5 Troubleshooting placement groups (PGs) #

As previously noted, a placement group is not necessarily problematic because its state is not active+clean. Generally, Ceph's ability to self-repair may not be working when placement groups get stuck. The stuck states include:

To identify stuck placement groups, run the following:

cephuser@adm > ceph pg dump_stuck [unclean|inactive|stale|undersized|degraded]

When you create a cluster and your cluster remains in active, active+remapped, or active+degraded status and never achieves an active+clean status, you likely have a problem with the configuration. As a general rule, you should run your cluster with more than one OSD and a pool size greater than 1 object replica.

5.2.1 Experimenting with a one node cluster #

 

Ceph no longer provides documentation for operating on a single node. Mounting client kernel modules on a single node containing a Ceph daemon can cause a deadlock due to issues with the Linux kernel itself (unless you use VMs for the clients). However, we recommend experimenting with Ceph in a 1-node configuration regardless of the limitations.

If you are trying to create a cluster on a single node, change the default of the osd crush chooseleaf type setting from 1 (meaning host or node) to 0 (meaning osd) in your Ceph configuration file before you create your monitors and OSDs. This tells Ceph that an OSD can peer with another OSD on the same host. If you are trying to set up a 1-node cluster and osd crush chooseleaf type is greater than 0, Ceph tries to pair the PGs of one OSD with the PGs of another OSD on another node, chassis, rack, row, or even datacenter depending on the setting.

Note

Do not mount kernel clients directly on the same node as your Ceph Storage Cluster, because kernel conflicts can arise. However, you can mount kernel clients within virtual machines (VMs) on a single node.

If you are creating OSDs using a single disk, you must create directories for the data manually first. For example:

cephuser@adm > ceph-deploy osd create --data {disk} {host}

5.2.2 Fewer OSDs than replicas #

 

If you have brought up two OSDs to an up and in state, but you still do not see active+clean placement groups, you may have an osd pool default size set to greater than 2. There are a few ways to address this situation. If you want to operate your cluster in an active+degraded state with two replicas, you can set the osd pool default min size to 2 so that you can write objects in an active+degraded state. You may also set the osd pool default size setting to 2 so that you only have two stored replicas (the original and one replica), in which case the cluster should achieve an active+clean state.

Note

You can make the changes at runtime. If you make the changes in your Ceph configuration file, you may need to restart your cluster.

cephuser@adm > ceph osd force-create-pg <pgid>

It is normal for placement groups to enter states such as degraded or peering following a failure. These states indicate the normal progression through the failure recovery process. However, if a placement group stays in one of these states for a long time this may be an indication of a larger problem. For this reason, the monitor will warn when placement groups get stuck in a non-optimal state. Specifically, check for:

You can explicitly list stuck placement groups with one of:

cephuser@adm > ceph pg dump_stuck stale
cephuser@adm > ceph pg dump_stuck inactive
cephuser@adm > ceph pg dump_stuck unclean

For stuck stale placement groups, ensure you have the right ceph-osd daemons running again. For stuck inactive placement groups, it is can be a peering problem. For stuck unclean placement groups, there can be something preventing recovery from completing, like unfound objects.

In certain cases, the ceph-osd peering process can run into problems, preventing a PG from becoming active and usable. For example, ceph health may report:

cephuser@adm > ceph health detail
  HEALTH_ERR 7 pgs degraded; 12 pgs down; 12 pgs peering; 1 pgs recovering;  \
  6 pgs stuck unclean; 114/3300 degraded (3.455%); 1/3 in osds are down
  ...
  pg 0.5 is down+peering
  pg 1.4 is down+peering
  ...
  osd.1 is down since epoch 69, last address 192.168.106.220:6801/8651

Query the cluster to determine exactly why the PG is marked down by executing the following:

cephuser@adm > ceph pg 0.5 query
  { "state": "down+peering",
    ...
    "recovery_state": [
         { "name": "Started\/Primary\/Peering\/GetInfo",
           "enter_time": "2012-03-06 14:40:16.169679",
           "requested_info_from": []},
         { "name": "Started\/Primary\/Peering",
           "enter_time": "2012-03-06 14:40:16.169659",
           "probing_osds": [
                 0,
                 1],
           "blocked": "peering is blocked due to down osds",
           "down_osds_we_would_probe": [
                 1],
           "peering_blocked_by": [
                 { "osd": 1,
                   "current_lost_at": 0,
                   "comment": "starting or marking this osd lost may let us proceed"}]},
         { "name": "Started",
           "enter_time": "2012-03-06 14:40:16.169513"}
     ]
  }

recovery_state section shows that peering is blocked due to down ceph-osd daemons, specifically osd.1. In this case, restart the ceph-osd to recover. Alternatively, if there is a catastrophic failure of osd.1 such as a disk failure, tell the cluster that it is lost and to cope as best it can.

Important

The cluster cannot guarantee that the other copies of the data are consistent and up to date.

To instruct Ceph to continue anyway:

cephuser@adm > ceph osd lost 1

Recovery will proceed.

