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

[William Lewis]

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
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