Ceph操作——監視OSD和PG
MONITORING PLACEMENT GROUP STATES
If you execute a command such as ceph health, ceph -s or ceph -w, you may notice that the cluster does not always echo backHEALTH 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
- You have just created a pool and placement groups haven’t peered yet.
- The placement groups are recovering.
- You have just added an OSD to or removed an OSD from the cluster.
- You have just modified your CRUSH map and your placement groups are migrating.
- There is inconsistent data in different replicas of a placement group.
- Ceph is scrubbing a placement group’s replicas.
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
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 are in a particular state such as active+clean (y).
vNNNNNN: x pgs: y active+clean; z bytes data, aa MB used, bb GB / cc GB avail
Note
It is common for Ceph to report multiple states for placement groups.
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:
- You are reaching your near full ratio or full ratio.
- Your data isn’t getting distributed across the cluster due to an error in your CRUSH configuration.
Placement Group IDs
Placement group IDs consist of the pool number (not pool name) followed by a period (.) and the placement group ID–a hexadecimal number. You can view pool numbers and their names from the output of ceph osd lspools. The default pool names data, metadataand rbd correspond to pool numbers 0, 1 and 2 respectively. A fully qualified placement group ID has the following form:
{pool-num}.{pg-id}
And it typically looks like this:
0.1f
To retrieve a list of placement groups, execute the following:
ceph pg dump
You can also format the output in JSON format and save it to a file:
ceph pg dump -o {filename} --format=json
To query a particular placement group, execute the following:
ceph pg {poolnum}.{pg-id} query
Ceph will output the query in JSON format.
{ "state": "active+clean", "up": [ 1, 0 ], "acting": [ 1, 0 ], "info": { "pgid": "1.e", "last_update": "4'1", "last_complete": "4'1", "log_tail": "0'0", "last_backfill": "MAX", "purged_snaps": "[]", "history": { "epoch_created": 1, "last_epoch_started": 537, "last_epoch_clean": 537, "last_epoch_split": 534, "same_up_since": 536, "same_interval_since": 536, "same_primary_since": 536, "last_scrub": "4'1", "last_scrub_stamp": "2013-01-25 10:12:23.828174" }, "stats": { "version": "4'1", "reported": "536'782", "state": "active+clean", "last_fresh": "2013-01-25 10:12:23.828271", "last_change": "2013-01-25 10:12:23.828271", "last_active": "2013-01-25 10:12:23.828271", "last_clean": "2013-01-25 10:12:23.828271", "last_unstale": "2013-01-25 10:12:23.828271", "mapping_epoch": 535, "log_start": "0'0", "ondisk_log_start": "0'0", "created": 1, "last_epoch_clean": 1, "parent": "0.0", "parent_split_bits": 0, "last_scrub": "4'1", "last_scrub_stamp": "2013-01-25 10:12:23.828174", "log_size": 128, "ondisk_log_size": 128, "stat_sum": { "num_bytes": 205, "num_objects": 1, "num_object_clones": 0, "num_object_copies": 0, "num_objects_missing_on_primary": 0, "num_objects_degraded": 0, "num_objects_unfound": 0, "num_read": 1, "num_read_kb": 0, "num_write": 3, "num_write_kb": 1 }, "stat_cat_sum": { }, "up": [ 1, 0 ], "acting": [ 1, 0 ] }, "empty": 0, "dne": 0, "incomplete": 0 }, "recovery_state": [ { "name": "Started\/Primary\/Active", "enter_time": "2013-01-23 09:35:37.594691", "might_have_unfound": [ ], "scrub": { "scrub_epoch_start": "536", "scrub_active": 0, "scrub_block_writes": 0, "finalizing_scrub": 0, "scrub_waiting_on": 0, "scrub_waiting_on_whom": [ ] } }, { "name": "Started", "enter_time": "2013-01-23 09:35:31.581160" } ] }
The following subsections describe common states in greater detail.
CREATING
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.
PEERING
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.
Authoratative History
Ceph will NOT acknowledge a write operation to a client, until all OSDs of the acting set persist the write operation. This practice ensures that at least one member of the acting set will have a record of every acknowledged write operation since the last successful peering operation.
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.
ACTIVE
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.
CLEAN
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.
DEGRADED
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 OSDs must peer again when the 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 thedown 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 300 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.
RECOVERING
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 isn’t always trivial, because a hardware failure might cause a cascading failure of multiple 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 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 (1 thread by default). The osd recovery thread timeout sets a thread timeout, because multiple OSDs may fail, restart and re-peer at staggered rates. The osd recovery max active setting limits the number of recovery requests an OSD will entertain simultaneously to prevent the OSD from failing to serve . The osd recovery max chunk setting limits the size of the recovered data chunks to prevent network congestion.
BACK FILLING
When a new OSD joins the cluster, CRUSH will reassign placement groups from OSDs in the cluster to the newly added OSD. Forcing the new OSD to accept the reassigned placement groups immediately can put excessive load on the new OSD. Back filling 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 may see one of several states: backfill_wait indicates that a backfill operation is pending, but isn’t underway yet; backfill indicates that a backfill operation is underway; and, backfill_too_full indicates that a backfill operation was requested, but couldn’t be completed due to insufficient storage capacity.
Ceph provides a number of settings to manage the load spike associated with reassigning placement groups to an OSD (especially a new OSD). By default, osd_max_backfills sets the maximum number of concurrent backfills to or from an OSD to 10. The osd backfillfull ratio enables an OSD to refuse a backfill request if the OSD is approaching its its full ratio (85%, by default). If an OSD refuses a backfill request, the osd backfill retry interval enables an OSD to retry the request (after 10 seconds, by default). OSDs can also set osd backfill scan min and osd backfill scan max to manage scan intervals (64 and 512, by default).
REMAPPED
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.
STALE
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 (e.g., a temporary network fault). By default, OSD daemons report their placement group, up thru, boot and failure statistics every half second (i.e., 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 your cluster, it is common to see the stale state until the peering process completes. After your 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.