Why rebalancing/recovery of small sized objects is slower?

[Prasad Krishnan <prasad.krishnan@xxxxxxxxxxxx>]

Dear Ceph Developers,

On our Ceph S3 Storage clusters we have found that the rate of recovery/backfill on the cluster with S3/RADOS object sizes between 10KB - 100KB takes much longer to recover compared to our other cluster whose S3 object sizes are usually a few tens of MB (with "rgw_obj_stripe_size" of 4MB, so RADOS objects would be 4MB or lesser).

We're exploring ways to improve the recovery speed by keeping the following factors constant (since tweaking them would lead to other issues):

1. Type of media - this would be HDD as moving all data to SSD would be prohibitively expensive
2. "osd_max_backfills" - We do not want to increase this as it leads to blocked requests and interferes with client I/O. We suspect that the disk RPS gets saturated if increased.
   
3. PG count - Increasing this would lead to more memory usage beyond what's available with the OSDs.

I came across the same question posted on this forum a few years back but seems to have no answers. Refer this and this.

Can the community help me understand what is theoretically causing this slowness? Is the overhead in recovering each RADOS object (grabbing a lock on the PG, txn overhead) so high that any increase in its number would decrease the recovery throughput?

Should I just tweak our workloads to not generate small sized S3/RADOS objects so that the MTTR would become better for our cluster?

Thanks,

Prasad Krishnan

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