From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Stromberg Subject: Re: RAID 5 of RAID 5's? Date: Mon, 13 Jun 2005 10:55:44 -0700 Message-ID: <1118685345.25437.12.camel@seki.nac.uci.edu> References: <1118187572.15459.96.camel@seki.nac.uci.edu> <42A93D30.6080900@apartia.fr> <1118436219.25226.252.camel@seki.nac.uci.edu> <20050612004523.GJ750@strugglers.net> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="=-88HvNUE8kovGPIp9Xn4X" Return-path: In-Reply-To: <20050612004523.GJ750@strugglers.net> Sender: linux-raid-owner@vger.kernel.org To: Andy Smith Cc: strombrg@dcs.nac.uci.edu, linux-raid@vger.kernel.org List-Id: linux-raid.ids --=-88HvNUE8kovGPIp9Xn4X Content-Type: text/plain Content-Transfer-Encoding: quoted-printable On Sun, 2005-06-12 at 00:45 +0000, Andy Smith wrote: > On Fri, Jun 10, 2005 at 01:43:39PM -0700, Dan Stromberg wrote: > > Consider: > >=20 > > You have a bunch of "bricks" that can shuffle data between a NAS head > > and a bunch of disks. > >=20 > > The disks are RAID'd (through the "bricks"), but if one of the bricks > > themselves dies, you're kinda stuck. > >=20 > > But if you RAID 5 the RAID 5's, then you don't end up with massive > > parity pounding, and your bricks aren't a single point of failure, and > > you don't lose as much space as if you mirrored. >=20 > Off the top of my head, this is what I am thinking, but I could well > have missed something... >=20 > Assume you have 50 disks. I guess we're talking about 200 or so, but the ideas are pretty similar in either case I imagine. > If you organise them as 10 five disk RAID 5s and then RAID 5 the > RAID 5s you end up with the capacity of (5-1)*(10-1)=3D36 disks. > Depending on your RAID technology, reads may be as fast as a 10-way > stripe. As far as I can see though, a write would have to be > striped to 10 RAID 5s, which would itself be striped to 5 disks > each, so it would be a 50-way write. Well... Does RAID not usually read from n-1 disks, and write to 2, in a typical RAID 5 config when writing a single block? > So, unless I have misunderstood, depending on how you split the RAID > 5s you'll get about 75% of the disk as opposed to 50% for RAID 10, > but the write performance and the reliability seem much worse. Hmmmmmm... What about these forms of RAID that are supposed to be able to say, lose n-4 disks for an up-to-4-disk failure? What would they be like on a 200 disk RAID array? --=-88HvNUE8kovGPIp9Xn4X Content-Type: application/pgp-signature; name=signature.asc Content-Description: This is a digitally signed message part -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.6 (GNU/Linux) iD8DBQBCrcigo0feVm00f/8RAmoCAJ4pNp6dgqcWfMHFeVzOR4I2ko0pJgCglaJn 9X+FpFBAJZ2vpYhtP9NaV2A= =ixbF -----END PGP SIGNATURE----- --=-88HvNUE8kovGPIp9Xn4X--