On Thu, Sep 02, 2010 at 01:21:05PM -0500, Douglas McClendon wrote: > 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. > Yes and no. Snapshotting is similar but not identical to a sparse device. > 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. > It has to be for snapshotting. The main idea of snapshotting is the possibility to do a rollback, ie that you can mount an earlier snapshot instead of the current one and re-start from there. This ability is lost if a later snapshot deletes blocks permanently; these blocks would have to be removed from _all_ snapshots to really free up space. But this would make rollback impossible. Tempting as it may sound, I fear it's not going to work in general. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 N�g GF: Markus Rex, HRB 16746 (AG N�g)
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