One small point: It's a bit easier to observe distinct WAL and DB
behavior when they are on separate partitions. I often do this for
benchmarking and testing though I don't know that it would be enough of
a benefit to do it in production.
Mark
On 11/09/2017 04:16 AM, Richard Hesketh wrote:
You're correct, if you were going to put the WAL and DB on the same device you should just make one partition and allocate the DB to it, the WAL will automatically be stored with the DB. It only makes sense to specify them separately if they are going to go on different devices, and that itself only makes sense if the WAL device will be much faster than the DB device, otherwise you're just making your setup more complex for no gain.
On 09/11/17 08:05, jorpilo wrote:
I get confused there because on the documentation:
http://docs.ceph.com/docs/master/rados/configuration/bluestore-config-ref/
"If there is more, provisioning a DB device makes more sense. The BlueStore journal will always be placed on the fastest device available, so using a DB device will provide the same benefit that the WAL device would while also allowing additional metadata to be stored there"
So I guess it doesn't make any sense to implicit put WAL and DB on a SSD, only with DB, the biggest you can, would be enough, unless you have 2 different kinds of SSD (for example a tiny Nvme and a SSD)
Am I right? Or would I get any benefit from setting implicit WAL partition on the same SSD?
-------- Mensaje original --------
De: Nick Fisk <nick@xxxxxxxxxx>
Fecha: 8/11/17 10:16 p. m. (GMT+01:00)
Para: 'Mark Nelson' <mnelson@xxxxxxxxxx>, 'Wolfgang Lendl' <wolfgang.lendl@xxxxxxxxxxxxxxxx>
Cc: ceph-users@xxxxxxxxxxxxxx
Asunto: Re: bluestore - wal,db on faster devices?
-----Original Message-----
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of
Mark Nelson
Sent: 08 November 2017 19:46
To: Wolfgang Lendl <wolfgang.lendl@xxxxxxxxxxxxxxxx>
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re: 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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