afsd(8)
NAME
afsd, afsd.fuse - Initializes the Cache Manager and starts related daemons
SYNOPSIS
afsd [-afsdb] [-backuptree]
[-biods <number of bkg I/O daemons (aix vm)>]
[-blocks <1024 byte blocks in cache>]
[-cachedir <cache directory>]
[-chunksize <log(2) of chunk size>]
[-confdir <configuration directory>]
[-daemons <number of daemons to use>]
[-dcache <number of dcache entries>] [-debug]
[-dynroot] [-dynroot-sparse] [-enable_peer_stats]
[-enable_process_stats] [-fakestat] [-fakestat-all]
[-files <files in cache>]
[-files_per_subdir <log(2) of files per dir> ]
[-help] [-logfile <Place to keep the CM log>]
[-mem_alloc_sleep] [-memcache]
[-mountdir <mount location>] [-nomount]
[-nosettime]
[-prealloc <number of 'small' preallocated blocks>]
[-rmtsys] [-rootvol <name of AFS root volume>]
[-rxbind] [-rxmaxmtu value for maximum MTU ]
[-rxpck value for rx_extraPackets ]
[-settime] [-shutdown]
[-splitcache <RW/RO ratio>]
[-stat <number of stat entries>] [-verbose]
[-disable-dynamic-vcaches]
[-volumes <number of volume entries>]
[-waitclose] [-rxmaxfrags <max # of fragments>]
DESCRIPTION
The afsd command initializes the Cache Manager on an AFS client machine
by transferring AFS-related configuration information into kernel
memory and starting several daemons. afsd.fuse is an experimental
variant that initializes a FUSE-based Cache Manager instead of one
based on a kernel module.
The afsd command performs the following actions:
* Sets a field in kernel memory that defines the machine's cell
membership. Some Cache Manager-internal operations and system calls
consult this field to learn which cell to execute in. (The AFS
command interpreters refer to the /etc/openafs/ThisCell file
instead.) This information is transferred into the kernel from the
/etc/openafs/ThisCell file and cannot be changed until the afsd
program runs again.
* Places in kernel memory the names and Internet addresses of the
database server machines in the local cell and (optionally) foreign
cells. The appearance of a cell's database server machines in this
list enables the Cache Manager to contact them and to access files
in the cell. Omission of a cell from this list, or incorrect
information about its database server machines, prevents the Cache
Manager from accessing files in it.
By default, the list of database server machines is transferred
into the kernel from the /etc/openafs/CellServDB file.
Alternatively, when the -afsdb option is used, the list of database
server machines is taken from the DNS SRV or AFSDB records for each
cell. After initialization, use the fs newcell command to change
the kernel-resident list without having to reboot.
* Mounts the root of the AFS filespace on a directory on the
machine's local disk, according to either the first field in the
/etc/openafs/cacheinfo file (the default) or the afsd command's
-mountdir argument. The conventional value is /afs.
* Determines which volume to mount at the root of the AFS file tree.
The default is the volume "root.afs"; use the -rootvol argument to
override it. Although the base (read/write) form of the volume name
is the appropriate value, the Cache Manager has a bias for
accessing the read-only version of the volume (by convention,
"root.afs.readonly") if it is available.
* Configures the cache on disk (the default) or in machine memory if
the -memcache argument is provided. In the latter case, the afsd
program allocates space in machine memory for caching, and the
Cache Manager uses no disk space for caching even if the machine
has a disk.
* Defines the name of the local disk directory devoted to caching,
when the -memcache argument is not used. If necessary, the afsd
program creates the directory (its parent directory must already
exist). It does not remove the directory that formerly served this
function, if one exists.
The second field in the /etc/openafs/cacheinfo file is the source
for this name. The standard value is /usr/vice/cache. Use the
-cachedir argument to override the value in the cacheinfo file.
* Sets the size of the cache. The default source for the value is the
third field in the /etc/openafs/cacheinfo file, which specifies a
number of kilobytes.
For a memory cache, the following arguments to the afsd command
override the value in the cacheinfo file:
* The -blocks argument, to specify a different number of kilobyte
blocks.
