From owner-freebsd-fs@FreeBSD.ORG Tue Nov 20 04:20:49 2012 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id EA48AA00 for ; Tue, 20 Nov 2012 04:20:49 +0000 (UTC) (envelope-from artemb@gmail.com) Received: from mail-la0-f54.google.com (mail-la0-f54.google.com [209.85.215.54]) by mx1.freebsd.org (Postfix) with ESMTP id 656B98FC12 for ; Tue, 20 Nov 2012 04:20:49 +0000 (UTC) Received: by mail-la0-f54.google.com with SMTP id j13so5260881lah.13 for ; Mon, 19 Nov 2012 20:20:48 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:cc:content-type; bh=XRbWs1UJAv5OciNTZJyM7uJsC7Qu5gg/la2CfeBxPo0=; b=wpG7fIuDsCPoN4CXufTwsG0WNoIlKZSuaUsUCi1gWdaldZWupxQST2ZiNHxhowJ11v t/KgJsw3sRZmZULWCqoM5NSAtdQTYF60AgPt1kBlxCZdjZl87G8G0TBYT9LWfyGdm1+B Q+hghReTOl6uZyRmWcVXDSmaNZXS8k/jNM67+oZMotE36qJcLOiBbEAVNZneworu/xGU IQmnEqWyc60ICuu1xHoxHHreMnScFldJaEzx4/IRSJBPsLKbJMH1nkspTsteKHgVfGFC SldqKdEk1ezaIQKStItWUOSAn7Vspz4oIpajh6q9r8PQlZiJL+OZFrzgS3tZ3pwGddKU oUOQ== MIME-Version: 1.0 Received: by 10.152.144.69 with SMTP id sk5mr13564062lab.22.1353385226196; Mon, 19 Nov 2012 20:20:26 -0800 (PST) Sender: artemb@gmail.com Received: by 10.112.80.103 with HTTP; Mon, 19 Nov 2012 20:20:26 -0800 (PST) In-Reply-To: <20121120040258.GA27849@neutralgood.org> References: <57ac1f$gf3rkl@ipmail05.adl6.internode.on.net> <50A31D48.3000700@shatow.net> <20121116044055.GA47859@neutralgood.org> <50A64694.5030001@egr.msu.edu> <20121117181803.GA26421@neutralgood.org> <20121117225851.GJ1462@egr.msu.edu> <20121120040258.GA27849@neutralgood.org> Date: Mon, 19 Nov 2012 20:20:26 -0800 X-Google-Sender-Auth: 5Cn1BIt05z2oaolH-F4gfYwwFEE Message-ID: Subject: Re: SSD recommendations for ZFS cache/log From: Artem Belevich To: kpneal@pobox.com Content-Type: text/plain; charset=ISO-8859-1 Cc: freebsd-fs X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 20 Nov 2012 04:20:50 -0000 On Mon, Nov 19, 2012 at 8:02 PM, wrote: > Advising people to use dedup when high dedup ratios are expected, and > advising people to otherwise not use dedup, is by itself incorrect advice. > Rather, dedup should only be enabled on a system with a large amount of > memory. The usual advice of 1G of ram per 1TB of disk is flat out wrong. > > Now, I do not know how much memory to give as a minimum. I suspect that > the minimum should be more like 16-32G, with more if large amounts of > deduped data are to be removed by destroying entire datasets. But that's > just a guess. For what it's worth, Oracle has published an article on memory sizing for dedupe. http://www.oracle.com/technetwork/articles/servers-storage-admin/o11-113-size-zfs-dedup-1354231.html In a nutshell, it's 320 bytes per record. Number of records will depend on your data set and the way it's been written. --Artem