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Re: [Xen-devel] Re: Bug#637234: linux-image-3.0.0-1-686-pae: I/O errors using ext4 under xen

On Thu, Aug 25, 2011 at 07:47:08AM +0100, Ian Campbell wrote:
> Hi Konrad,
> Does this look at all familiar? There is some more info in the full bug
> log at http://bugs.debian.org/637234 . In particular, contrary to the
> message below, the user subsequently confirmed that the issue appears to
> be Xen specific (doesn't happen on native or vmware) and that it arose
> between 2.6.39-2-686-pae and 3.0.0-1-686-pae.
> Could it be related to edf6ef59ec7e "xen-blkfront: Introduce
> BLKIF_OP_FLUSH_DISKCACHE support"? That looks like the only pertinent
> change between 2.6.39 and 3.0.

It shouldn't - from the look of it:
[    0.529412] blkfront: xvdc: barrier: enabled

it looks as if the 'feature-barrier' is used. Not 'feature-flush-cache' -
otherwise you would have seen a message about that.

But then.. 3.0 (and 2.6.39) don't do barriers anymore. However the backend
seems to do it. And from my understanding is that the barrier request
is a superset of a flush request so it should work. Put maybe that is
an incorrect assumption.

One way to make sure that is not the case is to disable barriers in the
guest. Meaning in /etc/fstab have something like this:

/dev/xvdc /blah     ext4    errors=remount-ro,barrier=0 0 1

The other question is what version of Dom0 are you running? Is it 2.6.32?

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