* The -dcache and -chunksize arguments together, to set both the
number of dcache entries and the chunk size (see below for
definition of these parameters). In this case, the afsd program
derives cache size by multiplying the two values. Using this
combination is not recommended, as it requires the issuer to
perform the calculation beforehand to determine the resulting
cache size.
* The -dcache argument by itself. In this case, the afsd program
derives cache size by multiplying the value specified by the
-dcache argument by the default memory cache chunk size of
eight kilobytes. Using this argument is not recommended, as it
requires the issuer to perform the calculation beforehand to
determine the resulting cache size.
For satisfactory memory cache performance, the specified value must
leave enough memory free to accommodate all other processes and
commands that can run on the machine. If the value exceeds the
amount of memory available, the afsd program exits without
initializing the Cache Manager and produces the following message
on the standard output stream:
afsd: memCache allocation failure at <number> KB
where <number> is how many kilobytes were allocated just before the
failure.
For a disk cache, use the -blocks argument to the afsd command to
override the value in the cacheinfo file. The value specified in
either way sets an absolute upper limit on cache size; values
provided for other arguments (such as -dcache and -chunksize) never
result in a larger cache. The afsd program rejects any setting
larger than 95% of the partition size, and exits after generating
an error message on the standard output stream, because the cache
implementation itself requires a small amount of disk space and
overfilling the partition can cause the client machine to panic.
To change the size of a disk cache after initialization without
rebooting, use the fs setcachesize command; the setting persists
until the afsd command runs again or the fs setcachesize command is
reissued. The fs setcachesize command does not work for memory
caches.
* Sets the size of each cache chunk, and by implication the amount of
data that the Cache Manager requests at a time from the File Server
(how much data per fetch RPC, since AFS uses partial file
transfer).
For a disk cache, a chunk is a Vn file and this parameter sets the
maximum size to which each one can expand. For a memory cache,
each chunk is a collection of contiguous memory blocks. The default
for a disk cache is between 256 KB and 1 MB depending on the size
of the cache. The default for a memory cache is 8 KB.
To override the default chunk size for either type of cache, use
the -chunksize argument to provide an integer to be used as an
exponent of two; see "OPTIONS" for details. For a memory cache, if
total cache size divided by chunk size leaves a remainder, the afsd
program rounds down the number of dcache entries, resulting in a
slightly smaller cache.
* Sets the number of chunks in the cache. For a memory cache, the
number of chunks is equal to the cache size divided by the chunk
size. For a disk cache, the number of chunks (Vn files) is set to
the largest of the following unless the -files argument is used to
set the value explicitly:
* 100
* 1.5 times the result of dividing cache size by chunk size
(cachesize/chunksize * 1.5)
* The result of dividing cachesize by 10 KB (cachesize/10240)
* Sets the number of dcache entries allocated in machine memory for
storing information about the chunks in the cache.
For a disk cache, the /usr/vice/cache/CacheItems file contains one
entry for each Vn file. By default, one half the number of these
entries (but not more that 2,000) are duplicated as dcache entries
in machine memory for quicker access.
For a memory cache, there is no CacheItems file so all information
about cache chunks must be in memory as dcache entries. Thus,
there is no default number of dcache entries for a memory cache;
instead, the afsd program derives it by dividing the cache size by
the chunk size.
To set the number of dcache entries, use the -dcache argument; the
specified value can exceed the default limit of 2,000. Using this
argument is not recommended for either type of cache. Increasing
the number of dcache entries for a disk cache sometimes improves
performance (because more entries are retrieved from memory rather
than from disk), but only marginally. Using this argument for a
memory cache requires the issuer to calculate the cache size by
multiplying this value by the chunk size.
* Sets the number of stat entries available in machine memory for
caching status information about cached AFS files. The default is
based on the size of the cache. Use the -stat argument to override
the default.
In addition to setting cache configuration parameters, the afsd program
starts the following daemons. (On most system types, these daemons
appear as nameless entries in the output of the UNIX ps command.)
* One callback daemon, which handles callbacks. It also responds to
the File Server's periodic probes, which check that the client
machine is still alive.
* One maintenance daemon, which performs the following tasks:
* Garbage collects obsolete data (for example, expired tokens)
from kernel memory.
* Synchronizes files.
* Refreshes information from read-only volumes once per hour.
