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 <hch@xxxxxxxxxxxxx> 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? -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
