From owner-freebsd-fs@freebsd.org Mon Aug 3 13:43:26 2015 Return-Path: Delivered-To: freebsd-fs@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id E62359B2F30 for ; Mon, 3 Aug 2015 13:43:26 +0000 (UTC) (envelope-from bfriesen@simple.dallas.tx.us) 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 B17371B34 for ; Mon, 3 Aug 2015 13:43:26 +0000 (UTC) (envelope-from bfriesen@simple.dallas.tx.us) 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 t73DVkAQ016078; Mon, 3 Aug 2015 08:31:47 -0500 (CDT) Date: Mon, 3 Aug 2015 08:31:46 -0500 (CDT) From: Bob Friesenhahn X-X-Sender: bfriesen@freddy.simplesystems.org To: Quartz cc: freebsd-fs@freebsd.org Subject: Re: ZFS: Disabling ARC? In-Reply-To: <55BF1270.10003@sneakertech.com> Message-ID: References: <55BC14B7.9010009@sneakertech.com> <9DBE58C6-8C42-498B-AB66-7D9BBDFAA90F@kraus-haus.org> <55BF1270.10003@sneakertech.com> 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]); Mon, 03 Aug 2015 08:31:47 -0500 (CDT) X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 03 Aug 2015 13:43:27 -0000 On Mon, 3 Aug 2015, Quartz wrote: >> If you are really worried about the ARC hogging RAM, then set a cap. >> The kernel tunables here are: > > I'm not worried about it hogging ram per se, but rather I'm a little confused > about where and when it helps, where it's useless or detrimental (if ever), > and consequently I don't really know when I should tune it or what to tune it > *to*. > > Basically, my question is the subject line of this thread: is there ever a > reason to attempt to disable ARC, and what would that situation probably look > like? ARC is intrinsic to the design of zfs and so it can not be entirely disabled. Without caching, zfs would perform terribly, and some caching is needed in order for transaction groups to work. The types of data (data/metadata) cached in the ARC are tunable on a filesystem basis. If you do not encounter a problem, then you should not worry about it. The whole point of the ARC algorithm is that it self-tunes itself to meet available resources and requirements. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/