[ceph-users] bluestore - wal,db on faster devices?

Nick Fisk nick at fisk.me.uk
Wed Nov 8 13:16:38 PST 2017


> -----Original Message-----
> From: ceph-users [mailto:ceph-users-bounces at lists.ceph.com] On Behalf Of
> Mark Nelson
> Sent: 08 November 2017 19:46
> To: Wolfgang Lendl <wolfgang.lendl at meduniwien.ac.at>
> Cc: ceph-users at lists.ceph.com
> Subject: Re: [ceph-users] bluestore - wal,db on faster devices?
> 
> Hi Wolfgang,
> 
> You've got the right idea.  RBD is probably going to benefit less since
you
> have a small number of large objects and little extra OMAP data.
> Having the allocation and object metadata on flash certainly shouldn't
hurt,
> and you should still have less overhead for small (<64k) writes.
> With RGW however you also have to worry about bucket index updates
> during writes and that's a big potential bottleneck that you don't need to
> worry about with RBD.

If you are running anything which is sensitive to sync write latency, like
databases. You will see a big performance improvement in using WAL on SSD.
As Mark says, small writes will get ack'd once written to SSD. ~10-200us vs
10000-20000us difference. It will also batch lots of these small writes
together and write them to disk in bigger chunks much more effectively. If
you want to run active workloads on RBD and want them to match enterprise
storage array with BBWC type performance, I would say DB and WAL on SSD is a
requirement.

> 
> Mark
> 
> On 11/08/2017 01:01 PM, Wolfgang Lendl wrote:
> > Hi Mark,
> >
> > thanks for your reply!
> > I'm a big fan of keeping things simple - this means that there has to
> > be a very good reason to put the WAL and DB on a separate device
> > otherwise I'll keep it collocated (and simpler).
> >
> > as far as I understood - putting the WAL,DB on a faster (than hdd)
> > device makes more sense in cephfs and rgw environments (more
> metadata)
> > - and less sense in rbd environments - correct?
> >
> > br
> > wolfgang
> >
> > On 11/08/2017 02:21 PM, Mark Nelson wrote:
> >> Hi Wolfgang,
> >>
> >> In bluestore the WAL serves sort of a similar purpose to filestore's
> >> journal, but bluestore isn't dependent on it for guaranteeing
> >> durability of large writes.  With bluestore you can often get higher
> >> large-write throughput than with filestore when using HDD-only or
> >> flash-only OSDs.
> >>
> >> Bluestore also stores allocation, object, and cluster metadata in the
> >> DB.  That, in combination with the way bluestore stores objects,
> >> dramatically improves behavior during certain workloads.  A big one
> >> is creating millions of small objects as quickly as possible.  In
> >> filestore, PG splitting has a huge impact on performance and tail
> >> latency.  Bluestore is much better just on HDD, and putting the DB
> >> and WAL on flash makes it better still since metadata no longer is a
> >> bottleneck.
> >>
> >> Bluestore does have a couple of shortcomings vs filestore currently.
> >> The allocator is not as good as XFS's and can fragment more over time.
> >> There is no server-side readahead so small sequential read
> >> performance is very dependent on client-side readahead.  There's
> >> still a number of optimizations to various things ranging from
> >> threading and locking in the shardedopwq to pglog and dup_ops that
> >> potentially could improve performance.
> >>
> >> I have a blog post that we've been working on that explores some of
> >> these things but I'm still waiting on review before I publish it.
> >>
> >> Mark
> >>
> >> On 11/08/2017 05:53 AM, Wolfgang Lendl wrote:
> >>> Hello,
> >>>
> >>> it's clear to me getting a performance gain from putting the journal
> >>> on a fast device (ssd,nvme) when using filestore backend.
> >>> it's not when it comes to bluestore - are there any resources,
> >>> performance test, etc. out there how a fast wal,db device impacts
> >>> performance?
> >>>
> >>>
> >>> br
> >>> wolfgang
> >>>
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> >
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