On Wed, Oct 21 2009 at 11:05pm -0400, Douglas McClendon <dmc.fedora@xxxxxxxxxxxxxxxxxxxxxx> wrote: > 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? The snapshot must faithfully maintain a copy of the origin's data relative to a particular point in time. You can't use changes to the origin (trim or any other change) to delete the exceptions that a snapshot is already maintaining. That would invalidate the whole intent of the snapshot. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
