On 09/02/2010 03:05 AM, Hannes Reinecke wrote:
Hi all,
now that we've got discard support in the block layer, are there plans
to update dm-snap to actually implement discard?
Looks like a valid addendum here; we could be freeing up unused blocks
thus freeing up space.
Especially helpful when using dm-snap to create a sparse device;
cf
http://www.mjmwired.net/kernel/Documentation/device-mapper/zero.txt
Thoughts?
Here was my similar suggestion and the discussion with Mike that followed-
http://www.spinics.net/lists/dm-devel/msg11632.html
I still feel like there is a disconnect between my thinking and Mike's,
as I think I have the same fundamental interest as you do Hannes.
I.e. a perfect example is a simple dm-snapshot used for a livecd root
filesystem. In that case, after boot, the user creates a new file in
the rootfs, and that causes exception chunks (terminology?) to be
created in the cow device of the dm-snapshot. Then, at a later point,
the user deletes that file. It then seems 100% reasonable for the
discard request to result in freeing the used resources in the cow
device. I see no semantic reason why that data need be kept around any
more than it need be kept around on an SSD after a file is deleted.
Now, where I think the disconnect comes in, is that perhaps the majority
use of snapshots in todays word is perhaps a snapshot-origin
(terminology?) where the exception store (not quite a 'cow device'?) is
not used to store the modified blocks, but instead, used to store the
original blocks. In _that_ case, things are different, perhaps more as
Mike described them.
Now, I do understand that Mike is the person actually neck deep in all
this code, so I could be completely misunderstanding the picture. But
I'm glad someone else also sees what I see here as far as potential
benefit and use case.
-dmc
Cheers,
Hannes
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