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[dm-devel] trimmable dm-snapshot?

Has anyone looked into the idea of dm-snapshots responding appropriately to trims from filesystems?

I.e. the efficiency problem of a dm-snapshotted ext filesystem having files created and then deleted? I.e. in such a scenario, resources in the cow device end up taken that could be freed if the dm layer could efficiently respond to trim notifications by discarding any useless exceptions?

I've been poking around pondering whether an offline quick hack might be possible with libext2fs and enough knowledge of the on-disk persistent snapshot format. I.e. just walk the exception chunks in the cow device, use libext2fs (sufficient? easiest way?) to determine whether all the fsblocks/sectors the chunk contains are all currently unneeded, and if so reclaiming that space (possibly by relocating the last exception. I'm still a distance from truly grokking the on-disk format along with the rest of the dm-snapshot and exception-store code).

Does any of this make sense? Been looked at? Seem like a reasonable avenue to pursue?


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