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

[Busby]

Hi Mike,
            Thank you for your reply. Does the thin pool can reuse the
blocks that the thin-lun freed( like ssd 's discard/trim , the lun
will discard the blocks to the pool ). As the filesystem  creates a
file (3 G bytes big, etc.) will cause the block allocation of the thin
pool (thin lun), if delete the file, will the space(the blocks,3 G
bytes ,etc. ) dynamically be reused now (as we think thin-lun knows
the blocks delete by the FS )? or have to delete the thin-lun then the
space will be reused (just thin provision only once, not support block
reclamation from thin-lun to pool while lun is using )? I want to know
something about the 'block (page) reclamation' before my test :)
           Thanks.

Regards,
Busby


2012/5/7, Mike Snitzer <snitzer at redhat.com>:
> 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
>