quotas(4)quotas(4)NAMEquotas - filesystem quota subsystem
DESCRIPTION
In most computing environments, disk space is not infinite. The
lbootable quotas subsystem provides a mechanism to control usage of disk
space. Quotas can be set for each individual user on any or 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; usually if one is imposed, both are. 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.
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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.
Administering the Quota System
Quotas is an lbootable subsystem that can be installed by the
administrator. A decision as to what 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. If possible, /tmp should be free of quotas.
XFS and EFS quota systems share many characteristics. However, overall
XFS offers a richer set of functionalities. We will explain the old EFS
quota system first and differences in XFS afterwards.
Administering the EFS Quota System
On EFS filesystems, a file by the name of quotas should be created in the
root of the filesystems that are to have quotas. This file should be of
size zero and should be writable only by root. After deciding on the
filesystems that will have quotas, the administrator then establishes
quotas for individual users. The edquota(1M) command is 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(1M) 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 fstab(4) for details. When the quotas subsystem is
installed,
/usr/etc/quotaon -a
can be automatically executed during system boot up time by /etc/rc2.
The script /etc/init.d/quotas handles enabling of quotas and it uses the
chkconfig(1M) command to check the quotas configuration flag to decide
whether or not to enable quotas. Users can circumvent the quotas
mechanism by giving the files away to other users with the use of
chown(1) or chown(2). The system administrator can prevent this by
setting the variable restricted_chown to a non-zero value (see intro(2)
and lboot(1M)).
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When quotas need to be disabled, the quotaoff(1M) command is used.
However, if the filesystem is about to be dismounted, the umount(1M)
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(1M) command is used to accomplish
this. It is not necessary to dismount 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.
quotacheck can be run automatically during boot up via the script
/etc/init.d/quotas. Since quotacheck can take some time, there is a
chkconfig flag called quotacheck that controls whether or not to run
quotacheck at system boot up time.
The superuser can use the quota command to examine the usage and quotas
of any user, and the repquota(1M) 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 EFS in many ways.
o There is no necessity for a quotas file 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 fstab(4) (or -o quota in mount(1M) ) turns on both quota
accounting and enforcement. The qnoenforce option must be used to
turn on accounting with limit enforcement disabled. quotaon(1M)
contains some examples of frequently used procedures.
o Turning on quotas on the root filesystem is slightly different from
the above. quotaon(1M) 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(1M) with the -s option to
monitor the effect of quotaon/off at various stages.
o quotacheck(1M) 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. Similarly,
/etc/chkconfig(1M) options quotas and quotacheck are not needed for
XFS filesystems.
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o repquota(1M) with the -s 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 repquota also provides a -e option to output the limits of all users
listed in /etc/passwd to a file that can later be read in by
edquota(1M). This is useful in recreating the limits of a large
number of users. A possible scenario would be (a) creating the
output file using repquota, (b) turning off quotas and deleting all
the quota information (including limits, etc), (c) mounting the XFS
filesystem back with quotas turned on, and (d) reading that file
containing limits of users using edquota. This procedure will help
compact the quota information. Keeping all the limits saved in a
file for later use will also help in case of a disaster because,
apart from xfsdump(1M), backup utilities do not handle quota
information.
o edquota(1M) 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.
Quota Implementation Notes
On EFS filesystems, 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.
While this name is not known to the system in any way, several of the
user level utilities "know" it, and choosing any other name is not wise.
The data in the quotas file is stored in a format different from
Berkeley's implementation. Any direct accesses to the file by programs
is not recommended. Instead, use the commands or the quotactl(2) system
call to access or modify the quota information.
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. In memory, the quota
information is kept hashed by user ID and filesystem and retained in an
LRU chain so recently released data can be easily reclaimed.
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.
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Note that the XFS quota system implementation is radically different from
that of EFS described above. XFS considers quota information as
filesystem metadata and uses journalling to provide a higher level
guarantee of consistency. Among other things, it is worth mentioning
that XFS can keep accounting information for a very large number of
active users efficiently.
BUGS
Currently, it is not possible to enable quotas on XFS realtime
filesystems.
FILES
/etc/init.d/quotas script to enable and disable quotas
/etc/rc2.d/S14quotas linked to /etc/init.d/quotas
/etc/config/quotas chkconfig flag
/etc/config/quotacheck chkconfig flag
SEE ALSOmount(1M), chkconfig(1M), edquota(1M), quota(1), quotacheck(1M), rc2(1M),
repquota(1M), quotactl(2), fstab(4).
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