[dm-devel] Does dm-thin support GC (garbage collection) ?

[Mike Snitzer]

On Mon, May 07 2012 at  3:28am -0400,
Busby <chaimvy at gmail.com> wrote:

> Hi All,
>          The dm-thin supports 'discard' now?Can I think this function
> likes SSD's 'TRIM'? If it just discard the block mapped but not
> remap(reallocate), the Pool's space will soon be used up . I want to
> know whether it supports block reuse ( can use ext4 with '--discard'
> test this? ) now or in the future.

thinp discard support was added during upstream's most recent 3.4 merge
window.  See:
http://git.kernel.org/linus/104655fd4dcebd500
http://git.kernel.org/linus/67e2e2b281812b5ca

As you can see in the commit header for this last commit, you have
control over how the discard (as issued by ext4, etc) is handled by
thinp.  It should be noted that if the underlying storage does not
support discard then the thin-pool will default to no_discard_passdown
-- otherwise it does default to passing discards down to the underlying
storage.

But you'll need 2 late fixes for thinp issues that have been found
during the 3.4-rcX development (these will be sent upstream shortly):
http://www.redhat.com/archives/dm-devel/2012-April/msg00027.html
http://www.redhat.com/archives/dm-devel/2012-April/msg00071.html

(I'll bounce both of these patches to your email so you have them,
please apply them before building/testing the latest 3.4-rcX kernel).

Regards,
Mike