Date: Thu, 22 May 2014 08:33:52 -0500 (CDT) From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> To: Karl Denninger <karl@denninger.net> Cc: freebsd-fs@freebsd.org Subject: Re: Turn off RAID read and write caching with ZFS? Message-ID: <alpine.GSO.2.01.1405220825290.1735@freddy.simplesystems.org> In-Reply-To: <537DF2F3.10604@denninger.net> References: <719056985.20140522033824@supranet.net> <537DF2F3.10604@denninger.net>
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On Thu, 22 May 2014, Karl Denninger wrote: > > Write-caching is very evil in a ZFS world, because ZFS checksums each block. > If the filesystem gets back an "OK" for a block not actually on the disk ZFS > will presume the checksum is ok. If that assumption proves to be false down > the road you're going to have a very bad day. I don't agree with the above statement. Non-volatile write caching is very beneficial for zfs since it allows transactions (particularly synchronous zil writes) to complete much quicker. This is important for NFS servers and for databases. What is important is that the cache either be non-volatile (e.g. battery-backed RAM) or absolutely observe zfs's cache flush requests. Volatile caches which don't obey cache flush requests can result in a corrupted pool on power loss, system panic, or controller failure. Some plug-in RAID cards have poorly performing firmware which causes problems. Only testing or experience from other users can help identify such cards so that they can be avoided or set to their least harmful configuration. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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