The XFS quota system differs to that of other filesystems in a
number of ways. Most importantly, XFS considers quota
information as filesystem metadata and uses journaling to provide
a higher level guarantee of consistency. As such, it is
administered differently, in particular:
1. The quotacheck command has no effect on XFS filesystems.
The first time quota accounting is turned on (at mount
time), XFS does an automatic quotacheck internally;
afterwards, the quota system will always be completely
consistent until quotas are manually turned off.
2. There is no need for quota file(s) in the root of the XFS
filesystem.
3. 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 to the mount
command turns on both (user) quota accounting and
enforcement. The "uqnoenforce" option must be used to
turn on user accounting with limit enforcement disabled.
4. Turning on quotas on the root filesystem is slightly
different from the above. For Linux XFS, the quota mount
flags must be passed in with the "rootflags=" boot
parameter.
5. It is useful to use the state to monitor the XFS quota
subsystem at various stages - it can be used to see if
quotas are turned on, and also to monitor the space
occupied by the quota system itself..
6. There is a mechanism built into xfsdump that allows quota
limit information to be backed up for later restoration,
should the need arise.
7. Quota limits cannot be set before turning on quotas on.
8. XFS filesystems keep quota accounting on the superuser
(user ID zero), and the tool will display the superuser's
usage information. However, limits are never enforced on
the superuser (nor are they enforced for group and project
ID zero).
9. XFS filesystems perform quota accounting whether the user
has quota limits or not.
10. XFS supports the notion of project quota, which can be
used to implement a form of directory tree quota (i.e. to
restrict a directory tree to only being able to use up a
component of the filesystems available space; or simply to
keep track of the amount of space used, or number of
inodes, within the tree).
