If you are experiencing apparent hung operations, the first task is to identify where the problem is occurring: in the client, the MDS, or the network connecting them. Start by looking to see if either side has stuck operations (Slow requests (MDS), below), and narrow it down from there.
If part of the CephFS metadata or data pools is unavaible and CephFS isn’t responding, it is probably because RADOS itself is unhealthy. Resolve those problems first (/rados/troubleshooting).
If an operation is hung inside the MDS, it will eventually show up in ceph health, identifying “slow requests are blocked”. It may also identify clients as “failing to respond” or misbehaving in other ways. If the MDS identifies specific clients as misbehaving, you should investigate why they are doing so. Generally it will be the result of 1) overloading the system (if you have extra RAM, increase the “mds cache size” config from its default 100000; having a larger active file set than your MDS cache is the #1 cause of this!) 2) running an older (misbehaving) client, or 3) underlying RADOS issues.
Otherwise, you have probably discovered a new bug and should report it to the developers!
You can list current operations via the admin socket by running:
ceph daemon mds.<name> dump_ops_in_flight
from the MDS host. Identify the stuck commands and examine why they are stuck. Usually the last “event” will have been an attempt to gather locks, or sending the operation off to the MDS log. If it is waiting on the OSDs, fix them. If operations are stuck on a specific inode, you probably have a client holding caps which prevent others from using it, either because the client is trying to flush out dirty data or because you’ve encountered a bug in CephFS’ distributed file lock code (the file “capabilities” [“caps”] system).
If it’s a result of a bug in the capabilities code, restarting the MDS is likely to resolve the problem.
If there are no slow requests reported on the MDS, and it isn’t reporting that clients are misbehaving, either the client has a problem or its requests aren’t reaching the MDS.
ceph-fuse also supports dump_ops_in_flight. See if it has any and where they are stuck.
To get more debugging information from ceph-fuse, try running in the foreground with logging to the console (-d) and enabling client debug (--debug-client=20), enabling prints for each message sent (--debug-ms=1).
If you suspect a potential monitor issue, enable monitor debugging as well (--debug-monc=20).
Unfortunately the kernel client does not support the admin socket, but it has similar (if limited) interfaces if your kernel has debugfs enabled. There will be a folder in sys/kernel/debug/ceph/, and that folder (whose name will look something like 28f7427e-5558-4ffd-ae1a-51ec3042759a.client25386880) will contain a variety of files that output interesting output when you cat them. These files are described below; the most interesting when debugging slow requests are probably the mdsc and osdc files.
If there are no stuck requests but you have file IO which isn’t progressing, you might have a...
Because CephFS has a “consistent cache”, if your network connection is disrupted for a long enough time, the client will be forcibly disconnected from the system. At this point, the kernel client is in a bind: it can’t safely write back dirty data, and many applications do not handle IO errors correctly on close(). At the moment, the kernel client will remount the FS, but outstanding filesystem IO may or may not be satisfied. In these cases, you may need to reboot your client system.
You can identify you are in this situation if dmesg/kern.log report something like:
Jul 20 08:14:38 teuthology kernel: [3677601.123718] ceph: mds0 closed our session Jul 20 08:14:38 teuthology kernel: [3677601.128019] ceph: mds0 reconnect start Jul 20 08:14:39 teuthology kernel: [3677602.093378] ceph: mds0 reconnect denied Jul 20 08:14:39 teuthology kernel: [3677602.098525] ceph: dropping dirty+flushing Fw state for ffff8802dc150518 1099935956631 Jul 20 08:14:39 teuthology kernel: [3677602.107145] ceph: dropping dirty+flushing Fw state for ffff8801008e8518 1099935946707 Jul 20 08:14:39 teuthology kernel: [3677602.196747] libceph: mds0 172.21.5.114:6812 socket closed (con state OPEN) Jul 20 08:14:40 teuthology kernel: [3677603.126214] libceph: mds0 172.21.5.114:6812 connection reset Jul 20 08:14:40 teuthology kernel: [3677603.132176] libceph: reset on mds0
This is an area of ongoing work to improve the behavior. Kernels will soon be reliably issuing error codes to in-progress IO, although your application(s) may not deal with them well. In the longer-term, we hope to allow reconnect and reclaim of data in cases where it won’t violate POSIX semantics (generally, data which hasn’t been accessed or modified by other clients).
A mount 5 error typically occurs if a MDS server is laggy or if it crashed. Ensure at least one MDS is up and running, and the cluster is active + healthy.
If the Ceph Client is behind the Ceph cluster, try to upgrade it:
sudo apt-get update && sudo apt-get install ceph-common
You may need to uninstall, autoclean and autoremove ceph-common and then reinstall it so that you have the latest version.