Chapter 3. Monitoring a Ceph storage cluster

After you start the Ceph storage cluster, and before you start reading or writing data, check the storage cluster’s health first.

You can watch events that are happening with the Ceph storage cluster using the command-line interface.

The used value reflects the actual amount of raw storage used. The xxx GB / xxx GB value means the amount available, the lesser of the two numbers, of the overall storage capacity of the cluster. The notional number reflects the size of the stored data before it is replicated, cloned or snapshotted. Therefore, the amount of data actually stored typically exceeds the notional amount stored, because Ceph creates replicas of the data and may also use storage capacity for cloning and snapshotting.

To check a cluster’s data usage and data distribution among pools, use the df option. It is similar to the Linux df command.

The SIZE/AVAIL/RAW USED in the ceph df and ceph status command output are different if some OSDs are marked OUT of the cluster compared to when all OSDs are IN. The SIZE/AVAIL/RAW USED is calculated from sum of SIZE (osd disk size), RAW USE (total used space on disk), and AVAIL of all OSDs which are in IN state. You can see the total of SIZE/AVAIL/RAW USED for all OSDs in ceph osd df tree command output.

[ceph: root@host01 /]#ceph df
--- RAW STORAGE ---
CLASS   SIZE    AVAIL     USED  RAW USED  %RAW USED
hdd    5 TiB  2.9 TiB  2.1 TiB   2.1 TiB      42.98
TOTAL  5 TiB  2.9 TiB  2.1 TiB   2.1 TiB      42.98

--- POOLS ---
POOL                        ID  PGS   STORED  OBJECTS     USED  %USED  MAX AVAIL
.mgr                         1    1  5.3 MiB        3   16 MiB      0    629 GiB
.rgw.root                    2   32  1.3 KiB        4   48 KiB      0    629 GiB
default.rgw.log              3   32  3.6 KiB      209  408 KiB      0    629 GiB
default.rgw.control          4   32      0 B        8      0 B      0    629 GiB
default.rgw.meta             5   32  1.7 KiB       10   96 KiB      0    629 GiB
default.rgw.buckets.index    7   32  5.5 MiB       22   17 MiB      0    629 GiB
default.rgw.buckets.data     8   32  807 KiB        3  2.4 MiB      0    629 GiB
default.rgw.buckets.non-ec   9   32  1.0 MiB        1  3.1 MiB      0    629 GiB
source-ecpool-86            11   32  1.2 TiB  391.13k  2.1 TiB  53.49    1.1 TiB

The ceph df detail command gives more details about other pool statistics such as quota objects, quota bytes, used compression, and under compression.

The RAW STORAGE section of the output provides an overview of the amount of storage the storage cluster manages for data.

The POOLS section of the output provides a list of pools and the notional usage of each pool. The output from this section DOES NOT reflect replicas, clones or snapshots. For example, if you store an object with 1 MB of data, the notional usage will be 1 MB, but the actual usage may be 3 MB or more depending on the number of replicas for example, size = 3, clones and snapshots.

Use the ceph osd df command to view OSD utilization stats.

[ceph: root@host01 /]# ceph osd df
ID CLASS WEIGHT  REWEIGHT SIZE    USE     DATA    OMAP    META    AVAIL   %USE VAR  PGS
 3   hdd 0.90959  1.00000  931GiB 70.1GiB 69.1GiB      0B    1GiB  861GiB 7.53 2.93  66
 4   hdd 0.90959  1.00000  931GiB 1.30GiB  308MiB      0B    1GiB  930GiB 0.14 0.05  59
 0   hdd 0.90959  1.00000  931GiB 18.1GiB 17.1GiB      0B    1GiB  913GiB 1.94 0.76  57
MIN/MAX VAR: 0.02/2.98  STDDEV: 2.91

You can check the status of the Red Hat Ceph Storage cluster from the command-line interface. The status sub command or the -s argument will display the current status of the storage cluster.

If the storage cluster has multiple Ceph Monitors, which is a requirement for a production Red Hat Ceph Storage cluster, then you can check the Ceph Monitor quorum status after starting the storage cluster, and before doing any reading or writing of data.

A quorum must be present when multiple Ceph Monitors are running.

Check the Ceph Monitor status periodically to ensure that they are running. If there is a problem with the Ceph Monitor, that prevents an agreement on the state of the storage cluster, the fault can prevent Ceph clients from reading and writing data.