* Does delayed writes for NFS clients if the machine is running
the NFS/AFS Translator.
* One cache-truncation daemon, which flushes the cache when free
space is required, by writing cached data and status information to
the File Server.
* One server connection daemon, which sends a probe to the File
Server every few minutes to check that it is still accessible. If
the -settime option is set, it also synchronizes the machine's
clock with the clock on a randomly-chosen file server machine.
There is always one server connection daemon.
* One or more background daemons that improve performance by pre-
fetching files and performing background (delayed) writes of saved
data into AFS.
The default number of background daemons is two, enough to service
at least five simultaneous users of the machine. To increase the
number, use the -daemons argument. A value greater than six is not
generally necessary.
* On some system types, one Rx listener daemon, which listens for
incoming RPCs.
* On some system types, one Rx event daemon, which reviews the Rx
system's queue of tasks and performs them as appropriate. Most
items in the queue are retransmissions of failed packets.
* On machines that run AIX with virtual memory (VM) integration, one
or more VM daemons (sometimes called I/O daemons, which transfer
data between disk and machine memory. The number of them depends on
the setting of the -biods and -daemons arguments:
* If the -biods argument is used, it sets the number of VM
daemons.
* If only the -daemons argument is used, the number of VM daemons
is twice the number of background daemons.
* If neither argument is used, there are five VM daemons.
afsd.fuse is a variant of afsd that, instead of initializing a Cache
Manager implemented as a kernel module, initializes a FUSE-based AFS
client. FUSE (Filesystem in USErspace) is a Linux-only mechanism for
providing a file system through a purely user-space daemon without a
kernel module component. afsd.fuse takes all of the same options as
afsd.
This command does not use the syntax conventions of the AFS command
suites. Provide the command name and all option names in full.
CAUTIONS
Before using the -shutdown parameter, use the standard UNIX umount
command to unmount the AFS root directory (by convention, /afs). On
Linux, unloading the AFS kernel module and then loading it again before
restarting AFS after -shutdown is recommended.
AFS has for years had difficulties with being stopped and restarted
without an intervening reboot. While most of these issues have been
ironed out, stopping and restarting AFS is not recommended unless
necessary and rebooting before restarting AFS is still the safest
course of action. This does not apply to Linux; it should be safe to
restart the AFS client on Linux without rebooting.
In contrast to many client-server applications, not all communication
is initiated by the client. When the AFS client opens a file, it
registers a callback with the AFS server. If the file changes, the
server notifies the client that the file has changed and that all
cached copies should be discarded. In order to enable full
functionality on the AFS client, including all command-line utilities,
the following UDP ports must be open on an firewalls between the client
and the server:
fileserver 7000/udp
cachemanager 7001/udp (OpenAFS client. Arla uses 4711/udp)
ptserver 7002/udp
vlserver 7003/udp
kaserver 7004/udp (not needed with Kerberos v5)
volserver 7005/udp
reserved 7006/udp (for future use)
bosserver 7007/udp
Clients will also need to be able to contact your Kerberos KDC to
authenticate. If you are using kaserver and klog, you need to allow
inbound and outbound UDP on ports >1024 (probably 1024<port<2048 would
suffice depending on the number of simultaneous klogs).
Be sure to set the UDP timeouts on the firewall to be at least twenty
minutes for the best callback performance.
afsd.fuse was first introduced in OpenAFS 1.5.74. It is only available
if OpenAFS was built with the "--enable-fuse-client" configure switch.
It should be considered experimental.
OPTIONS
-afsdb
Enable afsdb support. This will use DNS to lookup the SRV or AFSDB
records and use that for the database servers for each cell instead
of the values in the CellServDB file. This has the advantage of
only needing to update one set of DNS records to reconfigure the
AFS clients for a new database server as opposed to touching all of
the clients, and also allows one to access a cell without
preconfiguring its database servers in CellServDB. The format of
SRV records is defined in RFC 5864, and the AFSDB record format is
in RFC 1183.
-backuptree
Prefer backup volumes for mountpoints in backup volumes. This
option means that the AFS client will prefer to resolve mount
points to backup volumes when a parent of the current volume is a
backup volume. This is similar to the standard behaviour of
preferring read-only volumes over read-write volumes when the
parent volume is a read-only volume.
