Write barrier support with LVM2/md

[Martin Lucina <mato@xxxxxxxxxx>]

Hello,

Having recently upgraded several systems to Linux 2.6.16/2.6.18 I learnt
about the issues with modern drives and write caching.  Thanks for the
excellent FAQ entry on this.

Unfortunately it seems that write barriers do not work when using XFS on
top of LVM2 (device-mapper).

Trying an explicit mount with -o barrier gives:

Filesystem "dm-4": Disabling barriers, not supported by the underlying device

Is anyone working on a fix for this?  It might be a good idea to add to
the FAQ that write barriers are not supported on dm devices.

-mato