From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Thu, 28 Apr 2011 03:53:55 -0400 From: Christoph Hellwig Message-ID: <20110428075355.GA2190@infradead.org> References: <20110412234706.GA11244@redhat.com> <20cf301cbef67d323104a0c2ff52@google.com> <20110413224025.GA18589@redhat.com> <20110413234854.GA19793@redhat.com> <20110426173213.GA19604@redhat.com> <20110428001912.GA14659@redhat.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20110428001912.GA14659@redhat.com> Subject: Re: [linux-lvm] [dm-devel] [PATCH] dm snapshot: ignore discards issued to the snapshot-origin target Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: device-mapper development Cc: DarkNovaNick@gmail.com, LVM general discussion and development 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)