<P><LI>Volumes increase file availability through replication and backup.
<A NAME="IDX5564"></A>
<P>
<A NAME="IDX5565"></A>
<P><LI>Replication (placing copies of a volume on more than one file server
machine) makes the contents more reliably available; for details, see <A HREF="#HDRWQ15">Replication</A>. Entire sets of volumes can be backed up to tape and
restored to the file system; see <A HREF="auagd011.htm#HDRWQ248">Configuring the AFS Backup System</A> and <A HREF="auagd012.htm#HDRWQ283">Backing Up and Restoring AFS Data</A>. 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 <A HREF="auagd010.htm#HDRWQ201">Creating Backup Volumes</A>.
<P><LI>Volumes are the unit of resource management. A space quota
associated with each volume sets a limit on the maximum volume size.
See <A HREF="auagd010.htm#HDRWQ234">Setting and Displaying Volume Quota and Current Size</A>.
<A NAME="IDX5566"></A>
</UL>
<P><H3><A NAME="HDRWQ14" HREF="auagd002.htm#ToC_18">Mount Points</A></H3>
<A NAME="IDX5567"></A>
<P>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 <I>root directory</I>, and the
mechanism that associates the directory and volume is called a <I>mount
point</I>. 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.
<TABLE><TR><TD ALIGN="LEFT" VALIGN="TOP"><B>Note:</B></TD><TD ALIGN="LEFT" VALIGN="TOP">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.
</TD></TR></TABLE>
<P>
<A NAME="IDX5568"></A>
<A NAME="IDX5569"></A>
<A NAME="IDX5570"></A>
<A NAME="IDX5571"></A>
<P>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 <A HREF="#HDRWQ28">The Cache Manager</A>) 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.
<P>A volume is said to be <I>mounted</I> 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.
<P><H3><A NAME="HDRWQ15" HREF="auagd002.htm#ToC_19">Replication</A></H3>
<A NAME="IDX5572"></A>
<A NAME="IDX5573"></A>
<P><I>Replication</I> refers to making a copy, or <I>clone</I>, 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.
<P>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 <A HREF="auagd007.htm#HDRWQ50">When to Replicate Volumes</A>.
<P><H3><A NAME="HDRWQ16" HREF="auagd002.htm#ToC_20">Caching and Callbacks</A></H3>
<A NAME="IDX5574"></A>
<P>Just as replication increases system availability, <I>caching</I>
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
<I>cache</I> 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 <I>File Server process</I> 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.
<P>Caching improves the speed of data delivery to application programs in the
following ways:
<UL>
<P><LI>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.
<P><LI>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.
<A NAME="IDX5575"></A>
<A NAME="IDX5576"></A>
<P>
<A NAME="IDX5577"></A>
</UL>
<P>
<A NAME="IDX5578"></A>
<P>
<A NAME="IDX5579"></A>
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 <I>callback</I>.
<P>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:
<UL>
<P><LI>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.
<P><LI>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.
</UL>
<P>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.
<HR><H2><A NAME="HDRWQ17" HREF="auagd002.htm#ToC_21">AFS Server Processes and the Cache Manager</A></H2>
<A NAME="IDX5580"></A>
<A NAME="IDX5581"></A>
<P>As mentioned in <A HREF="#HDRWQ10">Servers and Clients</A>, 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.
<P>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.
<P>The <I>File Server</I>, 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.
<P>The <I>Basic OverSeer Server (BOS Server)</I> 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.
<P>The <I>Authentication Server</I> 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). It maintains the
Authentication Database.
<P>The <I>Protection Server</I> 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.
<P>The <I>Volume Server</I> 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.
<P>The <I>Volume Location Server (VL Server)</I> 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.
<P>The <I>Update Server</I> 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.
<P>The <I>Backup Server</I> 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.
<P>The <I>Salvager</I> 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.
<P>The <I>Network Time Protocol Daemon (NTPD)</I> 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 <I>Ubik</I>); see <A HREF="auagd008.htm#HDRWQ103">Configuring the Cell for Proper Ubik Operation</A>. The NTPD is controlled by the <B>runntp</B>
process.
<P>The <I>Cache Manager</I> 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.
