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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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