Re: Local SSD cache for ceph on each compute node.

[Ric Wheeler <rwheeler-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 03/29/2016 04:53 PM, Nick Fisk wrote:
>> -----Original Message-----
>> From: ceph-users [mailto:ceph-users-bounces-idqoXFIVOFJgJs9I8MT0rw@public.gmane.org] On Behalf Of
>> Ric Wheeler
>> Sent: 29 March 2016 14:40
>> To: Nick Fisk <nick-ksME7r3P/wO1Qrn1Bg8BZw@public.gmane.org>; 'Sage Weil' <sage-BnTBU8nroG7k1uMJSBkQmQ@public.gmane.org>
>> Cc: ceph-users-idqoXFIVOFJgJs9I8MT0rw@public.gmane.org; device-mapper development <dm-
>> devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>> Subject: Re: [ceph-users] Local SSD cache for ceph on each compute node.
>>
>> On 03/29/2016 04:35 PM, Nick Fisk wrote:
>>> One thing I picked up on when looking at dm-cache for doing caching
>>> with RBD's is that it wasn't really designed to be used as a writeback
>>> cache for new writes, as in how you would expect a traditional
>>> writeback cache to work. It seems all the policies are designed around
>>> the idea that writes go to cache only if the block is already in the
>>> cache (through reads) or its hot enough to promote. Although there did
>>> seem to be some tunables to alter this behaviour, posts on the mailing
>>> list seemed to suggest this wasn't how it was designed to be used. I'm
>>> not sure if this has been addressed since I last looked at it though.
>>>
>>> Depending on if you are trying to accelerate all writes, or just your
> "hot"
>>> blocks, this may or may not matter. Even <1GB local caches can make a
>>> huge difference to sync writes.
>> Hi Nick,
>>
>> Some of the caching policies have changed recently as the team has looked
>> at different workloads.
>>
>> Happy to introduce you to them if you want to discuss offline or post
>> comments over on their list: device-mapper development <dm-
>> devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>>
>> thanks!
>>
>> Ric
> Hi Ric,
>
> Thanks for the heads up, just from a quick flick through I can see there are
> now separate read and write promotion thresholds, so I can see just from
> that it would be a lot more suitable for what I intended. I might try and
> find some time to give it another test.
>
> Nick

Let us know how it works out for you, I know that they are very interested in 
making sure things are useful :)

ric