Use the administration socket to interact with a given daemon directly by using a UNIX socket file. For example, the socket enables you to:

In addition, using the socket is helpful when troubleshooting problems related to Ceph Monitors or OSDs.

Regardless, if the daemon is not running, a following error is returned when attempting to use the administration socket:

Error 111: Connection Refused

A Ceph OSD’s status is either in the storage cluster, or out of the storage cluster. It is either up and running, or it is down and not running. If a Ceph OSD is up, it can be either in the storage cluster, where data can be read and written, or it is out of the storage cluster. If it was in the storage cluster and recently moved out of the storage cluster, Ceph starts migrating placement groups to other Ceph OSDs. If a Ceph OSD is out of the storage cluster, CRUSH will not assign placement groups to the Ceph OSD. If a Ceph OSD is down, it should also be out.

If you execute a command such as ceph health, ceph -s or ceph -w, you might notice that the storage cluster does not always echo back HEALTH OK. Do not panic. With respect to Ceph OSDs, you can expect that the storage cluster will NOT echo HEALTH OK in a few expected circumstances:

An important aspect of monitoring Ceph OSDs is to ensure that when the storage cluster is up and running that all Ceph OSDs that are in the storage cluster are up and running, too.

To see if all OSDs are running, execute:

[ceph: root@host01 /]# ceph osd stat

or

[ceph: root@host01 /]# ceph osd dump

The result should tell you the map epoch, eNNNN, the total number of OSDs, x, how many, y, are up, and how many, z, are in:

eNNNN: x osds: y up, z in

If the number of Ceph OSDs that are in the storage cluster are more than the number of Ceph OSDs that are up. Execute the following command to identify the ceph-osd daemons that are not running:

[ceph: root@host01 /]# ceph osd tree

# id    weight  type name   up/down reweight
-1  3   pool default
-3  3       rack mainrack
-2  3           host osd-host
0   1               osd.0   up  1
1   1               osd.1   up  1
2   1               osd.2   up  1

If a Ceph OSD is down, connect to the node and start it. You can use Red Hat Storage Console to restart the Ceph OSD daemon, or you can use the command line.

systemctl start CEPH_OSD_SERVICE_ID

[root@host01 ~]# systemctl start ceph-499829b4-832f-11eb-8d6d-001a4a000635@osd.6.service

When CRUSH assigns placement groups to Ceph OSDs, it looks at the number of replicas for the pool and assigns the placement group to Ceph OSDs such that each replica of the placement group gets assigned to a different Ceph OSD. For example, if the pool requires three replicas of a placement group, CRUSH may assign them to osd.1, osd.2 and osd.3 respectively. CRUSH actually seeks a pseudo-random placement that will take into account failure domains you set in the CRUSH map, so you will rarely see placement groups assigned to nearest neighbor Ceph OSDs in a large cluster. We refer to the set of Ceph OSDs that should contain the replicas of a particular placement group as the Acting Set. In some cases, an OSD in the Acting Set is down or otherwise not able to service requests for objects in the placement group. When these situations arise, do not panic. Common examples include:

Ceph processes a client request using the Up Set, which is the set of Ceph OSDs that actually handle the requests. In most cases, the up set and the Acting Set are virtually identical. When they are not, it can indicate that Ceph is migrating data, an Ceph OSD is recovering, or that there is a problem, that is, Ceph usually echoes a HEALTH WARN state with a "stuck stale" message in such scenarios.

Before you can write data to a placement group, it must be in an active state, and it should be in a clean state. For Ceph to determine the current state of a placement group, the primary OSD of the placement group that is, the first OSD in the acting set, peers with the secondary and tertiary OSDs to establish agreement on the current state of the placement group. Assuming a pool with three replicas of the PG.

Figure 3.1. Peering

If you execute a command such as ceph health, ceph -s or ceph -w, you may notice that the cluster does not always echo back HEALTH OK. After you check to see if the OSDs are running, you should also check placement group states. You should expect that the cluster will NOT echo HEALTH OK in a number of placement group peering-related circumstances:

If one of the foregoing circumstances causes Ceph to echo HEALTH WARN, don’t panic. In many cases, the cluster will recover on its own. In some cases, you may need to take action. An important aspect of monitoring placement groups is to ensure that when the cluster is up and running that all placement groups are active, and preferably in the clean state.

