A little RAID experiment

[Stefan Ring]

> You now have persistent write cache.  Did you test with XFS barriers
> disabled?  If not you should.  You'll likely see a decent, possibly
> outstanding, performance improvement with your huge metadata
> modification workload, as XFS will no longer flush the cache frequently
> when writing to the journal log.

I've already done that test with the previous controller. It had a
BBWC, so it was persistent as well. And it was easy to enable or
disable it. Yes, everything was always done with barrier=0. And yes,
the cache made a big difference (about 3x).