From owner-freebsd-fs@FreeBSD.ORG Thu May 12 06:44:19 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 A35EF106564A for ; Thu, 12 May 2011 06:44:19 +0000 (UTC) (envelope-from daniel@digsys.bg) Received: from smtp-sofia.digsys.bg (smtp-sofia.digsys.bg [193.68.3.230]) by mx1.freebsd.org (Postfix) with ESMTP id 3422E8FC15 for ; Thu, 12 May 2011 06:44:18 +0000 (UTC) Received: from dcave.digsys.bg (dcave.digsys.bg [192.92.129.5]) (authenticated bits=0) by smtp-sofia.digsys.bg (8.14.4/8.14.4) with ESMTP id p4C6i9d6058598 (version=TLSv1/SSLv3 cipher=DHE-RSA-CAMELLIA256-SHA bits=256 verify=NO) for ; Thu, 12 May 2011 09:44:14 +0300 (EEST) (envelope-from daniel@digsys.bg) Message-ID: <4DCB81B8.6070301@digsys.bg> Date: Thu, 12 May 2011 09:44:08 +0300 From: Daniel Kalchev User-Agent: Mozilla/5.0 (X11; U; FreeBSD amd64; en-US; rv:1.9.2.15) Gecko/20110307 Thunderbird/3.1.9 MIME-Version: 1.0 To: freebsd-fs@freebsd.org References: <4DCA5620.1030203@dannysplace.net> <4DCB455C.4020805@dannysplace.net> In-Reply-To: <4DCB455C.4020805@dannysplace.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit 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 06:44:19 -0000 On 12.05.11 05:26, Danny Carroll wrote: > > - Don't use SSD for the Log device. Write speed tends to be a problem. It all depends on your usage. You need to experiment, unfortunately. > - SSD ok for cache if the sizing is right, but without TRIM, don't > expect to take full advantage of the SSD. I do not believe TRIM has any effect on L2ARC. Why? - TRIM is a technique to optimize future writes; - L2ARC is written at controlled, very low rate, I believe something like 8MB/sec. There is no SSD currently on the market, with or without TRIM that has any trouble sustaining that rate. - TRIM might introduce delays, it is very 'expensive' command. But that will surely wary by drive/manufacturer. - There is no way TRIM can influence reading from the flash media. Reading from L2ARC with low latency and high speed is it's main purpose anyway. > Remaining questions. > - Is there any advantage to using a spare partition on a SCSI or SATA > drive as L2Arc? Assuming it was in the machine already but doing nothing? Absolutely no advantage. You want L2ARC to be very low latency and high-bandwidth for random reading. Especially low-latency. This does not apply to rotating disks. Daniel