Re: [ceph-users] Possible improvements for a slow write speed (excluding independent SSD journals)

[Eneko Lacunza]

Hi,

I'm just writing to you to stress out what others have already said, 
because it is very important that you take it very seriously.

On 20/04/15 19:17, J-P Methot wrote:
On 4/20/2015 11:01 AM, Christian Balzer wrote:

This is similar to another thread running right now, but since our
current setup is completely different from the one described in the
other thread, I thought it may be better to start a new one.

We are running Ceph Firefly 0.80.8 (soon to be upgraded to 0.80.9). We
have 6 OSD hosts with 16 OSD each (so a total of 96 OSDs). Each OSD 
is a
Samsung SSD 840 EVO on which I can reach write speeds of roughly 400
MB/sec, plugged in jbod on a controller that can theoretically transfer
at 6gb/sec. All of that is linked to openstack compute nodes on two
bonded 10gbps links (so a max transfer rate of 20 gbps).

I sure as hell hope you're not planning to write all that much to this
cluster.
But then again you're worried about write speed, so I guess you do.
Those _consumer_ SSDs will be dropping like flies, there are a number of
threads about them here.

They also might be of the kind that don't play well with O_DSYNC, I 
can't
recall for sure right now, check the archives.
  Consumer SSDs universally tend to slow down quite a bit when not 
TRIM'ed
and/or subjected to prolonged writes, like those generated by a 
benchmark.
I see, yes it looks like these SSDs are not the best for the job. We 
will not change them for now, but if they start failing, we will 
replace them with better ones.
I tried to put a Samsung 840 Pro 256GB in a ceph setup. It is supposed 
to be quite better than the EVO right? It was total crap. No "not the 
best for the job". TOTAL CRAP. :)

It can't give any useful write performance for a Ceph OSD. Spec sheet 
numbers don't matter for this, they don't work for ceph OSD, period. And 
yes, the drive is fine and works like a charm in workstation workloads.

I suggest you at least get some intel S3700/S3610 and use them for the 
journal of those samsung drives, I think that could help performance a lot.

Cheers
Eneko

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