On Tue, Dec 18, 2012 at 01:20:20PM +0200, Brendan Hide wrote: > On 2012/12/17 06:23 PM, Hugo Mills wrote: > >On Mon, Dec 17, 2012 at 04:51:33PM +0100, Sebastien Luttringer wrote: > >>Hello, > snip > >>I get the feeling that RAID1 only allow one disk removing. Which is more > >>a RAID5 feature. > > The RAID-1 support in btrfs makes exactly two copies of each item > >of data, so you can lose at most one disk from the array safely. Lose > >any more, and you're likely to have lost data, as you've found out. > >>I'm afraid Btrfs raid1 will not be working before the end of the world. > > It does work (as you demonstrated with the first disk being > >removed) -- but just not as you thought it should. Now, you can argue > >that "RAID-1" isn't a good name to use here, but there's no good name > >in RAID terminology to describe what we actually have here. > Technically, btrfs's "RAID1" implementation is much closer to RAID1E > than traditional RAID1. See > http://en.wikipedia.org/wiki/Non-standard_RAID_levels#RAID_1E or > http://pic.dhe.ibm.com/infocenter/director/v5r2/index.jsp?topic=/serveraid_9.00/fqy0_craid1e.html > > Perhaps a new name, as with ZFS, might be appropriate. RAID-Z and > RAID-Z2, for example, could not adequately be described by any > existing RAID terminology and, technically, RAID-Z still isn't a > RAID in the classical sense.
Yeah, we did have a naming scheme proposed, with combinations of nCmSpP, where n is the number of copies held, m the number of stripes, and p the number of parity stripes. So btrfs RAID-1 is 2C, RAID-5 on 5 disks would be 4S1P, and RAID-10 on 4 disks would be 2C2S. I'd prefer to see that than some non-"standard" RAID-18KTHXBYE formulation. Plenty of room for shed-painting here, though. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- I believe that it's closely correlated with --- the aeroswine coefficient.
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