Re: xfs on san

[Matt Cuttler <mcuttler@bnl.gov>]

Rudy Gevaert wrote:
> Hi,
>
> I've read on the xfs homepage that one should disable the write back
> cache on the disks to prevent corruption.
>
> How does one do this when your filesystem is on a SAN ?
>
> Thanks in advance,
>
> Rudy
>
I'm assuming that you have some sort of Fiber Channel disk arrays
attached to your SAN. Most hardware of this sort, that I've dealt with
[1], has options which you can configure various cache settings on the
storage unit / enclosure (rather than the individual disks themselves).
The way to do so might be through a www interface, ssh session .etc..
However you (or the storage admin) normally configure your storage.

In general, write-behind cache *can* be dangerous, but the risk is
greatly reduced by controller/cache battery backup, redundant power
supplies etc. My point is, you may not want to disable write-behind
cache on a Good storage array. I mention this since you might just see
your write speeds drop to molasses if you disabled write cache.

Perhaps less risky would be write-through cache. But you may not gain
much (if any) performance - over no caching at all  - depending upon
your workload. If multiple reads need the same data at roughly the same
time, this might be a way to go.

If you don't have battery back up units, stable power etc, then yes, I
would recommend turning off the cache to see what kind of numbers you get..


1) Nexsan units such as ATABoyII, SATABlade..  Re-branded Infortran
units such as RAIDInc FalconII's.. Sun StorEdge..etc..

-Matt Cuttler.