On Sun, Mar 10, 2013 at 11:55:10PM +0000, sam tygier wrote: > On 09/03/13 20:31, Hugo Mills wrote: > > Some time ago, and occasionally since, we've discussed altering the > > "RAID-n" terminology to change it to an "nCmSpP" format, where n is the > > number of copies, m is the number of (data) devices in a stripe per copy, > > and p is the number of parity devices in a stripe. > > > > The current kernel implementation uses as many devices as it can in the > > striped modes (RAID-0, -10, -5, -6), and in this implementation, that is > > written as "mS" (with a literal "m"). The mS and pP sections are omitted > > if the value is 1S or 0P. > > > > The magic look-up table for old-style / new-style is: > > > > single 1C (or omitted, in btrfs fi df output) > > RAID-0 1CmS > > RAID-1 2C > > DUP 2CD > > RAID-10 2CmS > > RAID-5 1CmS1P > > RAID-6 1CmS2P > > Are these the only valid options?
Currently, yes. > Are 'sensible' new levels (eg 3C, mirrored to 3 disk or 1CmS3P, like > raid6 with but with 3 parity blocks) allowed? Not right now, but: - I don't know if the forthcoming 3c code will allow arbitrary values or not, but Chris has promised 3c. - Fixed S will definitely happen for the parity-RAID levels. I'm not sure about the stripe-RAID levels. - Higher P are mathematically possible, but (AIUI) awkward to construct efficient and effective ones (and it's a manual process to do so). I suspect that 3p may happen, but 4p may not for a long time. > Are any arbitrary levels allowed (some other comments in the thread > suggest no)? Currently, no, and I don't think there are immediate plans to generalise it, but I'd like to see that happen eventually. > Will there be a recommended (or supported) set? Quite likely, even with the limited (forthcoming) set of parameters. Using mSpP on an array of larger than some particular size is probably not going to be particularly good for performance, for example. Hugo. -- === 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 --- You stay in the theatre because you're afraid of having no --- money? There's irony...
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