From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lists.gentoo.org (pigeon.gentoo.org [208.92.234.80]) by finch.gentoo.org (Postfix) with ESMTP id B280C1381F3 for ; Fri, 21 Jun 2013 18:10:48 +0000 (UTC) Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id 23D96E0AD9; Fri, 21 Jun 2013 18:10:42 +0000 (UTC) Received: from catbert.rellim.com (catbert.rellim.com [204.17.205.1]) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by pigeon.gentoo.org (Postfix) with ESMTPS id 507F2E0AB8 for ; Fri, 21 Jun 2013 18:10:41 +0000 (UTC) Received: from localhost ([IPv6:2001:470:e815:0:230:48ff:fe34:5fe2]) (authenticated bits=0) by catbert.rellim.com (8.14.5/8.14.5) with ESMTP id r5LIAPRF010108 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NO); Fri, 21 Jun 2013 11:10:25 -0700 X-DKIM: OpenDKIM Filter v2.6.7 catbert.rellim.com r5LIAPRF010108 DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=rellim.com; s=catbert; t=1371838225; bh=/iMjN2p92f3uaFF4BJZzyCCyIRyqPoTclhsBdbbZPjs=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=UCQly0Rxmhqock4vbETyJ14ROQiRoIUEuvMiHzNIT5zVw7DqfSBDxWRpyB+gkx/z3 XeOj/cx0e+/W5NRnw0NZw== Date: Fri, 21 Jun 2013 11:10:24 -0700 From: "Gary E. Miller" To: gentoo-amd64@lists.gentoo.org Cc: rich0@gentoo.org Subject: Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value? Message-ID: <20130621111024.454abeeb.gem@rellim.com> In-Reply-To: References: Organization: Rellim X-Mailer: Claws Mail 3.9.0 (GTK+ 2.24.17; x86_64-pc-linux-gnu) Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-Id: Gentoo Linux mail X-BeenThere: gentoo-amd64@lists.gentoo.org Reply-to: gentoo-amd64@lists.gentoo.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg=PGP-SHA1; boundary="Sig_/iUR51GUyDt27798t._qsGVH"; protocol="application/pgp-signature" X-Archives-Salt: 5c4a753e-691d-401b-a5a7-18d9b2837532 X-Archives-Hash: 247c139307fbec20791724af0e820337 --Sig_/iUR51GUyDt27798t._qsGVH Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Yo Rich! On Fri, 21 Jun 2013 13:57:20 -0400 Rich Freeman wrote: > In theory it could be done that way, but every > raid1 implementation I've heard of makes writes to all drives > (obviously), but reads from only a single drive (assuming it is > correct). That means that read latency is greatly reduced since they > can be split across two drives which effectively means two heads per > "platter." Yes, that is what I see in practice. A much reduced average read time. And if you are really pressed for speed, add more stripes and get even more speed. > Also, raid1 typically does not include checksumming, so if > there is a discrepancy between the drives there is no way to know > which one is right. Uh, not exactly correct. Remember each HDD has ECC for each sector. If there is a read error the HDD will detecct the bad ECC and report the error to the RAID1 hardware/software. Then RAID1 is smart enough to try to read from the 2nd drive. > With raid5 at least you can always correct > discrepancies if you have all the disks Not really. If 2 disks fail in an n+1 RAID5 you are out of luck. Not as uncommon occurance as one might think. > (though as Duncan pointed out > in practice this only happens if you do an explicit scrub on mdadm). Which you should be doing at least weekly. Otherwise you only find out your disks have failed when you try to do a full copy or backup, and then you likely have multiple failures and you are out of luck. RGDS GARY --------------------------------------------------------------------------- Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701 gem@rellim.com Tel:+1(541)382-8588 --Sig_/iUR51GUyDt27798t._qsGVH Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.20 (GNU/Linux) iEYEARECAAYFAlHElxAACgkQBmnRqz71OvNQXwCbBG5agljhlUiaFOWX7eAAZ722 RDsAoOOwMVj4fUQSyKwMAzjdNc9flp5u =hjUh -----END PGP SIGNATURE----- --Sig_/iUR51GUyDt27798t._qsGVH--