[dm-devel] [ceph-users] Local SSD cache for ceph on each compute node.

[Nick Fisk]

> -----Original Message-----
> From: ceph-users [mailto:ceph-users-bounces at lists.ceph.com] On Behalf Of
> Ric Wheeler
> Sent: 29 March 2016 14:40
> To: Nick Fisk <nick at fisk.me.uk>; 'Sage Weil' <sage at newdream.net>
> Cc: ceph-users at lists.ceph.com; device-mapper development <dm-
> devel at redhat.com>
> 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 at redhat.com>
> 
> 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

> 
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