<P><H3><A NAME="HDRWQ18" HREF="auagd002.htm#ToC_22">The File Server</A></H3>
<A NAME="IDX5582"></A>
<P>The <I>File Server</I> 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:
<UL>
<P><LI>Delivering programs and data files to client workstations as requested and
storing them again when the client workstation finishes with them.
<P><LI>Maintaining the hierarchical directory structure that users create to
organize their files.
<P><LI>Handling requests for copying, moving, creating, and deleting files and
directories.
<P><LI>Keeping track of status information about each file and directory
(including its size and latest modification time).
<P><LI>Making sure that users are authorized to perform the actions they request
on particular files or directories.
<P><LI>Creating symbolic and hard links between files.
<P><LI>Granting advisory locks (corresponding to UNIX locks) on request.
</UL>
<P><H3><A NAME="HDRWQ19" HREF="auagd002.htm#ToC_23">The Basic OverSeer Server</A></H3>
<A NAME="IDX5583"></A>
<P>The <I>Basic OverSeer Server (BOS Server)</I> 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.
<P>The BOS Server runs on every file server machine. Its primary
function is to minimize system outages. It also
<UL>
<P><LI>Constantly monitors the other server processes (on the local machine) to
make sure they are running correctly.
<P><LI>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.
<A NAME="IDX5584"></A>
<P>
<A NAME="IDX5585"></A>
<P><LI>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.
<P><LI>Helps system administrators to manage system configuration
information. The BOS server automates the process of adding and
changing <I>server encryption keys</I>, 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 <A HREF="auagd008.htm#HDRWQ85">Common Configuration Files in the /usr/afs/etc Directory</A>.
</UL>
<P><H3><A NAME="HDRWQ20" HREF="auagd002.htm#ToC_24">The Authentication Server</A></H3>
<A NAME="IDX5586"></A>
<P>The <I>Authentication Server</I> performs two main functions related to
network security:
<UL>
<P><LI>Verifying the identity of users as they log into the system by requiring
that they provide a password. The Authentication Server grants the user
a <I>token</I> as proof to AFS server processes that the user has
authenticated. For more on tokens, see <A HREF="auagd007.htm#HDRWQ76">Complex Mutual Authentication</A>.
<P><LI>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.
</UL>
<P>In fulfilling these duties, the Authentication Server utilizes algorithms
and other procedures known as <I>Kerberos</I> (which is why many commands
used to contact the Authentication Server begin with the letter
<B>k</B>). This technology was originally developed by the
Massachusetts Institute of Technology's Project Athena.
<P>The Authentication Server also maintains the <I>Authentication
Database</I>, 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 <A HREF="auagd007.htm#HDRWQ75">A More Detailed Look at Mutual Authentication</A>.
<A NAME="IDX5587"></A>
<A NAME="IDX5588"></A>
<A NAME="IDX5589"></A>
<A NAME="IDX5590"></A>
<P><H3><A NAME="HDRWQ21" HREF="auagd002.htm#ToC_25">The Protection Server</A></H3>
<A NAME="IDX5591"></A>
<A NAME="IDX5592"></A>
<A NAME="IDX5593"></A>
<P>The <I>Protection Server</I> 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:
<UL>
<P><LI>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 <I>access control
list (ACL)</I> 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 <A HREF="auagd020.htm#HDRWQ562">Managing Access Control Lists</A>.
<A NAME="IDX5594"></A>
<P><LI>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.
<P><LI>Enabling users to define their own groups of users, recorded in the
<I>Protection Database</I> 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.
<P><LI>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.
</UL>
<A NAME="IDX5595"></A>
<A NAME="IDX5596"></A>
<P>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.
<P>The Protection Server also maps <I>usernames</I> (the name typed at the
login prompt) to <I>AFS user ID</I> numbers (<I>AFS UIDs</I>).
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 <A HREF="auagd007.htm#HDRWQ75">A More Detailed Look at Mutual Authentication</A>.
<P><H3><A NAME="HDRWQ22" HREF="auagd002.htm#ToC_26">The Volume Server</A></H3>
<A NAME="IDX5597"></A>
<P>The <I>Volume Server</I> 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). <A HREF="#HDRWQ13">Volumes</A> 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 <A HREF="#HDRWQ15">Replication</A>).
