Clients failing to respond to capability release

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I've seen this issue mentioned in the past, but with older releases. So
I'm wondering if anybody has any pointers.

The Ceph cluster is running Pacific 16.2.13 on Ubuntu 20.04. Almost all
clients are working fine, with the exception of our backup server. This
is using the kernel CephFS client on Ubuntu 22.04 with kernel 6.2.0 [1]
(so I suspect a newer Ceph version?).

The backup server has multiple (12) CephFS mount points. One of them,
the busiest, regularly causes this error on the cluster:

HEALTH_WARN 1 clients failing to respond to capability release
[WRN] MDS_CLIENT_LATE_RELEASE: 1 clients failing to respond to capability release
    mds.mds-server(mds.0): Client backupserver:cephfs-backupserver failing to respond to capability release client_id: 521306112

And occasionally, which may be unrelated, but occurs at the same time:

[WRN] MDS_SLOW_REQUEST: 1 MDSs report slow requests
    mds.mds-server(mds.0): 1 slow requests are blocked > 30 secs

The second one clears itself, but the first sticks until I can unmount
the filesystem on the client after the backup completes.

It appears that whilst it's in this stuck state there may be one or more
directory trees that are inaccessible to all clients. The backup server
is walking the whole tree but never gets stuck itself, so either the
inaccessible directory entry is caused after it has gone past, or it's
not affected. Maybe the backup server is holding a directory when it

It may be that an upgrade to Quincy resolves this, since it's more
likely to be inline with the kernel client version wise, but I don't
want to knee-jerk upgrade just to try and fix this problem.

Thanks for any advice.


[1] The reason for the newer kernel is that the backup performance from
CephFS was terrible with older kernels. This newer kernel does at least
resolve that issue.
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