From owner-freebsd-questions@FreeBSD.ORG Tue Jul 14 08:24:00 2009 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 44B601065670 for ; Tue, 14 Jul 2009 08:24:00 +0000 (UTC) (envelope-from m.seaman@infracaninophile.co.uk) Received: from smtp.infracaninophile.co.uk (gate6.infracaninophile.co.uk [IPv6:2001:8b0:151:1::1]) by mx1.freebsd.org (Postfix) with ESMTP id BE4A98FC18 for ; Tue, 14 Jul 2009 08:23:59 +0000 (UTC) (envelope-from m.seaman@infracaninophile.co.uk) Received: from happy-idiot-talk.infracaninophile.co.uk (localhost [IPv6:::1]) (authenticated bits=0) by smtp.infracaninophile.co.uk (8.14.3/8.14.3) with ESMTP id n6E8NpgL032660; Tue, 14 Jul 2009 09:23:53 +0100 (BST) (envelope-from m.seaman@infracaninophile.co.uk) X-DKIM: Sendmail DKIM Filter v2.8.3 smtp.infracaninophile.co.uk n6E8NpgL032660 DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=infracaninophile.co.uk; s=200708; t=1247559833; bh=4V35HxiEjpTSnNoublywwqxxFgEaQAaBF9LJ7vzxNes=; h=Message-ID:Date:From:MIME-Version:To:CC:Subject:References: In-Reply-To:Content-Type:Cc:Content-Type:Date:From:In-Reply-To: Message-ID:Mime-Version:References:To; z=Message-ID:=20<4A5C4091.3030208@infracaninophile.co.uk>|Date:=20T ue,=2014=20Jul=202009=2009:23:45=20+0100|From:=20Matthew=20Seaman= 20|Organization:=20Infracaninophi le|User-Agent:=20Thunderbird=202.0.0.22=20(X11/20090625)|MIME-Vers ion:=201.0|To:=20mahlerrd@yahoo.com|CC:=20Free=20BSD=20Questions=2 0list=20|Subject:=20Re:=20ZFS=20or= 20UFS=20for=204TB=20hardware=20RAID6?|References:=20<42310.1585.qm @web51008.mail.re2.yahoo.com>|In-Reply-To:=20<42310.1585.qm@web510 08.mail.re2.yahoo.com>|X-Enigmail-Version:=200.95.6|Content-Type:= 20multipart/signed=3B=20micalg=3Dpgp-sha256=3B=0D=0A=20protocol=3D "application/pgp-signature"=3B=0D=0A=20boundary=3D"------------eni g8D44B299A69B95A1567A379A"; b=CKkDb+6QNy+7aviqxxGXbmcRUJw0g/iLUaNJpEs8+QYk2XP8MZGC0bm0hxsMz7Wo0 qOjf7ns4C8tXO+04yvnycLqXhTBP7p3RUp2K4DwgWWOQUdZOnhMrJ3HL4AFuMf9+E9 Zdp0paBJzwMu/gIhYl+VONGlaIVlX7oEC8f5c7vg= X-Authentication-Warning: happy-idiot-talk.infracaninophile.co.uk: Host localhost [IPv6:::1] claimed to be happy-idiot-talk.infracaninophile.co.uk Message-ID: <4A5C4091.3030208@infracaninophile.co.uk> Date: Tue, 14 Jul 2009 09:23:45 +0100 From: Matthew Seaman Organization: Infracaninophile User-Agent: Thunderbird 2.0.0.22 (X11/20090625) MIME-Version: 1.0 To: mahlerrd@yahoo.com References: <42310.1585.qm@web51008.mail.re2.yahoo.com> In-Reply-To: <42310.1585.qm@web51008.mail.re2.yahoo.com> X-Enigmail-Version: 0.95.6 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="------------enig8D44B299A69B95A1567A379A" X-Virus-Scanned: clamav-milter 0.95.2 at happy-idiot-talk.infracaninophile.co.uk X-Virus-Status: Clean X-Spam-Status: No, score=-3.0 required=5.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VERIFIED,NO_RELAYS autolearn=ham version=3.2.5 X-Spam-Checker-Version: SpamAssassin 3.2.5 (2008-06-10) on happy-idiot-talk.infracaninophile.co.uk Cc: Free BSD Questions list Subject: Re: ZFS or UFS for 4TB hardware RAID6? X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 14 Jul 2009 08:24:00 -0000 This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig8D44B299A69B95A1567A379A Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable Richard Mahlerwein wrote: =20 > With 4 drives, you could get much, much higher performance out of > RAID10 (which is alternatively called RAID0+1 or RAID1+0 depending on > the manufacturer Uh -- no. RAID10 and RAID0+1 are superficially similar but quite differe= nt things. The main differentiator is resilience to disk failure. RAID10 ta= kes the raw disks in pairs, creates a mirror across each pair, and then strip= es across all the sets of mirrors. RAID0+1 divides the raw disks into two e= qual sets, constructs stripes across each set of disks, and then mirrors the two stripes. Read/Write performance is similar in either case: both perform well for=20 the sort of small randomly distributed IO operations you'ld get when eg. running a RDBMS. However, consider what happens if you get a disk failur= e. In the RAID10 case *one* of your N/2 mirrors is degraded but the other N-= 1 drives in the array operate as normal. In the RAID0+1 case, one of the 2 stripes is immediately out of action and the whole IO load is carried b= y the N/2 drives in the other stripe. Now consider what happens if a second drive should fail. In the RAID10 case, you're still up and running so long as the failed drive is one of the N-2 disks that aren't the mirror pair of the 1st failed drive. In the RAID0+1 case, you're out of action if the 2nd disk to fail is one of the N/2 drives from the working stripe. Or in other words, if two random disks fail in a RAID10, chances are the RAID will still work. If two arbitrarily selected disks fail in a RAID0+1 chances are basically even that the whole RAID is out of action[*]. I don't think I've ever seen a manufacturer say RAID1+0 instead of RAID10= , but I suppose all things are possible. My impression was that the 0+1=20 terminology was specifically invented to make it more visually distinctiv= e -- ie to prevent confusion between '01' and '10'. Cheers, Matthew [*] Astute students of probability will point out that this really only makes a difference for N > 4, and for N=3D4 chances are evens either way = that failure of two drives would take out the RAID. --=20 Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard Flat 3 PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate Kent, CT11 9PW --------------enig8D44B299A69B95A1567A379A Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.12 (FreeBSD) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEAREIAAYFAkpcQJcACgkQ8Mjk52CukIz1JQCeNeREHTenaloe/RskSFVGLMRf srwAoImZpbdpWoU2QXiC7scW7lJmfyYM =J4t9 -----END PGP SIGNATURE----- --------------enig8D44B299A69B95A1567A379A--