To see the status of all placement groups, execute:

[ceph: root@host01 /]# ceph pg stat

The result should tell you the placement group map version, vNNNNNN, the total number of placement groups, x, and how many placement groups, y, are in a particular state such as active+clean:

vNNNNNN: x pgs: y active+clean; z bytes data, aa MB used, bb GB / cc GB avail

Example Output:

244 active+clean+snaptrim_wait
 32 active+clean+snaptrim

In addition to the placement group states, Ceph will also echo back the amount of data used, aa, the amount of storage capacity remaining, bb, and the total storage capacity for the placement group. These numbers can be important in a few cases:

POOL_NUM.PG_ID

Example output:

0.1f

When you create a pool, it will create the number of placement groups you specified. Ceph will echo creating when it is creating one or more placement groups. Once they are created, the OSDs that are part of a placement group’s Acting Set will peer. Once peering is complete, the placement group status should be active+clean, which means a Ceph client can begin writing to the placement group.

When Ceph is Peering a placement group, Ceph is bringing the OSDs that store the replicas of the placement group into agreement about the state of the objects and metadata in the placement group. When Ceph completes peering, this means that the OSDs that store the placement group agree about the current state of the placement group. However, completion of the peering process does NOT mean that each replica has the latest contents.

With an accurate record of each acknowledged write operation, Ceph can construct and disseminate a new authoritative history of the placement group. A complete, and fully ordered set of operations that, if performed, would bring an OSD’s copy of a placement group up to date.

Once Ceph completes the peering process, a placement group may become active. The active state means that the data in the placement group is generally available in the primary placement group and the replicas for read and write operations.

When a placement group is in the clean state, the primary OSD and the replica OSDs have successfully peered and there are no stray replicas for the placement group. Ceph replicated all objects in the placement group the correct number of times.

When a client writes an object to the primary OSD, the primary OSD is responsible for writing the replicas to the replica OSDs. After the primary OSD writes the object to storage, the placement group will remain in a degraded state until the primary OSD has received an acknowledgement from the replica OSDs that Ceph created the replica objects successfully.

The reason a placement group can be active+degraded is that an OSD may be active even though it doesn’t hold all of the objects yet. If an OSD goes down, Ceph marks each placement group assigned to the OSD as degraded. The Ceph OSDs must peer again when the Ceph OSD comes back online. However, a client can still write a new object to a degraded placement group if it is active.

If an OSD is down and the degraded condition persists, Ceph may mark the down OSD as out of the cluster and remap the data from the down OSD to another OSD. The time between being marked down and being marked out is controlled by mon_osd_down_out_interval, which is set to 600 seconds by default.

A placement group can also be degraded, because Ceph cannot find one or more objects that Ceph thinks should be in the placement group. While you cannot read or write to unfound objects, you can still access all of the other objects in the degraded placement group.

For example, if there are nine OSDs in a three way replica pool. If OSD number 9 goes down, the PGs assigned to OSD 9 goes into a degraded state. If OSD 9 does not recover, it goes out of the storage cluster and the storage cluster rebalances. In that scenario, the PGs are degraded and then recover to an active state.

Ceph was designed for fault-tolerance at a scale where hardware and software problems are ongoing. When an OSD goes down, its contents may fall behind the current state of other replicas in the placement groups. When the OSD is back up, the contents of the placement groups must be updated to reflect the current state. During that time period, the OSD may reflect a recovering state.

Recovery is not always trivial, because a hardware failure might cause a cascading failure of multiple Ceph OSDs. For example, a network switch for a rack or cabinet may fail, which can cause the OSDs of a number of host machines to fall behind the current state of the storage cluster. Each one of the OSDs must recover once the fault is resolved.

Ceph provides a number of settings to balance the resource contention between new service requests and the need to recover data objects and restore the placement groups to the current state. The osd recovery delay start setting allows an OSD to restart, re-peer and even process some replay requests before starting the recovery process. The osd recovery threads setting limits the number of threads for the recovery process, by default one thread. The osd recovery thread timeout sets a thread timeout, because multiple Ceph OSDs can fail, restart and re-peer at staggered rates. The osd recovery max active setting limits the number of recovery requests a Ceph OSD works on simultaneously to prevent the Ceph OSD from failing to serve . The osd recovery max chunk setting limits the size of the recovered data chunks to prevent network congestion.

When a new Ceph OSD joins the storage cluster, CRUSH will reassign placement groups from OSDs in the cluster to the newly added Ceph OSD. Forcing the new OSD to accept the reassigned placement groups immediately can put excessive load on the new Ceph OSD. Backfilling the OSD with the placement groups allows this process to begin in the background. Once backfilling is complete, the new OSD will begin serving requests when it is ready.

