blkfront barrier empty write xvda op failed

When you run Ubuntu 12.04 VM on XenServer 6.0 (kernel you can get the following errors and your local file systems will mount read-only. Also see Debian #637234 or Ubuntu #824089.

blkfront: barrier: empty write xvda op failed
blkfront: xvda: barrier or flush: disabled
You also won't be able to remount read-write using "mount -o remount,rw /" as this will give you a kernel error like this
EXT4-fs (xvda1): re-mounted. Opts: errors=remount-ro
This problem more or less sporadically affects paravirtualized Ubuntu 12.04 VMs. Note that officially Ubuntu 12.04 is not listed as a supported system in the Citrix documentation. Note that this problem doesn't affect fully virtualized VMs.

The Solution: