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Identify_Ceph_Latency_Issues.md

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Identify Ceph Latency Issues

Examine the output of the ceph -s command to get context for potential issues causing latency.

Troubleshoot the underlying causes for the ceph -s command reporting slow PGs.

Prerequisites

This procedure requires admin privileges.

Procedure

  1. View the status of Ceph.

    ceph -s

    Example output:

    cluster:
   id:     73084634-9534-434f-a28b-1d6f39cf1d3d
   health: HEALTH_WARN
           1 filesystem is degraded
           1 MDSs report slow metadata IOs
           Reduced data availability: 15 pgs inactive, 15 pgs peering
           46 slow ops, oldest one blocked for 1395 sec, daemons [osd,2,osd,5,mon,ceph-1,mon,ceph-2,mon,ceph-3] have slow ops.

 services:
   mon: 3 daemons, quorum ceph-3,ceph-1,ceph-2 (age 38m)
   mgr: ceph-2(active, since 30m), standbys: ceph-3, ceph-1
   mds: cephfs:1/1 {0=ceph-2=up:replay} 2 up:standby
   osd: 15 osds: 15 up (since 93s), 15 in (since 23m); 10 remapped pgs
   rgw: 1 daemon active (ceph-1.rgw0)

 data:
   pools:   9 pools, 192 pgs
   objects: 15.93k objects, 55 GiB
   usage:   175 GiB used, 27 TiB / 27 TiB avail
   pgs:     7.812% pgs not active
            425/47793 objects misplaced (0.889%)
            174 active+clean
            8   peering
            7   remapped+peering
            2   active+remapped+backfill_wait
            1   active+remapped+backfilling

 io:
   client:   7.0 KiB/s wr, 0 op/s rd, 1 op/s wr
   recovery: 12 MiB/s, 3 objects/s

    The output can provide a lot of context to potential issues causing latency. In the example output above, the following troubleshooting information can be observed:

    • health - Shows latency and what daemons/OSDs are associated with it.
    • mds - MDS is functional, but it is in replay because of the slow ops.
    • osd - All OSDs are up and in. This could be related to a network issue or a single system issue if both OSDs are on the same box.
    • client - Shows the amount of IO/Throughput that clients using Ceph are performing. If health is not set to HEALTH_OK and traffic is passing through, then Ceph is functioning and re-balancing data because of typical hardware/network issues.
    • recovery - Shows recovery traffic as the system ensures all the copies of data are available to ensure data redundancy.

Fixes

Based on the output from ceph -s (using our example above) we can correlate some information to help determine our bottleneck.

  1. When reporting slow ops for OSDs, then it is good to find out if those OSDs are on the same node or different nodes.

    1. If the OSDs are on the same node, then look at networking or other hardware-related issues on that node.
    2. If the OSDs are on different nodes, then investigate networking issues.
      1. As an initial step, restart the OSDs, if the slow ops go away and do not return, then we can investigate the logs for possible software bugs or memory issues.
      2. If the slow ops come right back, then there is an issue with replication between the 2 OSDs which tends to be network-related.

  2. When reporting slow ops for MONs, then it is typically an issue with the process. The most common cause is either an abrupt clock skew or a hung mon/mgr process.

    1. The recommended remediation is to do a rolling restart of the Ceph MON and MGR daemons.

      1. Get MON and MGR daemons.

        ceph orch ps | grep -E 'mgr|mon'

      2. Restart each MON and MGR daemons. Make sure each daemon starts before restarting the next one.

        ceph orch daemon restart <daemon_name>

    2. Failover the active MDS server.

      ceph mds fail 0

  3. When reporting slow ops for MDS, then are multiple possible causes.

    1. If listed in addition to OSDs, then the root cause for this is typically the OSDs, and the process above should be used, followed by restarting the MDS daemons.
    2. If it is only listing MDS, then restart the MDS daemons. If the problem persists, then examine the logs in order to determine the root cause.
    3. See Troubleshoot Ceph MDS reporting slow requests and failure on client for additional steps to help identify MDS slow ops