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

[Eric Sandeen]

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?

If the write caches won't go away - or will be fully/gracefully destaged 
before they do, then nobarrier should be safe.

> 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?

nobarrier has no interaction with the OS page cache; all the "barrier" 
option (the default) does is enforce ordering for journal IO*, and in 
practice it does this by flushing the cache at points in time.

-Eric

*well, I think it flushes the drive cache on fsync, too, for data 
integrity (vs. the metadata integrity for the journal IO ordering)

> Thanks in advance for any clarification you can provide.
> 
> Regards
> 
> Will Lewis
> 
> 
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