Re: dm-cache module

[Mike Snitzer <snitzer@redhat.com>]

On Sat, Mar 20 2010 at  8:03pm -0400,
Olivier B. <dm.list@daevel.fr> wrote:

> Hi,
> 
> I'm looking at the dm-cache module from Ming Zhao (
> http://users.cis.fiu.edu/~zhaom/dmcache/index.html ), and would like
> to know if there is a problem to include it upstream.
> 
> It looks stable for a while, and some people seem to use it in production.

Such a DM target would be quite useful to have upstream given how
prevalent SSDs have become.

Other work has been a priority but a caching DM target is definitely on
the TODO.  If others can help take steps to make a robust caching DM
target a reality then we should be able to get an implementation
upstream sooner rather than later.

Just curious: where have you gotten this insight about dm-cache being
stable and used in production?  That should help dm-cache's cause if it
is submitted for review.

Seems Ming posted dm-cache to dm-devel (as a large monolithic patch) on
8/24/07.

dm-cache would need to be rebased to the latest upstream kernel.org
kernel (e.g. Linux >= 2.6.34-rc1).

Ideally dm-cache would be resubmitted to dm-devel incrementally such
that basic infrastructure is introduced in early patches and more
advanced capabilities (e.g. writeback) follow in later patches.  Each
patch should have a corresponding patch header the documents what the
patch provides.

Writeback support is arguably the most useful aspect of a caching DM
target so emphasis must be placed on its review/implementation.  Using
SSD as a persistent writeback cache should afford us much more fault
tolerance in the face of system crashes during writeback to the slower
media.

I know Heinz has taken steps to implement a caching DM target.  So
reconciling dm-cache's design and implementation with what Heinz has
would be a worthwhile component of the dm-cache review (assuming Heinz's
implementation is also posted for review).

Mike