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    SubjectRe: hunt for 2.6.37 dm-crypt+ext4 corruption? (was: Re: dm-crypt barrier support is effective)
    On Tue, Dec 7, 2010 at 2:33 PM, Chris Mason <chris.mason@oracle.com> wrote:
    > Excerpts from Jon Nelson's message of 2010-12-07 15:25:47 -0500:
    >> On Tue, Dec 7, 2010 at 2:02 PM, Chris Mason <chris.mason@oracle.com> wrote:
    >> > Excerpts from Jon Nelson's message of 2010-12-07 14:34:40 -0500:
    >> >> On Tue, Dec 7, 2010 at 12:52 PM, Chris Mason <chris.mason@oracle.com> wrote:
    >> >> >> postgresql errors. Typically, header corruption but from the limited
    >> >> >> visibility I've had into this via strace, what I see is zeroed pages
    >> >> >> where there shouldn't be.
    >> >> >
    >> >> > This sounds a lot like a bug higher up than dm-crypt.  Zeros tend to
    >> >> > come from some piece of code explicitly filling a page with zeros, and
    >> >> > that often happens in the corner cases for O_DIRECT and a few other
    >> >> > places in the filesystem.
    >> >> >
    >> >> > Have you tried triggering this with a regular block device?
    >> >>
    >> >> I just tried the whole set of tests, but with /dev/sdb directly (as
    >> >> ext4) without any crypt-y bits.
    >> >> It takes more iterations but out of 6 tests I had one failure: same
    >> >> type of thing, 'invalid page header in block ....'.
    >> >>
    >> >> I can't guarantee that it is a full-page of zeroes, just what I saw
    >> >> from the (limited) stracing I did.
    >> >
    >> > Fantastic. Now for our usual suspects:
    >> >
    >> > 1) Is postgres using O_DIRECT?  If yes, please turn it off
    >>
    >> According to strace, O_DIRECT didn't show up once during the test.
    >>
    >> > 2) Is postgres allocating sparse files?  If yes, please have it fully
    >> > allocate the file instead.
    >>
    >> That's a tough one. I don't think postgresql does that, but I'm not an
    >> expert here.
    >>
    >> > 3) Is postgres using preallocation (fallocate)?  If yes, please have it
    >> > fully allocate the file instead
    >>
    >> As far as strace is concerned, postgreql is not using fallocate in
    >> this version.
    >
    > Well, the only other usual suspect would be mmap.  Does the strace show
    > that you're using read/write for file IO or is it doing a lot of mmaps
    > on the files?

    I'm pretty sure postgresql uses regular file I/O and not mmap.

    --
    Jon
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