From owner-freebsd-fs@FreeBSD.ORG Mon Jan 10 17:30:42 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 71AC21065672; Mon, 10 Jan 2011 17:30:42 +0000 (UTC) (envelope-from bra@fsn.hu) Received: from people.fsn.hu (people.fsn.hu [195.228.252.137]) by mx1.freebsd.org (Postfix) with ESMTP id BCB428FC12; Mon, 10 Jan 2011 17:30:41 +0000 (UTC) Received: by people.fsn.hu (Postfix, from userid 1001) id 2717C7061F7; Mon, 10 Jan 2011 18:30:40 +0100 (CET) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.2 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MF-ACE0E1EA [pR: 14.1618] X-CRM114-CacheID: sfid-20110110_18303_780BD314 X-CRM114-Status: Good ( pR: 14.1618 ) X-Spambayes-Classification: ham; 0.00 Message-ID: <4D2B423F.2000403@fsn.hu> Date: Mon, 10 Jan 2011 18:30:39 +0100 From: Attila Nagy User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.23) Gecko/20090817 Thunderbird/2.0.0.23 Mnenhy/0.7.6.0 MIME-Version: 1.0 To: Pawel Jakub Dawidek References: <4D0A09AF.3040005@FreeBSD.org> <4D1F7008.3050506@fsn.hu> <4D29A198.4070107@fsn.hu> <20110110085756.GA1744@garage.freebsd.pl> In-Reply-To: <20110110085756.GA1744@garage.freebsd.pl> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: freebsd-fs@freebsd.org, freebsd-stable@freebsd.org Subject: Re: New ZFSv28 patchset for 8-STABLE 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: Mon, 10 Jan 2011 17:30:42 -0000 On 01/10/2011 09:57 AM, Pawel Jakub Dawidek wrote: > On Sun, Jan 09, 2011 at 12:52:56PM +0100, Attila Nagy wrote: > [...] >> I've finally found the time to read the v28 patch and figured out the >> problem: vfs.zfs.l2arc_noprefetch was changed to 1, so it doesn't use >> the prefetched data on the L2ARC devices. >> This is a major hit in my case. Enabling this again restored the >> previous hit rates and lowered the load on the hard disks significantly. > Well, not storing prefetched data on L2ARC vdevs is the default is > Solaris. For some reason it was changed by kmacy@ in r205231. Not sure > why and we can't ask him now, I'm afraid. I just sent an e-mail to What happened to him? > Brendan Gregg from Oracle who originally implemented L2ARC in ZFS why > this is turned off by default. Once I get answer we can think about > turning it on again. > I think it makes some sense as a stupid form of preferring random IO in the L2ARC instead of sequential. But if I rely on auto tuning and let prefetch enabled, even a busy mailserver will prefetch a lot of blocks and I think that's a fine example of random IO (also, it makes the system unusable, but that's another story). Having this choice is good, and in this case enabling this makes sense for me. I don't know any reasons about why you wouldn't use all of your L2ARC space (apart from sparing the quickly wearing out flash space and move disk heads instead), but I'm sure Brendan made this choice with a good reason. If you get an answer, please tell us. :) Thanks,