From: Eric Sandeen Subject: Re: do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target] Date: Thu, 28 Apr 2011 16:28:17 -0500 Message-ID: <4DB9DBF1.9060901@redhat.com> References: <20110412234706.GA11244@redhat.com> <20cf301cbef67d323104a0c2ff52@google.com> <20110413224025.GA18589@redhat.com> <20110413234854.GA19793@redhat.com> <20110426173213.GA19604@redhat.com> <20110428001912.GA14659@redhat.com> <20110428075355.GA2190@infradead.org> <20110428205935.GA24979@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Christoph Hellwig , dm-devel@redhat.com, DarkNovaNick@gmail.com, linux-lvm@redhat.com, linux-ext4@vger.kernel.org To: Mike Snitzer Return-path: Received: from mx1.redhat.com ([209.132.183.28]:5131 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932218Ab1D1V2d (ORCPT ); Thu, 28 Apr 2011 17:28:33 -0400 In-Reply-To: <20110428205935.GA24979@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On 4/28/11 3:59 PM, Mike Snitzer wrote: > [cc'ing linux-ext4] > > On Thu, Apr 28 2011 at 3:53am -0400, > Christoph Hellwig wrote: > >> 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) > > Eric Sandeen helped me see that having the DM snapshot-origin target > return success but actually ignore discards is just bad form. > > Especially when you consider that this exercise was motivated by the > fact that ext4 will disable discards on the first discard failure, see: > http://www.redhat.com/archives/dm-devel/2011-April/msg00070.html > > Eric and I think it is best to revert this commit: > a30eec2 ext4: stop issuing discards if not supported by device > > (though ideally ext4 would still WARN_ONCE per superblock with something > like: "discard failed, please consider disabling discard support") > > 1) The user asked for discards (with '-o discard' mount option) > - what is the real harm in coninuing to issue them even if it _seems_ > they aren't supported? TBH I sent a30eec2 on a whim. Seemed reasonable at the time, but if discard-ability changes over time, it may not be the best plan. > 2) assuming the entire block device uniformly supports discards can > be flawed (a DM device's discard support can vary based on logical > offset). I still think that concats of floppies, usb disks, and ssds should be rare, so I'm less concerned about that ;) I think Mike is right though, that if you do not do anything with a discard, you should return -EOPNOTSUPP, and not pretend that you honored it. We should, IMHO, deal with the truth of the matter at the filesystem caller. -Eric > Thoughts?