Re: block backend issues

[Marek Marczykowski <marmarek@invisiblethingslab.com>]

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On 08.06.2012 15:11, Marek Marczykowski wrote:
> Hey,
> 
> I've faced strange problem with block devices. When trying to read some file
> (from read-only ext3), everything looks good, except that file content is
> corrupted! But this can be coincidence (that "failed" reads doesn't hit
> filesystem metadata).
> fsck in dom0 on filesystem image returns no errors.
> fsck (with -nf flags) in domU on the device causes the kernel to output
> "blkfront: flush disk cache: empty write xvdd op failed", "blkfront: xvdd:
> barrier or flush: disable". And returns no filesystem errors. From that point,
> file reads return correct file content. For most cases dropping block cache
> (echo 3 > /proc/sys/vm/drop_caches) or remounting device also "fixes" the problem.
> 
> On RW device (with different size, filesystem and content), domU kernel
> complains about EXT4 errors.
> Doesn't observed such strange issues on device-mapper backed devices.
> 
> On 3.2.7 it worked, problem observed on 3.3.5 and 3.4 in dom0, regardless of
> domU kernel (tried 3.2.7, 3.3.5, 3.4.0).
> 
> I've suspected feature-flush-cache/feature-barrier, but when disabled its
> advertise in blkback code, problem still occurs.
> 
> Some details:
> dom0: 3.4.0-1.pvops.qubes.x86_64 (vanilla 3.4 + Konrad's patches for ACPI S3)
> domU: 3.3.5-1.pvops.qubes.x86_64 (vanilla 3.3.5 + Konrad's patches for ACPI S3)

(...)
Still the case on 3.4.1 with applied patches from Konrad's for-jens-3.5 branch.
I've compared file contents and it differs in (multiply of) 1024 bytes - the
same as filesystem block size. And only if block wasn't in pagecache in dom0.
When I flush VM pagecache (echo 1 > /proc/.../drop_caches) after trying to
read some files (actually md5sum -c), but not dom0 pagecache - problem
vanished. But if I clean also dom0 pagecache - problem returns.

Any clues welcomed...

-- 
Best Regards / Pozdrawiam,
Marek Marczykowski
Invisible Things Lab


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