<P><H3><A NAME="HDRWQ23" HREF="auagd002.htm#ToC_27">The Volume Location (VL) Server</A></H3>
<A NAME="IDX5598"></A>
<A NAME="IDX5599"></A>
<P>The <I>VL Server</I> maintains a complete list of volume locations in
the <I>Volume Location Database (VLDB)</I>. When the Cache Manager
(see <A HREF="#HDRWQ28">The Cache Manager</A>) 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.
<P>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 <A HREF="auagd010.htm#HDRWQ180">Volume Information in the VLDB</A>.
<A NAME="IDX5600"></A>
<P><H3><A NAME="HDRWQ24" HREF="auagd002.htm#ToC_28">The Update Server</A></H3>
<A NAME="IDX5601"></A>
<P>The <I>Update Server</I> 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.
<P>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
<I>binary distribution machine</I> for that type. The binary
distribution machine runs the <I>server portion</I> of the Update Server,
whereas all the other machines of that type run the <I>client portion</I>
of the Update Server. The client portions check frequently with the
server portion to see if they are running the right version of every
process; if not, the client portion 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 <A HREF="auagd008.htm#HDRWQ93">Binary Distribution Machines</A>.
<A NAME="IDX5602"></A>
<P>
<A NAME="IDX5603"></A>
<P>In cells that run the United States edition of AFS, 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 <A HREF="auagd008.htm#HDRWQ85">Common Configuration Files in the /usr/afs/etc Directory</A>). As with server process software, the need for
consistent system performance demands that all the machines have the same
version of these files. With the United States edition, the system
administrator needs to make changes to these files on one machine only, the
cell's <I>system control machine</I>, 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 <A HREF="auagd008.htm#HDRWQ94">The System Control Machine</A>.
<P><H3><A NAME="HDRWQ25" HREF="auagd002.htm#ToC_29">The Backup Server</A></H3>
<A NAME="IDX5604"></A>
<A NAME="IDX5605"></A>
<P>The <I>Backup Server</I> maintains the information in the <I>Backup
Database</I>. 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 <I>Backup System</I>.
<P>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' <I>Tape Coordinators</I>, which are the processes that
control the tape drives.
<P>Once the Backup System is configured, user and system data can be dumped
from volumes to tape. 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 <A HREF="auagd011.htm#HDRWQ248">Configuring the AFS Backup System</A> and <A HREF="auagd012.htm#HDRWQ283">Backing Up and Restoring AFS Data</A>.
<P><H3><A NAME="HDRWQ26" HREF="auagd002.htm#ToC_30">The Salvager</A></H3>
<A NAME="IDX5606"></A>
<P>The <I>Salvager</I> 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.
<P>As a system administrator, you can also invoke the Salvager as necessary,
even if the File Server or Volume Server has not failed. See <A HREF="auagd010.htm#HDRWQ232">Salvaging Volumes</A>.
<P><H3><A NAME="HDRWQ27" HREF="auagd002.htm#ToC_31">The Network Time Protocol Daemon</A></H3>
<A NAME="IDX5607"></A>
<P>The <I>Network Time Protocol Daemon (NTPD)</I> is not an AFS server
process per se, but plays an important role. It helps guarantee that
all of the file server machines agree on the time. The NTPD on one file
server machine acts as a synchronization site, generally learning the correct
time from a source outside the cell. The NTPDs on the other file server
machines refer to the synchronization site to set the internal clocks on their
machines.
<P>Keeping clocks synchronized is particularly important to the correct
operation of AFS's distributed database technology, which coordinates the
copies of the Authentication, Backup, Protection, and Volume Location
Databases; see <A HREF="auagd007.htm#HDRWQ52">Replicating the AFS Administrative Databases</A>. Client machines 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 <A HREF="auagd009.htm#HDRWQ151">The runntp Process</A>.
<P><H3><A NAME="HDRWQ28" HREF="auagd002.htm#ToC_32">The Cache Manager</A></H3>
<A NAME="IDX5608"></A>
<P>As already mentioned in <A HREF="#HDRWQ16">Caching and Callbacks</A>, the <I>Cache Manager</I> 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 remote procedure calls (RPCs) 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 <A HREF="#HDRWQ23">The Volume Location (VL) Server</A>). When the Cache Manager receives the requested file,
it caches it before passing data on to the application program.
<P>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.
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