During the backfill operations, you might see one of several states:

When a placement group cannot be backfilled, it can be considered incomplete.

Ceph provides a number of settings to manage the load spike associated with reassigning placement groups to a Ceph OSD, especially a new Ceph OSD. By default, osd_max_backfills sets the maximum number of concurrent backfills to or from a Ceph OSD to 10. The osd backfill full ratio enables a Ceph OSD to refuse a backfill request if the OSD is approaching its full ratio, by default 85%. If an OSD refuses a backfill request, the osd backfill retry interval enables an OSD to retry the request, by default after 10 seconds. OSDs can also set osd backfill scan min and osd backfill scan max to manage scan intervals, by default 64 and 512.

For some workloads, it is beneficial to avoid regular recovery entirely and use backfill instead. Since backfilling occurs in the background, this allows I/O to proceed on the objects in the OSD. You can force a backfill rather than a recovery by setting the osd_min_pg_log_entries option to 1, and setting the osd_max_pg_log_entries option to 2. Contact your Red Hat Support account team for details on when this situation is appropriate for your workload.

When the Acting Set that services a placement group changes, the data migrates from the old acting set to the new acting set. It may take some time for a new primary OSD to service requests. So it may ask the old primary to continue to service requests until the placement group migration is complete. Once data migration completes, the mapping uses the primary OSD of the new acting set.

While Ceph uses heartbeats to ensure that hosts and daemons are running, the ceph-osd daemons may also get into a stuck state where they aren’t reporting statistics in a timely manner. For example, a temporary network fault. By default, OSD daemons report their placement group, up thru, boot and failure statistics every half second, that is, 0.5, which is more frequent than the heartbeat thresholds. If the Primary OSD of a placement group’s acting set fails to report to the monitor or if other OSDs have reported the primary OSD down, the monitors will mark the placement group stale.

When you start the storage cluster, it is common to see the stale state until the peering process completes. After the storage cluster has been running for awhile, seeing placement groups in the stale state indicates that the primary OSD for those placement groups is down or not reporting placement group statistics to the monitor.

There are some temporary backfilling scenarios where a PG gets mapped temporarily to an OSD. When that temporary situation should no longer be the case, the PGs might still reside in the temporary location and not in the proper location. In which case, they are said to be misplaced. That’s because the correct number of extra copies actually exist, but one or more copies is in the wrong place.

For example, there are 3 OSDs: 0,1,2 and all PGs map to some permutation of those three. If you add another OSD (OSD 3), some PGs will now map to OSD 3 instead of one of the others. However, until OSD 3 is backfilled, the PG will have a temporary mapping allowing it to continue to serve I/O from the old mapping. During that time, the PG is misplaced, because it has a temporary mapping, but not degraded, since there are 3 copies.

pg 1.5: up=acting: [0,1,2]
ADD_OSD_3
pg 1.5: up: [0,3,1] acting: [0,1,2]

[0,1,2] is a temporary mapping, so the up set is not equal to the acting set and the PG is misplaced but not degraded since [0,1,2] is still three copies.

pg 1.5: up=acting: [0,3,1]

OSD 3 is now backfilled and the temporary mapping is removed, not degraded and not misplaced.

A PG goes into a incomplete state when there is incomplete content and peering fails, that is, when there are no complete OSDs which are current enough to perform recovery.

Lets say OSD 1, 2, and 3 are the acting OSD set and it switches to OSD 1, 4, and 3, then osd.1 will request a temporary acting set of OSD 1, 2, and 3 while backfilling 4. During this time, if OSD 1, 2, and 3 all go down, osd.4 will be the only one left which might not have fully backfilled all the data. At this time, the PG will go incomplete indicating that there are no complete OSDs which are current enough to perform recovery.

Alternately, if osd.4 is not involved and the acting set is simply OSD 1, 2, and 3 when OSD 1, 2, and 3 go down, the PG would likely go stale indicating that the mons have not heard anything on that PG since the acting set changed. The reason being there are no OSDs left to notify the new OSDs.

A placement group is not necessarily problematic just because it is not in a active+clean state. Generally, Ceph’s ability to self repair might not be working when placement groups get stuck. The stuck states include:

The Ceph client retrieves the latest cluster map and the CRUSH algorithm calculates how to map the object to a placement group, and then calculates how to assign the placement group to an OSD dynamically.