Under certain combinations of failures Ceph may complain about unfound objects:

cephuser@adm > ceph health detail
  HEALTH_WARN 1 pgs degraded; 78/3778 unfound (2.065%)
  pg 2.4 is active+degraded, 78 unfound

This means that the storage cluster knows that some objects (or newer copies of existing objects) exist, but it has not found copies of them. One example of how this might come about for a PG whose data is on ceph-osds 1 and 2:

In this example, 1 is aware that these object exist, but there is no live ceph-osd who has a copy. In this case, I/O to those objects blocks, and the cluster hopes that the failed node comes back soon. This is assumed to be preferable to returning an I/O error to the user.

Identify which objects are unfound by executing the following:

cephuser@adm > ceph pg 2.4 list_unfound [starting offset, in json]
  { "offset": { "oid": "",
       "key": "",
       "snapid": 0,
       "hash": 0,
       "max": 0},
   "num_missing": 0,
   "num_unfound": 0,
   "objects": [
      { "oid": "object 1",
        "key": "",
        "hash": 0,
        "max": 0 },
      ...
   ],
   "more": 0}

If there are too many objects to list in a single result, the more field is true and you can query for more.

Identify which OSDs have been probed or might contain data:

cephuser@adm > ceph pg 2.4 query
  "recovery_state": [
       { "name": "Started\/Primary\/Active",
         "enter_time": "2012-03-06 15:15:46.713212",
         "might_have_unfound": [
               { "osd": 1,
                 "status": "osd is down"}]},

In this case, for example, the cluster knows that osd.1 might have data, but it is down. The full range of possible states include:

Sometimes it takes some time for the cluster to query possible locations.

It is possible that there are other locations where the object can exist that are not listed. For example, if a ceph-osd is stopped and taken out of the cluster, the cluster fully recovers, and due to some future set of failures ends up with an unfound object, it will not consider the long-departed ceph-osd as a potential location to consider.

If all possible locations have been queried and objects are still lost, you may have to give up on the lost objects. This, again, is possible given unusual combinations of failures that allow the cluster to learn about writes that were performed before the writes themselves are recovered. To mark the unfound objects as lost:

cephuser@adm > ceph pg 2.5 mark_unfound_lost revert|delete

This the final argument specifies how the cluster should deal with lost objects. The delete option forgets about them entirely. The revert option (not available for erasure coded pools) either rolls back to a previous version of the object or (if it was a new object) forgets about it entirely. Use this with caution, as it may confuse applications that expected the object to exist.

It is possible for all OSDs that had copies of a given placement groups to fail. If that is the case, that subset of the object store is unavailable, and the monitor receives no status updates for those placement groups. To detect this situation, the monitor marks any placement group whose primary OSD has failed as stale. For example:

cephuser@adm > ceph health
  HEALTH_WARN 24 pgs stale; 3/300 in osds are down

Identify which placement groups are stale, and what were the last OSDs to store them by executing the following:

cephuser@adm > ceph health detail
  HEALTH_WARN 24 pgs stale; 3/300 in osds are down
  ...
  pg 2.5 is stuck stale+active+remapped, last acting [2,0]
  ...
  osd.10 is down since epoch 23, last address 192.168.106.220:6800/11080
  osd.11 is down since epoch 13, last address 192.168.106.220:6803/11539
  osd.12 is down since epoch 24, last address 192.168.106.220:6806/11861

For example, to get placement group 2.5 back online, this output shows that it was last managed by osd.0 and osd.2. Restarting the ceph-osd daemons allows the cluster to recover that placement group.

If you have many nodes in your cluster and only a few of them receive data, check the number of placement groups in your pool. See Sezione 12.7, «Verifica degli stati dei gruppi di posizionamento» for more information. Since placement groups get mapped to OSDs, a small number of placement groups will not distribute across the cluster. Create a pool with a placement group count that is a multiple of the number of OSDs. See Sezione 17.4, «Gruppi di posizionamento» for details.

If your cluster is up but some OSDs are down and you cannot write data, check to ensure that you have the minimum number of OSDs running for the placement group. If you do not have the minimum number of OSDs running, Ceph will not allow you to write data because there is no guarantee that Ceph can replicate your data.

If you receive an active+clean+inconsistent state, this may happen due to an error during scrubbing. Identify the inconsistent placement group(s) by executing the following:

cephuser@adm > ceph health detail
  HEALTH_ERR 1 pgs inconsistent; 2 scrub errors
  pg 0.6 is active+clean+inconsistent, acting [0,1,2]
  2 scrub errors

Or:

cephuser@adm > rados list-inconsistent-pg rbd
  ["0.6"]

