From owner-freebsd-fs@FreeBSD.ORG Wed Jan 23 16:22:49 2013 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by hub.freebsd.org (Postfix) with ESMTP id ECD9C500 for ; Wed, 23 Jan 2013 16:22:49 +0000 (UTC) (envelope-from mad@madpilot.net) Received: from winston.madpilot.net (winston.madpilot.net [78.47.75.155]) by mx1.freebsd.org (Postfix) with ESMTP id A7E69E64 for ; Wed, 23 Jan 2013 16:22:49 +0000 (UTC) Received: from winston.madpilot.net (localhost [127.0.0.1]) by winston.madpilot.net (Postfix) with ESMTP id 3YrsGv1fT1zFX1H for ; Wed, 23 Jan 2013 17:22:47 +0100 (CET) X-Virus-Scanned: amavisd-new at madpilot.net Received: from winston.madpilot.net ([127.0.0.1]) by winston.madpilot.net (winston.madpilot.net [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id Fts7KA5XHnfC for ; Wed, 23 Jan 2013 17:22:44 +0100 (CET) Received: from vwg82.hq.ignesti.it (unknown [80.74.176.55]) by winston.madpilot.net (Postfix) with ESMTPSA for ; Wed, 23 Jan 2013 17:22:44 +0100 (CET) Message-ID: <51000E55.6070901@madpilot.net> Date: Wed, 23 Jan 2013 17:22:45 +0100 From: Guido Falsi User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:17.0) Gecko/20130115 Thunderbird/17.0.2 MIME-Version: 1.0 To: freebsd-fs@freebsd.org Subject: Re: RFC: Suggesting ZFS "best practices" in FreeBSD References: <314B600D-E8E6-4300-B60F-33D5FA5A39CF@sarenet.es> <565CB55B-9A75-47F4-A88B-18FA8556E6A2@samsco.org> <81460DE8-89B4-41E8-9D93-81B8CC27AA87@baaz.fr> In-Reply-To: <81460DE8-89B4-41E8-9D93-81B8CC27AA87@baaz.fr> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit 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: Wed, 23 Jan 2013 16:22:50 -0000 On 01/23/13 16:16, Jean-Yves Moulin wrote: > Hi, > > > On 22 Jan 2013, at 15:33 , Scott Long wrote: > >> Agree 200%. Despite the best effort of sales and marketing people, RAID cards do not make good HBAs. At best they add latency. At worst, they add a lot of latency and extra failure modes. > > > But what about battery-backed cache RAID card ? They offer a non-volatile cache that improves writes. And this cache is safe because of the battery. These feature doesn't exist on bare disks. > Safe is optimistic. The cache can keep the memory alive for a 36-48 hours at most usually. In this (short) time frame you need to find identical hardware on which to move the disks and the controller without detaching the batteries. This in fact mostly means you need a second server without disks just in case you need a recovery. Also, expected battery life will decrease with time. Some vendors now sell solid state cache memory which can hold data indefinitely. This is a more sensible approach(and looks very similar to a dedicated ZIL device to me). It does not remove the need to find identical hardware on which to move disk and controllers to recover the array though. This is the one aspect in which open sourced software raid is better: any hardware with the enough connectors of the correct kind will do for recovery...well and enough RAM also. -- Guido Falsi