Given enough load, that fast Jornal will get filled and you will only be as fast as the back disk can flush (and at the same time service reads). That the the situation we are in right now. We are still seeing better performance than a raw spindle, but only 150 IOPs, not 15000 IOPS that the SSD can do. You are still ultimately bound by the back end disk.
Robert LeBlanc
Sent from a mobile device please excuse any typos.
On Oct 20, 2015 2:34 AM, "Luis Periquito" <periquito@xxxxxxxxx> wrote:
On Tue, Oct 20, 2015 at 3:26 AM, Haomai Wang <haomaiwang@xxxxxxxxx> wrote:
> The fact is that journal could help a lot for rbd use cases,
> especially for small ios. I don' t think it will be bottleneck. If we
> just want to reduce double write, it doesn't solve any performance
> problem.
>
One trick I've been using in my ceph clusters is hiding a slow write
backend behind a fast journal device. The write performance will be of
the fast (and small) journal device. This only helps on write, but it
can make a huge difference.
I've even made some tests showing (within 10%, RBD and S3) that the
backend device doesn't matter and the write performance is exactly the
same as that of the journal device fronting all the writes.
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