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.

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 extended attributes appear on a directory 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, set the extended attribute on a CephFS directory with a value:

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 limit:

$ getfattr -n ceph.quota.max_bytes /some/dir
# file: dir1/
ceph.quota.max_bytes="100000000"
$
$ getfattr -n ceph.quota.max_files /some/dir
# file: dir1/
ceph.quota.max_files="10000"

Note

Running getfattr /some/dir -d -m - for a CephFS directory will print none of the CephFS extended attributes. This is because the CephFS kernel and FUSE clients hide this information from the listxattr(2) system call. Instead, a specific CephFS extended attribute can be viewed by running getfattr /some/dir -n ceph.<some-xattr>.

To remove a quota, set the value of extended attribute to 0:

$ setfattr -n ceph.quota.max_bytes -v 0 /some/dir
$ getfattr /some/dir -n ceph.quota.max_bytes
dir1/: ceph.quota.max_bytes: No such attribute
$
$ setfattr -n ceph.quota.max_files -v 0 /some/dir
$ getfattr dir1/ -n ceph.quota.max_files
dir1/: ceph.quota.max_files: No such attribute

Space Usage Reporting and CephFS Quotas

When the root directory of the CephFS mount has quota set on it, the available space on the CephFS reported by space usage report tools (like df) is based on quota limit. That is, available space = quota limit - used space instead of available space = total space - used space.

This behaviour can be disabled by setting following option in client section of ceph.conf:

client quota df = false

Limitations

  1. 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.

  2. 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.

  3. Quotas are implemented in the kernel client 4.17 and higher. Quotas are supported by the userspace client (libcephfs, ceph-fuse). Linux kernel clients >= 4.17 support CephFS quotas but only on mimic+ clusters. Kernel clients (even recent versions) will fail to handle quotas on older clusters, even if they may be able to set the quotas extended attributes.

  4. 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.

    Kernel clients need access to the parent of the directory inode on which quotas are configured in order to enforce them. If quota is configured on a directory path (e.g., /home/volumes/group), the kclient needs to have access to the parent (e.g., /home/volumes).

    An example command to create such an user is as below:

    $ ceph auth get-or-create client.guest mds 'allow r path=/home/volumes, allow rw path=/home/volumes/group' mgr 'allow rw' osd 'allow rw tag cephfs metadata=*' mon 'allow r'

    See also: https://tracker.ceph.com/issues/55090

  5. Snapshot file data which has since been deleted or changed does not count towards the quota. See also: http://tracker.ceph.com/issues/24284

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

Note

In cases where CephFS extended attributes are set on a CephFS directory (for example, /some/dir), running getfattr /some/dir -d -m - will not print those CephFS extended attributes. This is because CephFS kernel and FUSE clients hide this information from the listxattr(2) system call. You can access a specific CephFS extended attribute by running getfattr /some/dir -n ceph.<some-xattr> instead.