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:
  • According to the Debian bug report the correct solution is to upgrade the dom0 kernel to at least 2.6.32-41.
  • To solve the problem without upgrading the dom0 kernel: reboot until you get a writable filesystem and add "barrier=0" to the mount options of all your local filesystems.
  • Alternatively: do not use paravirtualization :-)