On Tue, Oct 20, 2015 at 8:47 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote:
> On Tue, 20 Oct 2015, Z Zhang wrote:
>> Hi Guys,
>>
>> I am trying latest ceph-9.1.0 with rocksdb 4.1 and ceph-9.0.3 with
>> rocksdb 3.11 as OSD backend. I use rbd to test performance and following
>> is my cluster info.
>>
>> [ceph@xxx ~]$ ceph -s
>> cluster b74f3944-d77f-4401-a531-fa5282995808
>> health HEALTH_OK
>> monmap e1: 1 mons at {xxx=xxx.xxx.xxx.xxx:6789/0}
>> election epoch 1, quorum 0 xxx
>> osdmap e338: 44 osds: 44 up, 44 in
>> flags sortbitwise
>> pgmap v1476: 2048 pgs, 1 pools, 158 MB data, 59 objects
>> 1940 MB used, 81930 GB / 81932 GB avail
>> 2048 active+clean
>>
>> All the disks are spinning ones with write cache turning on. Rocksdb's
>> WAL and sst files are on the same disk as every OSD.
>
> Are you using the KeyValueStore backend?
>
>> Using fio to generate following write load:
>> fio -direct=1 -rw=randwrite -ioengine=sync -size=10M -bs=4K -group_reporting -directory /mnt/rbd_test/ -name xxx.1 -numjobs=1
>>
>> Test result:
>> WAL enabled + sync: false + disk write cache: on will get ~700 IOPS.
>> WAL enabled + sync: true (default) + disk write cache: on|off will get only ~25 IOPS.
>>
>> I tuned some other rocksdb options, but with no lock.
>
> The wip-newstore-frags branch sets some defaults for rocksdb that I think
> look pretty reasonable (at least given how newstore is using rocksdb).
>
>> I tracked down the rocksdb code and found each writer's Sync operation
>> would take ~30ms to finish. And as shown above, it is strange that
>> performance has no much difference no matters disk write cache is on or
>> off.
>>
>> Do your guys encounter the similar issue? Or do I miss something to
>> cause rocksdb's poor write performance?
>
> Yes, I saw the same thing. This PR addresses the problem and is nearing
> merge upstream:
>
> https://github.com/facebook/rocksdb/pull/746
>
cool, it looks reasonable for performance degraded
> There is also an XFS performance bug that is contributing to the problem,
are you refer to
this(http://comments.gmane.org/gmane.comp.file-systems.ceph.devel/27645)?
I think newstore also meet this situation.
> but it looks like Dave Chinner just put together a fix for that.
>
> But... we likely won't be using KeyValueStore in its current form over
> rocksdb (or any other kv backend). It stripes object data over key/value
> pairs, which IMO is not the best approach.
>
> sage
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>
--
Best Regards,
Wheat
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