From owner-freebsd-fs@FreeBSD.ORG Thu May 12 13:57:30 2011 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id CD1C3106564A for ; Thu, 12 May 2011 13:57:30 +0000 (UTC) (envelope-from bfriesen@simple.dallas.tx.us) Received: from blade.simplesystems.org (blade.simplesystems.org [65.66.246.74]) by mx1.freebsd.org (Postfix) with ESMTP id 90A348FC0A for ; Thu, 12 May 2011 13:57:29 +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 p4CDvHqT015705; Thu, 12 May 2011 08:57:17 -0500 (CDT) Date: Thu, 12 May 2011 08:57:17 -0500 (CDT) From: Bob Friesenhahn X-X-Sender: bfriesen@freddy.simplesystems.org To: Jeremy Chadwick In-Reply-To: <20110512033626.GA52047@icarus.home.lan> Message-ID: References: <4DCA5620.1030203@dannysplace.net> <4DCB455C.4020805@dannysplace.net> <20110512033626.GA52047@icarus.home.lan> 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, 12 May 2011 08:57:17 -0500 (CDT) Cc: freebsd-fs@freebsd.org Subject: Re: ZFS: How to enable cache and logs. X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 12 May 2011 13:57:30 -0000 On Wed, 11 May 2011, Jeremy Chadwick wrote: > > Bob, please correct me if I'm wrong, but as I understand it a log device > (ZIL) effectively limits the overall write speed of the pool itself. > Consumer-level SSDs do not have extremely high write performance (and it > gets worse without TRIM; again a 70% decrease in write speed in some > cases). It is certainly a factor. However, large block writes (something like 128K, I don't remember exactly) bypass the dedicated log device and instead are written to the main store (with only a reference being added to the dedicated device). The reason this is done is for the exact reason you point out. The SSD has a very fast seek and zero rotational latency but being a singular resource it suffers from bandwidth limitations. The main store usually suffers from multi-millisecond seeks and rotational latency but offers linearly scalable and substantial write performance for larger writes. Matt Ahrens has described this a few times on the zfs-discuss list and there is mention of it on slide 15 of the presentation found at "http://www.slideshare.net/edigit/zfs-presentation". The large write feature of the ZIL is a reason why we should appreciate modern NFS's large-write capability and avoid anchient NFS. It is worth mentioning that the ZIL is a write-only device which is only read when the system boots or a pool is imported. The writes are usually "write and forget" since zfs uses them to improve its ability to cache larger transaction groups. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/