Re: Preconditioning an RBD image

[Nick Fisk <nick@xxxxxxxxxx>]

Hi Peter,

 

Interesting graph. Out of interest, when you use bcache, do you then just leave the journal collocated on the combined bcache device and rely on the writeback to provide journal performance, or do you still create a separate partition on whatever SSD/NVME you use, effectively giving triple write overhead?

 

Nick

 

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Peter Maloney
Sent: 22 March 2017 10:06
To: Alex Gorbachev <ag@xxxxxxxxxxxxxxxxxxx>; ceph-users <ceph-users@xxxxxxxxxxxxxx>
Subject: Re: Preconditioning an RBD image

 

Does iostat (eg.  iostat -xmy 1 /dev/sd[a-z]) show high util% or await during these problems?

Ceph filestore requires lots of metadata writing (directory splitting for example), xattrs, leveldb, etc. which are small sync writes that HDDs are bad at (100-300 iops), and SSDs are good at (cheapo would be 6k iops, and not so crazy DC/NVMe would be 20-200k iops and more). So in theory, these things are mitigated by using an SSD, like bcache on your osd device. You could also try something like that, at least to test.

I have tested with bcache in writeback mode and found hugely obvious differences seen by iostat, for example here's my before and after (heavier load due to converting week 49-50 or so, and the highest spikes being the scrub infinite loop bug in 10.2.3):

http://www.brockmann-consult.de/ganglia/graph.php?cs=10%2F25%2F2016+10%3A27&ce=03%2F09%2F2017+17%3A26&z=xlarge&hreg[]=ceph.*&mreg[]=sd[c-z]_await&glegend=show&aggregate=1&x=100

But when you share a cache device, you get a single point of failure (and bcache, like all software, can be assumed to have bugs too). And I recommend vanilla kernel 4.9 or later which has many bcache fixes, or Ubuntu's 4.4 kernel which has the specific fixes I checked for.

On 03/21/17 23:22, Alex Gorbachev wrote:

I wanted to share the recent experience, in which a few RBD volumes, formatted as XFS and exported via Ubuntu NFS-kernel-server performed poorly, even generated an "out of space" warnings on a nearly empty filesystem.  I tried a variety of hacks and fixes to no effect, until things started magically working just after some dd write testing.

 

The only explanation I can come up with is that preconditioning, or thickening, the images with this benchmarking is what caused the improvement.

 

Ceph is Hammer 0.94.7 running on Ubuntu 14.04, kernel 4.10 on OSD nodes and 4.4 on NFS nodes.

 

Regards,

Alex

Storcium

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Alex Gorbachev

Storcium

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