[dm-devel] [PATCH] dm snapshot: ignore discards issued to the snapshot-origin target
Christoph Hellwig
hch at infradead.org
Thu Apr 28 07:53:55 UTC 2011
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)
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