12 Troubleshooting CephFS #

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 and narrow it down from there.

If part of the CephFS metadata or data pools is unavailable and CephFS is not responding, it is probably because RADOS itself is unhealthy.

Check the cluster's status with the following command:

cephuser@adm > ceph status

Ceph will print the cluster status. Review Chapter 2, Troubleshooting logging and debugging, Chapter 6, Troubleshooting Ceph Monitors and Ceph Managers, Chapter 5, Troubleshooting placement groups (PGs), and Chapter 4, Troubleshooting OSDs for tips on what may be causing the issue.

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:

cephuser@adm > ceph daemon mds.NAME dump_ops_in_flight

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 cannot 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 will not violate POSIX semantics.

The kernel in SUSE Linux Enterprise Server 15 SP2 includes a CephFS client that is able to take full advantage of all the features available on an SES7 cluster. All relevant features and bug fixes are backported to this operating system.

However, it may be necessary to access CephFS from other systems that may provide an older CephFS client, which may not support all the features required by an SUSE Enterprise Storage 7 cluster. When this happens, the kernel client will fail to mount the file system and will emit messages similar to the one shown below:

[ 4187.023633] libceph: mon0 192.168.122.150:6789 feature set mismatch, my 107b84a842aca < server's 40107b84a842aca, missing 400000000000000
[ 4187.023838] libceph: mon0 192.168.122.150:6789 missing required protocol features

The message above means that the MON identified 0x400000000000000 as the missing feature in the client (the value 0x107b84a842aca represents all the features supported by the client, while 0x40107b84a842aca represents the minimum set of features required by the cluster). From the following table, which shows the complete list of feature bits, we can see that the missing feature bit 58 (2^58 = 0x400000000000000) is CRUSH_TUNABLES5, NEW_OSDOPREPLY_ENCODING, or FS_FILE_LAYOUT_V2 (all these three features share the same feature bit).

A possible solution to allow an old kernel client to mount a recent CephFS is to modify the cluster CRUSH profile. CRUSH profiles define a set of CRUSH tunables that are named after the Ceph versions in which they were introduced. For example, the firefly tunables are first supported in the Firefly release (0.80), and older clients will not be able to access the cluster. Thus, to fix the problem shown above, the following command can be used:

cephuser@adm > ceph osd crush tunables hammer

This will adjust the CRUSH profile to the behaviour it had for the Hammer (0.94) release. Note however that this is not the optimal behaviour for the cluster. To change back to the optimal profile, run the following command:

cephuser@adm > ceph osd crush tunables optimal

The following table lists the available CRUSH profiles and which CRUSH tunables versions (the CRUSH_TUNABLE feature bits in the previous table) they correspond to. It also identifies the minimum kernel version required to use for each profile. Note however that Operating System vendors may choose to backport features to their kernels, so these kernel versions are valid for mainline kernels only. The kernel client included in SUSE Linux Enterprise Server 15 SP2, for example, includes backports of features and bug fixes relevant for usage in SES7 clusters.