Good data point on not trimming when non active+clean PGs are present. So am I reading this correct? It grew to 32GB? Did it end up growing beyond that, what was the max? Also is only ~18PGs per OSD a reasonable amount of PGs per OSD? I would think about quadruple that would be ideal. Is this an artifact of a steadily growing cluster or a design choice?
On Sat, Feb 3, 2018 at 10:50 AM, Wido den Hollander <wido@xxxxxxxx> wrote:
Hi,
I just wanted to inform people about the fact that Monitor databases can grow quite big when you have a large cluster which is performing a very long rebalance.
I'm posting this on ceph-users and ceph-large as it applies to both, but you'll see this sooner on a cluster with a lof of OSDs.
Some information:
- Version: Luminous 12.2.2
- Number of OSDs: 2175
- Data used: ~2PB
We are in the middle of migrating from FileStore to BlueStore and this is causing a lot of PGs to backfill at the moment:
33488 active+clean
4802 active+undersized+degraded+remapped+backfill_wait
1670 active+remapped+backfill_wait
263 active+undersized+degraded+remapped+backfilling
250 active+recovery_wait+degraded
54 active+recovery_wait+degraded+remapped
27 active+remapped+backfilling
13 active+recovery_wait+undersized+degraded+remapped
2 active+recovering+degraded
This has been running for a few days now and it has caused this warning:
MON_DISK_BIG mons srv-zmb03-05,srv-zmb04-05,srv-zmb05-05,srv-zmb06-05,srv-zmb0 7-05 are using a lot of disk space
mon.srv-zmb03-05 is 31666 MB >= mon_data_size_warn (15360 MB)
mon.srv-zmb04-05 is 31670 MB >= mon_data_size_warn (15360 MB)
mon.srv-zmb05-05 is 31670 MB >= mon_data_size_warn (15360 MB)
mon.srv-zmb06-05 is 31897 MB >= mon_data_size_warn (15360 MB)
mon.srv-zmb07-05 is 31891 MB >= mon_data_size_warn (15360 MB)
This is to be expected as MONs do not trim their store if one or more PGs is not active+clean.
In this case we expected this and the MONs are each running on a 1TB Intel DC-series SSD to make sure we do not run out of space before the backfill finishes.
The cluster is spread out over racks and in CRUSH we replicate over racks. Rack by rack we are wiping/destroying the OSDs and bringing them back as BlueStore OSDs and letting the backfill handle everything.
In between we wait for the cluster to become HEALTH_OK (all PGs active+clean) so that the Monitors can trim their database before we start with the next rack.
I just want to warn and inform people about this. Under normal circumstances a MON database isn't that big, but if you have a very long period of backfills/recoveries and also have a large number of OSDs you'll see the DB grow quite big.
This has improved significantly going to Jewel and Luminous, but it is still something to watch out for.
Make sure your MONs have enough free space to handle this!
Wido
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Respectfully,
Wes Dillingham
Research Computing | Senior CyberInfrastructure Storage Engineer
Harvard University | 38 Oxford Street, Cambridge, Ma 02138 | Room 204
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