Xen, XFS and the barrier error

Probably nobody is going to hit this but since I’ve lost half a morning puzzling my mind about this error, I think it would be useful to blog about it.
We have some old Xen machine running Debian Lenny with Xen 3.2 and while upgrading a VM using a XFS root partition to Debian Wheezy (it was a Squeeze), I’ve got this nice error:

[ 5.024330] blkfront: xvda1: barrier or flush: disabled
[ 5.024338] end_request: I/O error, dev xvda1, sector 4196916
[ 5.024343] end_request: I/O error, dev xvda1, sector 4196916
[ 5.024360] XFS (xvda1): metadata I/O error: block 0x400a34 (“xlog_iodone”) error 5 buf count 3072
[ 5.024369] XFS (xvda1): xfs_do_force_shutdown(0x2) called from line 1007 of file /build/linux-s5x2oE/linux-3.2.46/fs/xfs/xfs_log.c. Return address = 0xffffffffa009fed5
[ 5.024394] XFS (xvda1): Log I/O Error Detected. Shutting down filesystem
[ 5.024401] XFS (xvda1): Please umount the filesystem and rectify the problem(s)
[ 5.024411] XFS (xvda1): xfs_log_force: error 5 returned.
[ 5.024419] XFS (xvda1): xfs_do_force_shutdown(0x1) called from line 1033 of file /build/linux-s5x2oE/linux-3.2.46/fs/xfs/xfs_buf.c. Return address = 0xffffffffa005f8a7

this strange error simply means that the underlying device doesn’t provide barrier support! The simple, quick fix (once you know it) is to specify in the fstab nobarrier option. I think that I found this problem because in Linux 3.2 the barrier option is enabled by default, while it was off in previous kernel versions.