On Wed, Nov 20, 2013 at 04:43:57PM +0000, Duncan wrote: > Hugo Mills posted on Wed, 20 Nov 2013 08:09:58 +0000 as excerpted: > > > RAID-0: min 2 devices > > RAID-10: min 4 devices > > RAID-5: min 2 devices (I think) > > RAID-6: min 3 devices (I think) > > RAID-5 should be 3-device minimum (each stripe consisting of two data > segments and one parity segment, each on a different device).
You can successfully run RAID-5 on two devices: one data device(*), plus its parity. The parity check of a single piece of data is that data, so it's equivalent to RAID-1 in that configuration. IIRC, the MD-RAID code allows this; I can't remember if the btrfs RAID code does or not, but it probably should do if it doesn't. > And RAID-6 similarly four devices (two data and two parity). Similarly for RAID-6: it's a single data device(*), plus an XOR-based parity (effectively a mirror), plus a more complex parity calculation. > Perhaps it's time I get that wiki account and edit some of this stuff > myself... Do check the assumptions first. :) Hugo. (*) Yeah, OK, rotate the data/parity position as you move through the stripes because it's not RAID-4. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- In my day, we didn't have fancy high numbers. We had "nowt", --- "one", "twain" and "multitudes".
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