FLUSHSNAP_ACKs are not always sent back on unexpected FLUSHSNAP
There is a PR to make the MDS always send back a FLUSHSNAP_ACK: https://github.com/ceph/ceph/pull/4523
But I guess the clients are issuing these at times when the MDS doesn't expect them, and then the clients hang up. We should fix this on both sides if possible so it works if either end is upgraded.
#1 Updated by Zheng Yan over 4 years ago
client creates cap_snap and sends FLUSHSNAP message to MDS even when snap notification contains no new snap (snap removal). that's why MDS mau receive unexpected FLUSHSNAP message. client releases cap_snap and related resource only after it receives FLUSHSNAP_ACK, For kclient, it does not hang, the symptom of this bug is that kernel warns busy inodes after umount.
The kclient does not make sure cap snap get flushed properly on umount. I'm working on fixing it.