-biods <number of I/O daemons>
Sets the number of VM daemons dedicated to performing I/O
operations on a machine running a version of AIX with virtual
memory (VM) integration. If both this argument and the -daemons
argument are omitted, the default is five. If this argument is
omitted but the -daemons argument is provided, the number of VM
daemons is set to twice the value of the -daemons argument.
-blocks <blocks in cache>
Specifies the number of kilobyte blocks to be made available for
caching in the machine's cache directory (for a disk cache) or
memory (for a memory cache), overriding the default defined in the
third field of the /etc/openafs/cacheinfo file. For a disk cache,
the value cannot exceed 95% of the space available in the cache
partition. If using a memory cache, do not combine this argument
with the -dcache argument, since doing so can possibly result in a
chunk size that is not an exponent of 2.
-cachedir <cache directory>
Names the local disk directory to be used as the cache. This value
overrides the default defined in the second field of the
/etc/openafs/cacheinfo file.
-chunksize <chunk size>
Sets the size of each cache chunk. The integer provided, which must
be from the range 0 to 30, is used as an exponent on the number 2.
If not supplied, a default chunksize will be determined based on
the cache type and cache size, and will range from 13 (8KB) for
memory cache and 18 to 20 (256 KB to 1MB) for disk cache. A value
of 0 or less, or greater than 30, sets chunk size to the
appropriate default. Values less than 10 (which sets chunk size to
a 1 KB) are not recommended. Combining this argument with the
-dcache argument is not recommended because it requires that the
issuer calculate the cache size that results.
-chunksize is an important option when tuning for performance.
Setting this option to larger values can increase performance when
dealing with large files.
-confdir <configuration directory>
Names a directory other than the /etc/openafs directory from which
to fetch the cacheinfo, ThisCell, and CellServDB configuration
files.
-daemons <number of daemons to use>
Specifies the number of background daemons to run on the machine.
These daemons improve efficiency by doing prefetching and
background writing of saved data. This value overrides the default
of 2, which is adequate for a machine serving up to five users.
Values greater than 6 are not generally more effective than 6.
Note: On AIX machines with integrated virtual memory (VM), the
number of VM daemons is set to twice the value of this argument, if
it is provided and the -biods argument is not. If both arguments
are omitted, there are five VM daemons.
-dcache <number of dcache entries>
Sets the number of dcache entries in memory, which are used to
store information about cache chunks. For a disk cache, this
overrides the default, which is 50% of the number of Vn files
(cache chunks). For a memory cache, this argument effectively sets
the number of cache chunks, but its use is not recommended, because
it requires the issuer to calculate the resulting total cache size
(derived by multiplying this value by the chunk size). Do not
combine this argument with the -blocks argument, since doing so can
possibly result in a chunk size that is not an exponent of 2.
-debug
Generates a highly detailed trace of the afsd program's actions on
the standard output stream. The information is useful mostly for
debugging purposes.
-dynroot
The standard behaviour of the AFS client without the -dynroot
option is to mount the root.afs volume from the default cell on the
/afs path. The /afs folder and root.afs volume traditionally shows
the folders for ThisCell and other cells as configured by the AFS
cell administrator.
The -dynroot option changes this. Using this option, the AFS client
does not mount the root.afs volume on /afs. Instead it uses the
contents of the CellServDB file to populate the listing of cells in
/afs. This is known as a DYNamic ROOT. A cell is not contacted
until the path /afs/cellname if accessed. This functions similarly
to an automounter. The main advantage of using -dynroot is that
the AFS client will start properly even without network access,
whereas the client not using -dynroot will freeze upon startup if
cannot contact the default cell specified in ThisCell and mount the
root.afs volume. Dynamic root mode is also sometimes called
travelling mode because it works well for laptops which don't
always have network connectivity.
Two advantages of not using dynroot are that listing /afs will
usually be faster because the contents of /afs are limited to what
the AFS administrator decides and that symbolic links are
traditionally created by the AFS administrator to provide a short
name for the cell (i.e. cellname.domain.com is aliased to
cellname). However, with dynroot, the local system administrator
can limit the default contents of /afs by installing a stripped-
down CellServDB file, and if dynroot is in effect, the CellAlias
file can be used to provide shortname for common AFS cells which
provides equivalent functionality to the most commonly used
symbolic links.
