From owner-freebsd-questions@FreeBSD.ORG Thu Aug 7 08:31:56 2014 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 76F13B60 for ; Thu, 7 Aug 2014 08:31:56 +0000 (UTC) Received: from sdf.lonestar.org (mx.sdf.org [192.94.73.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client CN "mx.sdf.org", Issuer "SDF.ORG" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id 558EE25CF for ; Thu, 7 Aug 2014 08:31:55 +0000 (UTC) Received: from sdf.org (IDENT:bennett@sdf.lonestar.org [192.94.73.15]) by sdf.lonestar.org (8.14.8/8.14.5) with ESMTP id s778VhGa004559 (using TLSv1/SSLv3 with cipher DHE-RSA-AES256-GCM-SHA384 (256 bits) verified NO); Thu, 7 Aug 2014 08:31:44 GMT Received: (from bennett@localhost) by sdf.org (8.14.8/8.12.8/Submit) id s778VhJc015365; Thu, 7 Aug 2014 03:31:43 -0500 (CDT) From: Scott Bennett Message-Id: <201408070831.s778VhJc015365@sdf.org> Date: Thu, 07 Aug 2014 03:31:43 -0500 To: freebsd@qeng-ho.org Subject: Re: gvinum raid5 vs. ZFS raidz References: <201408020621.s726LsiA024208@sdf.org> <53DCDBE8.8060704@qeng-ho.org> <201408060556.s765uKJA026937@sdf.org> <53E1FF5F.1050500@qeng-ho.org> In-Reply-To: <53E1FF5F.1050500@qeng-ho.org> User-Agent: Heirloom mailx 12.4 7/29/08 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: freebsd-questions@freebsd.org X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 07 Aug 2014 08:31:56 -0000 Arthur Chance wrote: > On 06/08/2014 06:56, Scott Bennett wrote: > > Arthur Chance wrote: > >> > >> [stuff deleted --SB] > > I wonder if what varies is the amount of space taken up by the > > checksums. If there's a checksum for each block, then the block size > > would change the fraction of the space lost to checksums, and the parity > > for the checksums would thus also change. Enough to matter? Maybe. > > I'm not a file system guru, but my (high level) understanding is as > follows. Corrections from anyone more knowledgeable welcome. > > 1. UFS and ZFS both use tree structures to represent files, with the > data stored at the leaves and bookkeeping stored in the higher nodes. > Therefore the overhead scales as the log of the data size, which is a > negligible fraction for any sufficiently large amount of data. > > 2. UFS doesn't have data checksums, it relies purely on the hardware > checksums. (This is the area I'm least certain of.) What hardware checksums are there? I wasn't aware that this sort of hardware kept any. > > 3. ZFS keeps its checksums in a Merkel tree > (http://en.wikipedia.org/wiki/Merkle_tree) so the checksums are held in > the bookkeeping blocks, not in the data blocks. This simply changes the > constant multiplier in front of the logarithm for the overhead. Also, I > believe ZFS doesn't use fixed size data blocks, but aggregates writes > into blocks of up to 128K. > > Personally, I don't worry about the overheads of checksumming as the > cost of the parity stripe(s) in raidz is dominant. It's a cost well > worth paying though - I have a 3 disk raidz1 pool and a disk went bad > within 3 months of building it (the manufacturer turned out to be having > a few problems at the time) but I didn't lose a byte. > Good testimonial. I'm not worried about the checksum space either. I figure the benefits make it cheap at the price. Of more concern to me now is how I'm going to come up with at least two more 2 TB drives to set up a raidz2 with a tolerably small fraction of the total space being tied up in combined ZFS overhead (i.e., bookkeeping, parity, checksums, etc.) Scott Bennett, Comm. ASMELG, CFIAG ********************************************************************** * Internet: bennett at sdf.org *xor* bennett at freeshell.org * *--------------------------------------------------------------------* * "A well regulated and disciplined militia, is at all times a good * * objection to the introduction of that bane of all free governments * * -- a standing army." * * -- Gov. John Hancock, New York Journal, 28 January 1790 * **********************************************************************