Unix administration stuff that was hard to find out using Google and caused a lot of try and error. Hereby given back to the net...
Ubuntu 12.04 on Xen: blkfront: barrier: empty write xvda op failed
Submitted by Lars Windolf on 6. June 2012 - 17:52.
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.