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Re: [dm-devel] Discard support for dm-snap

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;


Here was my similar suggestion and the discussion with Mike that followed-


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.




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