QUOTA on XFS
____________
PREFACE
For an additional source of information on Linux quota, you can refer to
the (currently out-of-date) Linux Quota HOWTO at:
http://www.linuxdoc.org/HOWTO/mini/Quota.html
If you are looking to get started quickly, you can skip down to the
section GETTING STARTED below. It is recommended that you at least
read the "Administering the XFS Quota System" section below, however.
DESCRIPTION
In most computing environments, disk space is not infinite. The quota
subsystem provides a mechanism to control usage of disk space. Quotas
can be set for each individual user on any/all of the local filesystems.
The quotas subsystem warns users when they exceed their allotted limit,
but allows some extra space for current work (hard limit/soft limit).
In addition, XFS filesystems with limit enforcement turned off can be
used as an effective disk usage accounting system.
Users' Views of Disk Quotas
To most users, disk quotas are either of no concern or a fact of life
that cannot be avoided. There are two possible quotas that can be
imposed - a limit can be set on the amount of space a user can occupy,
and there may be a limit on the number of files (inodes) he can own.
The quota(1) command provides information on the quotas that have been
set by the system administrators and current usage.
There are four numbers for each limit: current usage, soft limit
(quota), hard limit, and time limit. The soft limit is the number of 1K
blocks (or files) that the user is expected to remain below. The hard
limit cannot be exceeded. If a user's usage reaches the hard limit,
further requests for space (or attempts to create a file) fail with an
EDQUOT/ENOSPC error.
When a user exceeds the soft limit, the timer is enabled. Any time the
quota drops below the soft limits, the timer is disabled. If the timer
pops, the particular limit that has been exceeded is treated as if the
hard limit has been reached, and no more resources are allocated to the
user. The only way to reset this condition, short of turning off limit
enforcement or increasing the limit, is to reduce usage below quota.
Only the superuser can set the time limits and this is done on a per
filesystem basis.
Surviving When the Quota Limit Is Reached
In most cases, the only way for a user to recover from over-quota
conditions is to abort whatever activity is in progress on the filesystem
that has reached its limit, remove sufficient files to bring the limit
back below quota, and retry the failed program.
However, if a user is in the editor and a write fails because of an over
quota situation, that is not a suitable course of action. It is most
likely that initially attempting to write the file has truncated its
previous contents, so if the editor is aborted without correctly writing
the file, not only are the recent changes lost, but possibly much, or
even all, of the contents that previously existed.
There are several possible safe exits for a user caught in this
situation. He can use the editor ! shell escape command to examine his
file space and remove surplus files. Alternatively, using csh(1), he can
suspend the editor, remove some files, then resume it. A third
possibility is to write the file to some other filesystem (perhaps to a
file on /tmp) where the user's quota has not been exceeded. Then after
rectifying the quota situation, the file can be moved back to the
filesystem it belongs on.
ADMINISTRATION
Quotas is a configurable subsystem that is optionally built into the kernel.
A decision as to which filesystems need to have quotas enabled needs to be
made. Usually, only filesystems that house users' home directories or other
user files need to be subjected to the quota system. It is recommended that
the filesystem housing /tmp should be free of quotas.
XFS and the Linux VFS quota systems (e.g. as used on ext2 filesystems) share
many characteristics. We begin with an overview of how the Linux VFS quota
system is administered.
Administering the Linux VFS Quota System
On most filesystems, quota file(s) should be created in the root of
those filesystems that are to have quotas. These files should be of
size zero and should be readable and writable only by root. After
deciding on the filesystems that will have quotas, the administrator
then establishes quotas for individual users.
The edquota(8) and/or setquota(8) commands are used to actually set
the limits desired upon each user. Where a number of users are to be
given the same quotas (a common occurrence) the -p option to edquota
allows this to be easily accomplished. Unless explicitly given a quota,
users have no limits set on the amount of disk they can use or the number
of files they can create.
Once the quotas are set and ready to operate, the system must be informed
to enforce quotas on the desired filesystems. This is accomplished with
the quotaon(8) command. For quotas to be accurate, it should be enabled
on a local filesystem immediately after the filesystem has been mounted.
quotaon either enables quotas for a particular filesystem or, with the -a
option, enables quotas for each filesystem indicated in /etc/fstab as
using quotas. See mount(8) for details. When the quota package is
installed,
/usr/etc/quotaon -a
can be automatically executed during system boot up time by the startup
scripts.
When quotas need to be disabled, the quotaoff(8) command is used.
However, if the filesystem is about to be dismounted, the umount(8)
command disables quotas immediately before the filesystem is unmounted.
This is actually an effect of the umount(2) system call, and it
guarantees that the quota system is not disabled if the umount would fail
because the filesystem is not idle.
Periodically (certainly after each reboot and when quotas are first
enabled for a filesystem), the records retained in the quota file should
be checked for consistency with the actual number of blocks and files
allocated to the user. The quotacheck(8) command is used to accomplish
this. It is not necessary to unmount the filesystem or disable the
quota system to run this command, though on active filesystems inaccurate
results may occur. This does no real harm in most cases; another run of
quotacheck when the filesystem is idle corrects any inaccuracy. The
startup scripts can be configured to run quotacheck automatically.
The superuser can use the quota command to examine the usage and quotas
of any user, and the repquota(8) command can be used to check the usages
and limits for all users on a filesystem.
Administering the XFS Quota System
The XFS quota system is different from that of the Linux VFS in many ways.
o There is no need for quota file(s) in the root of the XFS filesystem.
o XFS distinguishes between quota accounting and limit enforcement.
