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Applies to SUSE Enterprise Storage 7

12 Troubleshooting CephFS Edit source

12.1 Slow or stuck operations Edit source

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.

12.2 Checking RADOS health Edit source

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.

12.3 MDS Edit source

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:

  • Overloading the system

  • Running an older (misbehaving) client

  • Underlying RADOS issues

12.3.1 Identifying MDS slow requests Edit source

You can list current operations via the admin socket by running the following command from the MDS host:

cephuser@adm > ceph daemon mds.NAME dump_ops_in_flight

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. This can be because the client is trying to flush out dirty data or because you have encountered a bug in CephFS’ distributed file lock code (the file “capabilities” [“caps”] system).

If it is 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 is not reporting that clients are misbehaving, either the client has a problem or its requests are not reaching the MDS.

12.4 Kernel mount debugging Edit source

12.4.1 Slow requests Edit source

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 contains 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.

bdi

BDI info about the Ceph system (blocks dirtied, written, etc)

caps

Counts of file caps structures in-memory and used

client_options

Dumps the options provided to the CephFS mount

dentry_Iru

Dumps the CephFS dentries currently in-memory

mdsc

Dumps current requests to the MDS

mdsmap

Dumps the current MDSMap epoch and MDSes

mds_sessions

Dumps the current sessions to MDSes

monc

Dumps the current maps from the monitor, and any subscriptions held

monmap

Dumps the current monitor map epoch and monitors

osdc

Dumps the current ops in-flight to OSDs (ie, file data IO)

osdmap

Dumps the current OSDMap epoch, pools, and OSDs

12.5 Disconnecting and remounting the file system Edit source

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.

12.6 Mounting Edit source

12.6.1 Mount I/O error Edit source

A mount 5 (EIO, I/O error) 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.

12.6.2 Mount out of memory error Edit source

A mount 12 error (ENOMEM, out of memory) with cannot allocate memory usually occurs if you have a version mismatch between the Ceph Client version and the Ceph Storage Cluster version. Check the versions using:

ceph -v

If the Ceph Client is behind the Ceph cluster, try to upgrade it:

sudo zypper up
sudo zypper in ceph-common

You may need to uninstall, autoclean and autoremove ceph-common and then reinstall it so that you have the latest version.

12.7 Mounting CephFS using old kernel clients Edit source

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).

Table 12.1: Ceph features
FeatureBitValue
UID00x1
NOSRCADDR10x2
FLOCK30x8
SUBSCRIBE240x10
MONNAMES50x20
RECONNECT_SEQ60x40
DIRLAYOUTHASH70x80
OBJECTLOCATOR80x100
PGID6490x200
INCSUBOSDMAP100x400
PGPOOL3110x800
OSDREPLYMUX120x1000
OSDENC130x2000
SERVER_KRAKEN140x4000
MONENC150x8000
CRUSH_TUNABLES180x40000
SERVER_LUMINOUS210x200000
RESEND_ON_SPLIT210x200000
RADOS_BACKOFF210x200000
OSDMAP_PG_UPMAP210x200000
CRUSH_CHOOSE_ARGS210x200000
MSG_AUTH230x800000
CRUSH_TUNABLES2250x2000000
CREATEPOOLID260x4000000
REPLY_CREATE_INODE270x8000000
SERVER_M280x10000000
MDSENC290x20000000
OSDHASHPSPOOL300x40000000
MON_SINGLE_PAXOS310x80000000
OSD_CACHEPOOL350x800000000
CRUSH_V2360x1000000000
EXPORT_PEER370x2000000000
OSD_ERASURE_CODES380x4000000000
OSD_OSD_TMAP2OMAP380x4000000000
OSDMAP_ENC390x8000000000
MDS_INLINE_DATA400x10000000000
CRUSH_TUNABLES3410x20000000000
OSD_PRIMARY_AFFINITY410x20000000000
MSGR_KEEPALIVE2420x40000000000
OSD_POOLRESEND430x80000000000
ERASURE_CODE_PLUGINS_V2440x100000000000
OSD_FADVISE_FLAGS460x400000000000
MDS_QUOTA470x800000000000
CRUSH_V4480x1000000000000
MON_METADATA500x4000000000000
OSD_BITWISE_HOBJ_SORT510x8000000000000
OSD_PROXY_WRITE_FEATURES520x10000000000000
ERASURE_CODE_PLUGINS_V3530x20000000000000
OSD_HITSET_GMT540x40000000000000
HAMMER_0_94_4550x80000000000000
NEW_OSDOP_ENCODING560x100000000000000
MON_STATEFUL_SUB570x200000000000000
MON_ROUTE_OSDMAP570x200000000000000
OSDSUBOP_NO_SNAPCONTEXT570x200000000000000
SERVER_JEWEL570x200000000000000
CRUSH_TUNABLES5580x400000000000000
NEW_OSDOPREPLY_ENCODING580x400000000000000
FS_FILE_LAYOUT_V2580x400000000000000
FS_BTIME590x800000000000000
FS_CHANGE_ATTR590x800000000000000
MSG_ADDR2590x800000000000000
OSD_RECOVERY_DELETES600x1000000000000000
CEPHX_V2610x2000000000000000
RESERVED620x4000000000000000

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.

Table 12.2: CRUSH profiles
CRUSH ProfileCeph ReleaseCRUSH TunableMinimum Kernel Version
argonaut0.48CRUSH_TUNABLES3.6
bobtail0.56CRUSH_TUNABLES23.9
firefly0.80CRUSH_TUNABLES33.15
hammer0.94CRUSH_V44.1
jewel10.2.0CRUSH_TUNABLES54.5
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