If there is an issue with the cephfs kernel client, the most important thing is figuring out whether the problem is with the client or the MDS. Generally, this is easy to work out. If the kernel client broke directly, there will be output in dmesg. Collect it and any appropriate kernel state. If the problem is with the MDS, there will be hung requests that the client is waiting on. Look in /sys/kernel/debug/ceph/*/ and cat the mdsc file to get a listing of requests in progress. If one of them remains there, the MDS has probably “forgotten” it. We can get hints about what’s going on by dumping the MDS cache: ceph mds tell 0 dumpcache /tmp/dump.txt
And if high logging levels are set on the MDS, that will almost certainly hold the information we need to diagnose and solve the issue.