Date: Thu, 22 May 2014 03:38:24 -0700 From: Jeff Chan <jeffc@supranet.net> To: freebsd-fs@freebsd.org Subject: Turn off RAID read and write caching with ZFS? Message-ID: <719056985.20140522033824@supranet.net>
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As mentioned before we have a server with the LSI 2208 RAID chip which apparently doesn't seem to have HBA firmware available. (If anyone knows of one, please let me know.) Therefore we are running each drive as separate, individual RAID0, and we've turned off the RAID harware read and write caching on the claim it performs better with ZFS, such as: http://forums.freenas.org/index.php?threads/disable-cache-flush.12253/ " cyberjock, Apr 7, 2013 AAh. You have a RAID controller with on-card RAM. Based on my testing with 3 different RAID controllers that had RAM and benchmark and real world tests, here's my recommended settings for ZFS users: 1. Disable your on-card write cache. Believe it or not this improves write performance significantly. I was very disappointed with this choice, but it seems to be a universal truth. I upgraded one of the cards to 4GB of cache a few months before going to ZFS and I'm disappointed that I wasted my money. It helped a LOT on the Windows server, but in FreeBSD it's a performance killer. :(" 2. If your RAID controller supports read-ahead cache, you should be setting to either "disabled", the most "conservative"(smallest read-ahead) or "normal"(medium size read-ahead). I found that "conservative" was better for random reads from lots of users and the "normal" was better for things where you were constantly reading a file in order(such as copying a single very large file). If you choose anything else for the read-ahead size the latency of your zpool will go way up because any read by the zpool will be multiplied by 100x because the RAID card is constantly reading a bunch of sectors before and after the one sector or area requested." Does anyone have any comments or test results about this? I have not attempted to test it independently. Should we run with RAID hardware caching on or off? Cheers, Jeff C. -- Jeff Chan mailto:jeffc@supranet.net http://www.jeffchan.com/
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