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Write barriers and hardware RAID

To: xfs@xxxxxxxxxxx
Subject: Write barriers and hardware RAID
From: J Pälve <xtc@xxxxxxx>
Date: Fri, 17 Jul 2009 13:35:52 +0300 (EEST)
User-agent: Alpine 1.10 (LRH 962 2008-03-14)

I'm setting up a server to provide storage for a couple of VMware ESXi servers. I'm using the latest stable Debian and I'm leaning towards using NFS with XFS. However, I'm concerned about data integrity in the event of power-out (we have UPS but they only last so long). Here are the specific questions I have:

- The XFS FAQ states that with battery backup'd RAID hardware, both write barriers and individual disk cache should be turned off. However, I'm getting better benchmark results with both turned on. What I'm wondering is, will write barriers work as intended when used with hardware RAID controller (PERC 6/E)? Googling only turned up results relating to software RAID.

- The XFS FAQ also states that virtualization products prevent write barriers from working correctly. Is this still the case (specifically with ESXi 3.5 and later) and is there anything that can be done about it? Does VMFS somehow work around this, or is the problem then just "out of sight, out of mind"?


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