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Quotas
======
CephFS allows quotas to be set on any directory in the system. The
quota can restrict the number of *bytes* or the number of *files*
stored beneath that point in the directory hierarchy.
Limitations
-----------
#. *Quotas are cooperative and non-adversarial.* CephFS quotas rely on
the cooperation of the client who is mounting the file system to
stop writers when a limit is reached. A modified or adversarial
client cannot be prevented from writing as much data as it needs.
Quotas should not be relied on to prevent filling the system in
environments where the clients are fully untrusted.
#. *Quotas are imprecise.* Processes that are writing to the file
system will be stopped a short time after the quota limit is
reached. They will inevitably be allowed to write some amount of
data over the configured limit. How far over the quota they are
able to go depends primarily on the amount of time, not the amount
of data. Generally speaking writers will be stopped within 10s of
seconds of crossing the configured limit.
#. *Quotas are not yet implemented in the kernel client.* Quotas are
supported by the userspace client (libcephfs, ceph-fuse) but are
not yet implemented in the Linux kernel client.
#. *Quotas must be configured carefully when used with path-based
mount restrictions.* The client needs to have access to the
directory inode on which quotas are configured in order to enforce
them. If the client has restricted access to a specific path
(e.g., ``/home/user``) based on the MDS capability, and a quota is
configured on an ancestor directory they do not have access to
(e.g., ``/home``), the client will not enforce it. When using
path-based access restrictions be sure to configure the quota on
the directory the client is restricted too (e.g., ``/home/user``)
or something nested beneath it.
Configuration
-------------
Like most other things in CephFS, quotas are configured using virtual
extended attributes:
* ``ceph.quota.max_files`` -- file limit
* ``ceph.quota.max_bytes`` -- byte limit
If the attributes appear on a directory inode that means a quota is
configured there. If they are not present then no quota is set on
that directory (although one may still be configured on a parent directory).
To set a quota::
setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir # 100 MB
setfattr -n ceph.quota.max_files -v 10000 /some/dir # 10,000 files
To view quota settings::
getfattr -n ceph.quota.max_bytes /some/dir
getfattr -n ceph.quota.max_files /some/dir
Note that if the value of the extended attribute is ``0`` that means
the quota is not set.
To remove a quota::
setfattr -n ceph.quota.max_bytes -v 0 /some/dir
setfattr -n ceph.quota.max_files -v 0 /some/dir
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