From owner-freebsd-fs@FreeBSD.ORG Tue May 26 18:59:04 2009 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 E02A21065672 for ; Tue, 26 May 2009 18:59:04 +0000 (UTC) (envelope-from peterjeremy@optushome.com.au) Received: from mail18.syd.optusnet.com.au (mail18.syd.optusnet.com.au [211.29.132.199]) by mx1.freebsd.org (Postfix) with ESMTP id 6AA508FC16 for ; Tue, 26 May 2009 18:59:03 +0000 (UTC) (envelope-from peterjeremy@optushome.com.au) Received: from server.vk2pj.dyndns.org (c122-106-216-167.belrs3.nsw.optusnet.com.au [122.106.216.167]) by mail18.syd.optusnet.com.au (8.13.1/8.13.1) with ESMTP id n4QIx1p4010214 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Wed, 27 May 2009 04:59:01 +1000 X-Bogosity: Ham, spamicity=0.000000 Received: from server.vk2pj.dyndns.org (localhost.vk2pj.dyndns.org [127.0.0.1]) by server.vk2pj.dyndns.org (8.14.3/8.14.3) with ESMTP id n4QIx0wP098773; Wed, 27 May 2009 04:59:00 +1000 (EST) (envelope-from peter@server.vk2pj.dyndns.org) Received: (from peter@localhost) by server.vk2pj.dyndns.org (8.14.3/8.14.3/Submit) id n4QIx03a098772; Wed, 27 May 2009 04:59:00 +1000 (EST) (envelope-from peter) Date: Wed, 27 May 2009 04:59:00 +1000 From: peterjeremy@optushome.com.au To: Artis Caune Message-ID: <20090526185900.GA98171@server.vk2pj.dyndns.org> References: <9e20d71e0905230537ibcaf852g1dc32b6ffc3a681d@mail.gmail.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="9jxsPFA5p3P2qPhR" Content-Disposition: inline In-Reply-To: <9e20d71e0905230537ibcaf852g1dc32b6ffc3a681d@mail.gmail.com> X-PGP-Key: http://members.optusnet.com.au/peterjeremy/pubkey.asc User-Agent: Mutt/1.5.19 (2009-01-05) Cc: Randy Bush , freebsd-fs Subject: Re: raidz2 a bit big 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: Tue, 26 May 2009 18:59:05 -0000 --9jxsPFA5p3P2qPhR Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2009-May-23 15:37:14 +0300, Artis Caune wrote: >2009/5/23 Randy Bush : >> a dozen 2tb drives in a raidz2 >Reads on such configurations are very slow. Not really. Assuming each disk is capable of X IOPS and assuming non-degraded mode, you can still issue (N-2)*X random reads/sec because the parity stripes are not needed/used. (Compared to N*X random reads/sec for a mirrored configuration). Degraded reads _are_ very slow because you need to read most of the spindles (I'm not sure of the exact recovery mechanism for RAIDZ2 but it's probably close to X random reads/sec). Write performance _is_ poor - a write requires at least 2 (and maybe 3) physical reads and 3 physical writes. But you can still perform multiple parallel writes across the RAIDZ2 set, giving you roughly N*X/6 random writes/sec. (Compared to N*X/2 random writes/sec for a mirrored configuration). In general, you would not choose a RAIDZ{1,2} configuration where write performance was an issue - the benefit you get is greater storage utilisation. >with 4 disks in raidz2 (total 3 raidz2 vdevs) you get 300 IOPS Actually 600 reads/sec if all the sets are non-degraded. Unless you are particularly worried by dual disk failures, you would probably be better off using a 2+2 disk mirror configuration, rather than a 4-disk RAIDZ2 setup. --=20 Peter Jeremy --9jxsPFA5p3P2qPhR Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.11 (FreeBSD) iEYEARECAAYFAkocO/QACgkQ/opHv/APuIf3cQCghUaytxCzyWDGj0/G+HEsQVG7 xQ0An33d5I184PAIH5VZzAXN7Adru/S/ =bAY5 -----END PGP SIGNATURE----- --9jxsPFA5p3P2qPhR--