Re: [dm-devel] [PATCH] dm snapshot: ignore discards issued to the snapshot-origin target

[Christoph Hellwig <hch infradead org>]

On Wed, Apr 27, 2011 at 08:19:13PM -0400, Mike Snitzer wrote:
> Discards pose a problem for the snapshot-origin target because they are
> treated as writes.  Treating a discard as a write would trigger a
> copyout to the snapshot.  Such copyout can prove too costly in the face
> of otherwise benign scenarios (e.g. create a snapshot and then mkfs.ext4
> the origin -- mkfs.ext4 discards the entire volume by default, which
> would copyout the entire origin volume to the snapshot).

You also need to make sure that we don't claim discard_zeroes_data for
the origin volume in this case.  Especially as ext4 started to rely
on this actually working (very bad idea IMHO, but that's another story)