After a reboot of the MDS Server it can happen that the CephFS Filesystem becomes read-only:
HEALTH_WARN 1 MDSs are read only
[WRN] MDS_READ_ONLY: 1 MDSs are read only
mds.XXX(mds.0): MDS in read-only mode
[https://tracker.ceph.com/issues/58082](https://tracker.ceph.com/issues/58082)
In the MDS log you will find following entry
log_channel(cluster) log [ERR] : failed to commit dir 0x1 object, errno -22 mds.0.11963 unhandled write error (22) Invalid argument, force readonly... mds.0.cache force file system read-only log_channel(cluster) log [WRN] : force file system read-only mds.0.server force_clients_readonly
This is a known upstream issue thought the fix is still not merged
As a workaround you can use following steps:
ceph config set mds mds_dir_max_commit_size 80
ceph fs fail <fs_name>
ceph fs set <fs_name> joinable true
If not successful you may need to increase the mds_dir_max_commit_size, e.g. to 160