When the dynamic root (-dynroot, -dynroot-sparse) and the fake stat
(-fakestat, -fakestat-all) modes are in effect, the cache manager
provides a special directory named /afs/.:mount which allows access
to volumes by volume name or ID. The /afs/.:mount directory
appears to be empty, but any name in the form of cell:volume will
be resolved as a read-write mount point to the specified volume.
This dynamic mount feature is recommended only for temporary access
to a volume. Linux-based cache managers provide this dynamic mount
feature even when dynamic root (-dynroot, -dynroot-sparse) is not
in effect.
-dynroot-sparse
In addition to operating in the manner described for dynroot above,
cells other than the local cell are not shown by default until a
lookup occurs. Cell aliases as set in the CellAliases file are
shown as normal, although they may appear to be dangling links
until traversed.
-enable_peer_stats
Activates the collection of Rx statistics and allocates memory for
their storage. For each connection with a specific UDP port on
another machine, a separate record is kept for each type of RPC
(FetchFile, GetStatus, and so on) sent or received. To display or
otherwise access the records, use the Rx Monitoring API.
-enable_process_stats
Activates the collection of Rx statistics and allocates memory for
their storage. A separate record is kept for each type of RPC
(FetchFile, GetStatus, and so on) sent or received, aggregated over
all connections to other machines. To display or otherwise access
the records, use the Rx Monitoring API.
-fakestat
Return fake values for stat calls on cross-cell mounts. This option
makes an "ls -l" of /afs much faster since each cell isn't
contacted, and this and the -fakestat-all options are useful on Mac
OS X so that the Finder program doesn't try to contact every AFS
cell the system knows about.
Note that, for the purposes of -fakestat, local cellular mounts
count as "cross-cell" mounts. That is, if the local cell is
"localcell", a mount for "localcell:root.cell" will count as a
"cross-cell" mount and so stat calls for it will be faked with
-fakestat. In practice, local cellular mounts are rare and
generally discouraged, so this should not generally make a
difference.
-fakestat-all
Return fake values for stat calls on all mounts, not just cross-
cell mounts. This and the -fakestat options are useful on Mac OS X
so that the Finder program doesn't hang when browsing AFS
directories.
-files <files in cache>
Specifies the number of Vn files to create in the cache directory
for a disk cache, overriding the default that is calculated as
described in "DESCRIPTION". Each Vn file accommodates a chunk of
data, and can grow to a maximum size of 64 KB by default. Do not
combine this argument with the -memcache argument.
-files_per_subdir <files per cache subdirectory>
Limits the number of cache files in each subdirectory of the cache
directory. The value of the option should be the base-two log of
the number of cache files per cache subdirectory (so 10 for 1024
files, 14 for 16384 files, and so forth).
-help
Prints the online help for this command. All other valid options
are ignored.
-logfile <log file location>
This option is obsolete and no longer has any effect.
-mem_alloc_sleep
This option is obsolete and no longer has any effect.
-memcache
Initializes a memory cache rather than a disk cache. Do not combine
this flag with the -files argument.
-mountdir <mount location>
Names the local disk directory on which to mount the root of the
AFS filespace. This value overrides the default defined in the
first field of the /etc/openafs/cacheinfo file. If a value other
than the /afs directory is used, the machine cannot access the
filespace of cells that do use that value.
-nomount
Do not mount AFS on startup. The afs global mount must be mounted
via some other means. This is useful on Mac OS X where /afs is
sometimes mounted in /Network/afs like other network file systems.
-nosettime
This option is obsolete and no longer has any effect. The
operating system provided time keeping services should be used to
maintain the system time.
-prealloc <number of preallocated blocks>
Specifies the number of pieces of memory to preallocate for the
Cache Manager's internal use. The default initial value is 400, but
the Cache Manager dynamically allocates more memory as it needs it.
-rmtsys
Initializes an additional daemon to execute AFS-specific system
calls on behalf of NFS client machines. Use this flag only if the
machine is an NFS/AFS translator machine serving users of NFS
client machines who execute AFS commands.
