Excerpts from Mike Snitzer's message of 2010-11-08 09:58:09 -0500: > On Sun, Nov 07 2010 at 6:05pm -0500, > Andi Kleen <andi@xxxxxxxxxxxxxx> wrote: > > > On Sun, Nov 07, 2010 at 10:39:23PM +0100, Milan Broz wrote: > > > On 11/07/2010 08:45 PM, Andi Kleen wrote: > > > >> I read about barrier-problems and data getting to the partition when > > > >> using dm-crypt and several layers so I don't know if that could be > > > >> related > > > > > > > > Barriers seem to be totally broken on dm-crypt currently. > > > > > > Can you explain it? > > > > e.g. the btrfs mailing list is full of corruption reports > > on dm-crypt and most of the symptoms point to broken barriers. > > [cc'ing linux-btrfs, hopefully in the future dm-devel will get cc'd when > concerns about DM come up on linux-btrfs (or other lists)] > > I spoke with Josef Bacik and these corruption reports are apparently > against older kernels (e.g. <= 2.6.33). I say <= 2.6.33 because: We've consistently seen reports about corruptions on power hits with dm-crypt. The logs didn't have any messages about barriers failing, but the corruptions were still there. The most likely cause is that barriers just aren't getting through somehow. > > https://btrfs.wiki.kernel.org/index.php/Gotchas states: > "btrfs volumes on top of dm-crypt block devices (and possibly LVM) > require write-caching to be turned off on the underlying HDD. Failing to > do so, in the event of a power failure, may result in corruption not yet > handled by btrfs code. (2.6.33)" > > But Josef was not aware of any reports with kernels newer than 2.6.32 > (F12). > > Josef also noted that until last week btrfs wouldn't retry another > mirror in the face of some corruption, the fix is here: > http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-unstable.git;a=commit;h=cb44921a09221 > > This obviously doesn't fix any source of corruption but it makes btrfs > more resilient when it encounters the corruption. Right. > > > > Barriers/flush change should work, if it is broken, it is not only dm-crypt. > > > (dm-crypt simply relies on dm-core implementation, when barrier/flush > > > request come to dmcrypt, all previous IO must be already finished). > > > > Possibly, at least it doesn't seem to work. > > Can you please be more specific? What test(s)? What kernel(s)? > > Any pointers to previous (and preferably: recent) reports would be > appreciated. > > The DM barrier code has seen considerable change recently (via flush+fua > changes in 2.6.37). Those changes have been tested quite a bit > (including ext4 consistency after a crash). > > But even prior to those flush+fua changes DM's support for barriers > (Linux >= 2.6.31) was held to be robust. No known (at least no > reported) issues with DM's barrier support. I think it would be best to move forward with just hammering on the dm-crypt barriers: http://oss.oracle.com/~mason/barrier-test This script is the best I've found so far to reliably trigger corruptions with barriers off. I'd start with ext3 + barriers off just to prove it corrupts things, then move to ext3 + barriers on. -chris -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
