Feature #41209
mds: create a configurable snapshot limit
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Description
Add a new config option that imposes a limit on the number of snapshots in a directory. Zheng has found in the past that a directory with too many snapshots will result in performance issues on the MDS. Verify this is still the case. A test case should be prepared that ensures the limit is enforced and that it the limit is applied per-directory (i.e. one directory hitting its limit should not affect another).
History
#1 Updated by Zheng Yan 9 days ago
mds.0 has snaptable, which contains information about all snapshots. Each mds has a snapclient, which also caches snapshot information. From there, you can get total number of snapshot in the FS.
ceph inodes have rstat, rstat.rsnaps is number of nested snapshots under a directory (root inode's rstat.rsnaps also is total number of snapshots in the FS).
To limit snapshot count on a directory, check snapshot count when making new snaps. In Server::handle_client_mksnap, check snapshot count after locks are acquired.
#2 Updated by Milind Changire 8 days ago
Thanks for the hint Zheng.
How could the MDS return the status "too many snaps" to the caller ?
There's no error code in the system errno.h to denote this condition.
The attempt to create a snapshot beyond the limit for the directory cannot be silently ignored, now that we plan to impose a configurable limit.
Any suggestion for a default limit ?
5 snapshots per directory should be enough ?
#3 Updated by Zheng Yan 8 days ago
Milind Changire wrote:
Thanks for the hint Zheng.
How could the MDS return the status "too many snaps" to the caller ?
There's no error code in the system errno.h to denote this condition.
how about -E2BIG
The attempt to create a snapshot beyond the limit for the directory cannot be silently ignored, now that we plan to impose a configurable limit.
Any suggestion for a default limit ?
5 snapshots per directory should be enough ?
I suggest 100
#4 Updated by Milind Changire 8 days ago
Zheng Yan wrote:
Milind Changire wrote:
Thanks for the hint Zheng.
How could the MDS return the status "too many snaps" to the caller ?
There's no error code in the system errno.h to denote this condition.how about -E2BIG
-EMLINK seems reasonable too
The attempt to create a snapshot beyond the limit for the directory cannot be silently ignored, now that we plan to impose a configurable limit.
Any suggestion for a default limit ?
5 snapshots per directory should be enough ?I suggest 100