-rootvol <name of AFS root volume>
Names the read/write volume corresponding to the root directory for
the AFS file tree (which is usually the /afs directory). This value
overrides the default of the "root.afs" volume. This option is
ignored if -dynroot is given.
-rxbind
Bind the Rx socket (one interface only).
-rxmaxfrags <max # of fragments>
Set a limit for the maximum number of UDP fragments Rx will send
per Rx packet, and the maximum number of fragments Rx thinks it can
receive when advertising its receive size to peers. Practically
speaking, setting this option means that you will not see Rx data
packets that are broken into more than N fragments, where N is the
value specified for this option. Setting this option to 1
effectively prevents fragmentation, and can be useful when dealing
with networking equipment that does not properly handle UDP
fragments.
Note that this option just specifies a maximum. The actual number
of fragments seen on the wire may be less than what is specified,
depending on the configuration of the peer.
-rxmaxmtu <value for maximum MTU>
Set a limit for the largest maximum transfer unit (network packet
size) that the AFS client on this machine will be willing to
transmit. This switch can be used where an artificial limit on the
network precludes packets as large as the discoverable MTU from
being transmitted successfully.
-rxpck <value for rx_extraPackets>
Set rx_extraPackets to this value. This sets the number of extra Rx
packet structures that are available to handle Rx connections. This
value should be increased if the "rxdebug 127.0.0.1 -port 7001
-rxstats" command shows no free Rx packets. Increasing this value
may improve OpenAFS client performance in some circumstances.
-settime
The -settime option is deprecated and should no longer be
specified. The operating system provided time keeping services
should be used to maintain the system time.
The -settime option enables the native AFS time synchronization.
If the -settime option is specified, then a randomly selected a
file server machine in the local cell is used as the source for the
correct time. Every five minutes thereafter, the local clock is
adjusted (if necessary) to match the file server machine's clock.
-shutdown
Shuts down the Cache Manager. Before calling afsd with this option,
unmount the AFS file system with umount.
-splitcache <RW/RO Ratio>
This allows the user to set a certain percentage of the AFS cache
be reserved for read/write content and the rest to be reserved for
read-only content. The ratio should be written as a fraction. For
example, "-splitcache 75/25" devotes 75% of your cache space to
read/write content and 25% to read-only.
-stat <number of stat entries>
Specifies the number of entries to allocate in the machine's memory
for recording status information about the AFS files in the cache.
If this value is not specified, the number of stat entires will be
autotuned based on the size of the disk cache.
-verbose
Generates a detailed trace of the afsd program's actions on the
standard output stream.
-volumes <number of volume entries>
Specifies the number of memory structures to allocate for storing
volume location information. The default value is 200.
-disable-dynamic-vcaches
By default, dynamic vcache overrides the -stat option by using the
value of -stat (or the default) as the initial size of the stat (or
vcache) pool and increases the pool dynamically as needed on
supported platforms. This flag will disable this new functionality
and honor the '-stat' setting.
-waitclose
Has no effect on the operation of the Cache Manager. The behavior
it affected in previous versions of the Cache Manager, to perform
synchronous writes to the File Server, is now the default behavior.
To perform asynchronous writes in certain cases, use the fs
storebehind command.
EXAMPLES
The afsd command is normally included in the machine's AFS
initialization file, rather than typed at the command shell prompt. For
most disk caches, the appropriate form is
% /etc/openafs/afsd
The following command is appropriate when enabling a machine to act as
an NFS/AFS Translator machine serving more than five users.
% /etc/openafs/afsd -daemons 4 -rmtsys
The following command initializes a memory cache and sets chunk size to
16 KB (2^14).
% /etc/openafs/afsd -memcache -chunksize 14
PRIVILEGE REQUIRED
The issuer must be logged in as the local superuser root.
SEE ALSO
fs_newcell(1), afs_cache(5), CellServDB(5), cacheinfo(5) RFC 5864 <http://www.ietf.org/rfc/rfc5864.txt> RFC 1183 <http://www.ietf.org/rfc/rfc1183.txt>
COPYRIGHT
IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved. This documentation is covered by the IBM Public License Version 1.0. It was converted from HTML to POD by software written by Chas Williams and Russ Allbery, based on work by Alf Wachsmann and Elizabeth Cassell.
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Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
