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Xen block drivers and barrier support

I'm running into problems with using DRBD as a backend for a DomU. It says:

[ 3.801589] blkfront: barrier: write xvda2 op failed
[ 3.801597] blkfront: xvda2: barrier or flush: disabled
[ 3.801611] end_request: I/O error, dev xvda2, sector 52171168
[ 3.801630] end_request: I/O error, dev xvda2, sector 52171168
[ 3.801642] Buffer I/O error on device xvda2, logical block 6521396
[ 3.801652] lost page write due to I/O error on xvda2
[ 3.801755] Aborting journal on device xvda2.
[ 3.804415] EXT3-fs (xvda2): error: ext3_journal_start_sb: Detected aborted journal
[ 3.804434] EXT3-fs (xvda2): error: remounting filesystem read-only
[ 3.814754] journal commit I/O error

For more detail about my error, see [1]

In short:

Dom0 is Debian 6, Xen 4.0.1, kernel 2.6.32-5-xen-amd64, DRBD 8.3.7.
DomU is Ubuntu 12.04, kernel 3.2.0-24-generic (pvops).

It doesn't matter if I set no-disk-barrier in DRBD.

Looking at [2] and [3], it seems that it has to do with false advertising of barriers. But, when I configure DRBD not to use barriers (and xenstore-ls says feature-barrier=0), the DomU still fails with the above error.

There is also a Ubuntu lauchpad bugreport which was classified as invalid.

I have two main questions:

1): If I understand correctly, when not using barriers, I also don't use flush-cache because that feature isn't advertised (xenstore-ls doesn't mention it, and there was a bug somewhere, right?). So, does that mean when I disable barrier in fstab, I run the risk of data loss (upon power failure, for instance)?

2): Exactly where is this bug? I'm getting completely confused about xen-blkback, xen-blkfront, lvm, etc. And where does it need to be fixed? The xen-blkback on the Dom0? Xen-blkfront on the DomU? So my Debian host, my Ubuntu guest...?


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