3ware hardware raid with battery backup and the impact on barrier and no write cache options.

[Justin Piszcz]

On Mon, 2 Nov 2009, William Lewis wrote:

> Hi,
>
> I am in the process of setting up an XFS file system on underlying hardware
> consisting of a 3ware 9550SXU (+ battery backup module) and 4 x Seagate
> ST31500341AS 1.5TB hard drives in Raid 5 configuration.
>
> Reading your FAQ at http://xfs.org/index.php/XFS_FAQ I understand that it is
> advisable to mount the file system with nobarrier to improve performance.
> However going on to read about recommended settings for write cache, the
> advice for 3ware hardware doesn't seem to account for the fact that there
> are 2 levels of write cache in play, that in the 3ware card itself protected
> by the battery and the write cache of the disks themselves, which as far as
> I can understand is also protected by the battery backup (in the correct
> storage modes - balanced/protection) because the 3ware card uses write
> journaling to keep track of pending write operations in the disks' cache.
> Therefore unless I am misunderstanding something the most benefit is to be
> gained by mounting with nobarrier and having the write cache turned on?
>
> One thing I am not clear about is if nobarrier interacts with the page cache
> at all and if the lack of barrier leaves you with a situation in which
> pending writes can be lost from main memory on power failure?
>
> Thanks in advance for any clarification you can provide.
>
> Regards
>
> Will Lewis
>

I am also curious, I have a 16 drive RAID-6 configuration on a 
9650SE-16ML and using -o nobarrier or mounting normal the speed/benchmarks 
seemed to be the same.  Either barriers are not enabled by default for 
3ware RAID arrays or they make no difference in performance?

Justin.