Re: Raid 10 chunksize

On Thu, 2 Apr 2009, James Mansion wrote:

> Might have to give the whole disk to ZFS with Solaris to give it
> confidence to enable write cache

Confidence, sure, but not necessarily performance at the same time. The
ZFS Kool-Aid gets bitter sometimes too, and I worry that its reputation
causes people to just trust it when they should be wary. If there's
anything this thread does, I hope it helps demonstrate how easy it is to
discover reality doesn't match expectations at all in this very messy
area. Trust No One! Keep Your Laser Handy!

There's a summary of the expected happy ZFS actions at
http://www.opensolaris.org/jive/thread.jspa?messageID=19264& and a good
cautionary tale of unhappy ZFS behavior in this area at
http://blogs.digitar.com/jjww/2006/12/shenanigans-with-zfs-flushing-and-intelligent-arrays/
and its follow-up
http://blogs.digitar.com/jjww/2007/10/back-in-the-sandbox-zfs-flushing-shenanigans-revisted/

Systems with a hardware write cache are pretty common on this list, which
makes the situation described there not that unlikely to run into. The
official word here is at

http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide#FLUSH

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD