With the pg_autoscaler Manager module enabled the cluster shows a "many more objects per pg than average" warning

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Environment

SUSE Enterprise Storage 7
SUSE Enterprise Storage 6

Situation

The cluster is configured with RBD (Rados Block Device) images using an EC (Erasure Coded) data pool and thus a replicated metadata pool.

The cluster health status reports a warning similar to the following example:

MANY_OBJECTS_PER_PG <$X> pools have many more objects per pg than average
pool <$pool_name> objects per pg (209000) is more than 10.7800 times cluster average (19400)

Resolution

Consider setting the "target_size_ratio" for the metadata pools to 0 using the command:

ceph osd pool set <$ins_metadatapool_name> target_size_ratio 0

Cause

The metadata pools were configured with the same "target_size_ratio" as the EC data pools.

Additional Information

Since the metadata pools have the same ratio set as the data pools, the pg_autoscaler MGR (Manager) module expects all data for the cluster to be evenly distributed across also the metadata pools and assigns a high PG (Placement Group) count also for the metadata pools. Since these metadata pools however normally will contain very little actual data and objects when compared to the data pools, this results in the health warning.

The current pg_autoscaler values for the pools, including the current ratios, can be seen with:

ceph osd pool autoscale-status

To first test what will happen without any actual changes occurring, set one of the metadata pools to "warn" mode:

ceph osd pool set <$ins_metadatapool_name> pg_autoscale_mode warn

When now adjusting the "target_size_ratio" for this metadata pool, the new PG count that the pool will get if the mode is set to "on" will be shown with the status command under the "NEW PG_NUM" column heading. If the value looks sane (32 PGs for example), the setting can be implemented by setting the autoscale mode back to "on".

For additional information regarding the pg_autoscaler MGR module, see the SES Online documentation and also the upstream Ceph Documentation.

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