From owner-freebsd-fs@FreeBSD.ORG Thu May 22 13:35:02 2014 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 0D17763D for ; Thu, 22 May 2014 13:35:02 +0000 (UTC) Received: from blade.simplesystems.org (blade.simplesystems.org [65.66.246.74]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id C555C2283 for ; Thu, 22 May 2014 13:35:01 +0000 (UTC) Received: from freddy.simplesystems.org (freddy.simplesystems.org [65.66.246.65]) by blade.simplesystems.org (8.14.4+Sun/8.14.4) with ESMTP id s4MDXq5N004747; Thu, 22 May 2014 08:33:52 -0500 (CDT) Date: Thu, 22 May 2014 08:33:52 -0500 (CDT) From: Bob Friesenhahn X-X-Sender: bfriesen@freddy.simplesystems.org To: Karl Denninger Subject: Re: Turn off RAID read and write caching with ZFS? In-Reply-To: <537DF2F3.10604@denninger.net> Message-ID: References: <719056985.20140522033824@supranet.net> <537DF2F3.10604@denninger.net> User-Agent: Alpine 2.01 (GSO 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.2 (blade.simplesystems.org [65.66.246.90]); Thu, 22 May 2014 08:33:52 -0500 (CDT) Cc: freebsd-fs@freebsd.org X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 22 May 2014 13:35:02 -0000 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/