Wiki entry for disk write caches and RAID controllers

[Michael Monnerie]

I updated the sections about "disk write cache" and "RAID controllers", 
please review and comment.

http://xfs.org/index.php/XFS_FAQ

I merged my original paragraph about "disk write cache" with the "What 
is the problem with the write cache on journaled filesystems?" text.

On Donnerstag 05 Februar 2009 Dave Chinner wrote:
> What I missed was the "barriers turned on" - I was referring
> (context not quoted) to the fact that RAID5 is not unіque in it's
> ability to trash the filesystem on powerfail. You are right,
> barriers on a single disk should prevent filesystem corruption
> and will prevent loss of synchronously written data. only
> asynchronously written data will get lost (just like all the
> stuff sitting in RAM).

So can we say that 1 disk can let write cache on with barriers enabled?

For the Xyratex case: If all write caches are off (or write through), 
does it matter if you use [no]barrier? I guess no, just want to be sure.

mfg zmi
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