Quota accounting must be turned on at the time of mounting the XFS
filesystem. However, it is possible to turn on/off limit
enforcement any time quota accounting is turned on. The "quota"
option in mount(8) turns on both (user) quota accounting and
enforcement. The "uqnoenforce" option must be used to turn on
user accounting with limit enforcement disabled. quotaon(8)
contains some examples of frequently used procedures.
o Turning on quotas on the root filesystem is slightly different from
the above. quotaon(8) must be used on the root XFS filesystem
first; quotas will be turned on the next time the system is
rebooted. It is useful to use repquota(8) with the -v option to
monitor the effect of quotaon/off at various stages.
o quotacheck(8) has no effect on XFS filesystems. The first time
quota accounting is turned on, XFS does an automatic quotacheck
internally; afterwards, the quota system will always be completely
consistent until quotas are manually turned off.
o repquota(8) with the -v option can be used to monitor the status of
the quota system of an XFS filesystem. This can be used to see if
quotas are turned on, given an XFS filesystem. It can also be used
to monitor the space occupied by the quota system itself.
o Refer to the xfsdump(8), xfsdq(8), and xfsrq(8) man pages, from the
xfsdump package, which describes a mechanism built into xfsdump that
allows quota limit information to be backed up and easily restored.
o edquota(8) and setquota(8) cannot be used to set quota limits before
turning on quotas on the filesystem concerned.
o XFS filesystems keep quota accounting on the superuser, and quota -v
will display the superuser's usage information. However, limits are
never enforced on the superuser.
o XFS filesystems keep quota accounting whether the user has quota
limits or not.
IMPLEMENTATION NOTES
On filesystems using Linux VFS quota, disk quota usage information is
stored in a file on the filesystem that the quotas are to be applied to.
Conventionally, this file is called quotas, and resides at the root of
the filesystem.
The system is informed of the existence of the quota file by the quotactl
system call. It then reads the quota entries for any open files owned by
users. Each subsequent open of a file in the filesystem is accompanied
by a pairing with its quota information.
Each time a block is accessed or released and each time an inode is
allocated or freed, the quota system gets told about it and, in the case
of allocations, gets the opportunity to deny the allocation.
Note that the XFS quota system implementation is radically different
to the Linux VFS described above. XFS considers quota information as
filesystem metadata and uses journaling to provide a higher level
guarantee of consistency.
GETTING STARTED
To use quota under XFS you will need the following:
o An XFS aware kernel;
o Quota must be enabled at the time the kernel is built
(CONFIG_QUOTA, at the start of the Filesystems menu);
o XFS quota must be enabled at the time XFS is built
(CONFIG_XFS_QUOTA, below XFS in the Filesystems menu);
o Userspace quota tools which are aware of XFS quota.
User tools which support XFS can be downloaded from the Linux
quota project at:
http://sourceforge.net/projects/linuxquota/
The quota tools from version 3.01-pre2 onward have full support
for XFS. They can be downloaded via cvs or as a source tarball.
Building the user tools from source:
# autoconf
# ./configure --prefix=/usr --mandir=/usr/share/man
# make
# make install
EXAMPLES
>>> Enabling quota enforcement on a non-root XFS filesystem
[root@troppo]# echo /dev/hdb10 /mnt/xqm xfs rw,usrquota 0 0 >>/etc/fstab
[root@troppo]# mount /mnt/xqm
>>> Set limits for a user (can also use edquota, interactively)
[root@troppo]# setquota nathans /mnt/xqm 600 800 15 20
>>> Report current user quota
[root@troppo]# repquota /mnt/xqm
*** Report for user quotas on device /dev/hdb10
Block grace time: 7days Inode grace time: 7days
Block limits File limits
user used soft hard grace used soft hard grace
root -- 1552 0 0 11 0 0
nathans -- 440 600 800 8 15 20
pcpqa -- 880 0 0 1 0 0
>>> Push user over quota (see "File limits" above for user "nathans")
[nathans@troppo]$ sh
sh-2.04$ for i in 0 1 2 3 4 5 6 7 8 9 10 11 12 13
> do
> touch /mnt/xqm/blat/file${i}
> done
touch: creating `/mnt/xqm/blat/file12': Disk quota exceeded
touch: creating `/mnt/xqm/blat/file13': Disk quota exceeded
sh-2.04$ ^D
>>> Report current user quota again
[root@troppo]# repquota /mnt/xqm
*** Report for user quotas on device /dev/hdb10
Block grace time: 7days Inode grace time: 7days
Block limits File limits
user used soft hard grace used soft hard grace
root -- 1560 0 0 11 0 0
nathans -+ 440 600 800 20 15 20 7 days
pcpqa -- 880 0 0 1 0 0
>>> From the users point of view:
[nathans@troppo]$ quota
quota
Disk quotas for user nathans (uid 16302):
Filesystem blocks quota limit grace files quota limit grace
/dev/hdb10 440 600 800 20* 15 20 7 days
>>> Run warnquota(8) via cron(8) to periodically inform users of violations.
CAVEATS
The XFS allocation mechanism will always reserve the maximum amount of
space required before proceeding with a space allocation. If insufficient
space for this reservation is available, due to the users block quota limit
being reached for example, this may result in the allocation failing even
though there is sufficient space. Quota enforcement can thus sometimes
happen in situations where the user is under quota and the end result of
some operation would still have left the user under quota had the operation
been allowed to run its course. This is an unavoidable side of affect of
the way XFS operates, so should be kept in mind when assigning users block
limits. This additional overhead is typically in the range of tens of
filesystem-sized blocks.
On IRIX, XFS supports project quota. This is not (ever) likely to be
supported on Linux/XFS, as the concept of a project is specific to IRIX.
A filesystem that has used user quota on IRIX, however, can be migrated
to Linux, and vice-versa, as the ondisk format is shared between both
versions of XFS (and Linux/XFS is "endian clean").
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