On Mon, Nov 08 2010 at 12:59pm -0500, Chris Mason <chris.mason@xxxxxxxxxx> wrote: > 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. Can't blame anyone for assuming as much (although it does create FUD) but in practice (testing dm-crypt with ext4 using your barrier-test script) I have not been able to see any evidence that dm-crypt's barrier support is ineffective. Could be that the barrier-test script isn't able to reproduce the unique failure case that btrfs does (on power failure)? > > > > 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. I still think we need specific bug reports that detail workloads and if possible reproducers. > > 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. I started with ext4 + barrier=0,journal_async_commit and could reliably cause directory corruption (~75% of the time). I then switched to barrier=1 and could not cause corruption. I then added dm-crypt and got the same results: with barrier=1 I could not cause directory corruption. barrier=0 resulted in directory corruption (again ~75% of the time), no corruption occurred with barrier=1. Both 2.6.36 (original barrier code) and latest 2.6.37-rc1+ (new flush+fua code) were tested. 6 iterations of barrier=0 and 10 iterations of barrier=1. So my hope is we can now put this general dm-crypt barrier doubt to one side and work together on identifying the cause of corruption when dm-crypt is paired with btrfs. Thanks, Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