There is only one consistent state, but in the worst case, there could be different inconsistencies in multiple perspectives found in more than one objects. If an object named foo in PG 0.6 is truncated, the output is:

cephuser@adm > rados list-inconsistent-obj 0.6 --format=json-pretty

  {
      "epoch": 14,
      "inconsistents": [
          {
              "object": {
                  "name": "foo",
                  "nspace": "",
                  "locator": "",
                  "snap": "head",
                  "version": 1
              },
              "errors": [
                  "data_digest_mismatch",
                  "size_mismatch"
              ],
              "union_shard_errors": [
                  "data_digest_mismatch_info",
                  "size_mismatch_info"
              ],
              "selected_object_info": "0:602f83fe:::foo:head(16'1 client.4110.0:1 dirty|data_digest|omap_digest s 968 uv 1 dd e978e67f od ffffffff alloc_hint [0 0 0])",
              "shards": [
                  {
                      "osd": 0,
                      "errors": [],
                      "size": 968,
                      "omap_digest": "0xffffffff",
                      "data_digest": "0xe978e67f"
                  },
                  {
                      "osd": 1,
                      "errors": [],
                      "size": 968,
                      "omap_digest": "0xffffffff",
                      "data_digest": "0xe978e67f"
                  },
                  {
                      "osd": 2,
                      "errors": [
                          "data_digest_mismatch_info",
                          "size_mismatch_info"
                      ],
                      "size": 0,
                      "omap_digest": "0xffffffff",
                      "data_digest": "0xffffffff"
                  }
              ]
          }
      ]
  }

In this case, we can learn from the output that the only inconsistent object is named foo, and it has inconsistencies. The inconsistencies fall into two categories:

Repair the inconsistent placement group by executing:

cephuser@adm > ceph pg repair placement-group-ID

This command overwrites the bad copies with the authoritative ones. In most cases, Ceph is able to choose authoritative copies from all available replicas using some predefined criteria but this does not always work. For example, the stored data digest could be missing, and the calculated digest will be ignored when choosing the authoritative copies. Use the above command with caution.

If read_error is listed in the errors attribute of a shard, the inconsistency is likely due to disk errors. You might want to check your disk used by that OSD.

If you receive active+clean+inconsistent states periodically due to clock skew, you may consider configuring your NTP daemons on your monitor hosts to act as peers.

When CRUSH fails to find enough OSDs to map to a PG, it will show as a 2147483647 which is ITEM_NONE or no OSD found. For instance:

[2,1,6,0,5,8,2147483647,7,4]

cephuser@adm > ceph osd erasure-code-profile set myprofile k=5 m=3
cephuser@adm > ceph osd pool create erasurepool erasure myprofile

cephuser@adm > ceph osd crush rule ls
  [
      "replicated_rule",
      "erasurepool"]
  $ ceph osd crush rule dump erasurepool
  { "rule_id": 1,
    "rule_name": "erasurepool",
    "ruleset": 1,
    "type": 3,
    "min_size": 3,
    "max_size": 20,
    "steps": [
          { "op": "take",
            "item": -1,
            "item_name": "default"},
          { "op": "chooseleaf_indep",
            "num": 0,
            "type": "host"},
          { "op": "emit"}]}

cephuser@adm > ceph osd erasure-code-profile set myprofile crush-failure-domain=osd
cephuser@adm > ceph osd pool create erasurepool erasure myprofile

cephuser@adm > ceph osd crush rule dump erasurepool
  { "rule_name": "erasurepool",
    "ruleset": 1,
    "type": 3,
    "min_size": 3,
    "max_size": 20,
    "steps": [
          { "op": "take",
            "item": -1,
            "item_name": "default"},
          { "op": "chooseleaf_indep",
            "num": 0,
            "type": "host"},
          { "op": "emit"}]}
  $ ceph osd getcrushmap > crush.map
  got crush map from osdmap epoch 13
  $ crushtool -i crush.map --test --show-bad-mappings \
     --rule 1 \
     --num-rep 9 \
     --min-x 1 --max-x $((1024 * 1024))
  bad mapping rule 8 x 43 num_rep 9 result [3,2,7,1,2147483647,8,5,6,0]
  bad mapping rule 8 x 79 num_rep 9 result [6,0,2,1,4,7,2147483647,5,8]
  bad mapping rule 8 x 173 num_rep 9 result [0,4,6,8,2,1,3,7,2147483647]

# crushtool --decompile crush.map > crush.txt

step set_choose_tries 100

  rule erasurepool {
          ruleset 1
          type erasure
          min_size 3
          max_size 20
          step set_chooseleaf_tries 5
          step set_choose_tries 100
          step take default
          step chooseleaf indep 0 type host
          step emit
  }

# crushtool --compile crush.txt -o better-crush.map

# crushtool -i better-crush.map --test --show-bad-mappings \
     --show-choose-tries \
     --rule 1 \
     --num-rep 9 \
     --min-x 1 --max-x $((1024 * 1024))
  ...
  11:        42
  12:        44
  13:        54
  14:        45
  15:        35
  16:        34
  17:        30
  18:        25
  19:        19
  20:        22
  21:        20
  22:        17
  23:        13
  24:        16
  25:        13
  26:        11
  27:        11
  28:        13
  29:        11
  30:        10
  31:         6
  32:         5
  33:        10
  34:         3
  35:         7
  36:         5
  37:         2
  38:         5
  39:         5
  40:         2
  41:         5
  42:         4
  43:         1
  44:         2
  45:         2
  46:         3
  47:         1
  48:         0
  ...
  102:         0
  103:         1
  104:         0
  ...