On Fri, Dec 12, 2014 at 03:16:03AM -0800, Robert White wrote: > On 12/12/2014 01:06 AM, David Taylor wrote: > >The above quote is discussing two device RAID5, you are discussing > >three device RAID5. > > Heresy! (yes, some humor is required here.) > > There is no such thing as a "two device RAID5". That's what RAID1 is for. > > Saying "The above quote is discussing a two device RAID5" is exactly > like saying "The above quote is discussing a two wheeled tricycle". > > You might as well be talking about three-octet IP addresses. That is > you could make a network address out of three octets, but it > wouldn't' be an IP address. It would be something else with the > wrong name attached.
OK. Sounds like I need to dust off the change-of-nomenclature patch again. The argument here is about the 1c1s1p configuration. Is there a problem with that? Hugo. > I challenge you... nay I _defy_ you... to find a single authority on > disk storage anywhere on this planet (except, apparently, this list > and its directly attached people and materials) that discusses, > describes, or acknowledges the existence of a "two device RAID5" > while not discussing a system with an arity of 3 degraded by the > absence of one media. > > All these words have standardized definitions. > > [That's not hyperbole. I searched for several hours and could not > find _any_ reference anywhere to construction of a RAID5 array using > only two devices that did not involve airity-3 and a > dummy/missing/failed psudo target. So if you can find any reference > to doing this _anywhere_ outside of BTRFS I'd like to see it. > Genuinely.] > > THAT SAID... > > I really can find no reason the math wouldn't work using only two > drives. It would be a terrific waste of CPU cycles and storage space > to construct the stripe buffers and do the XORs instead of just > copying the data, but the math would work. > > So, um, "well I'll be damned". > > Perhaps is just a tautological belief that someone here didn't buy > into. Like how people keep partitioning drives into little slices > for things because thats the preserved wisdom from early eighties. > > I think constructing a non-degraded-mode two device thing and > calling it RAID5 will surprise virtually _everyone_ on the planet. > > In every other system. And I do mean _every_ other system, if I had > two media and I put them under RAID-5 I'd be required to specify the > third drive as some sort failed device (the block device equivalent > of /dev/null but that returns error results for all operations > instead of successes.) See the reserved keyword "missing" in the > mdadm documentation etc. > > That is, If I put two 1TiB disks into a RAID-5 I'd expect to get a > 2TiB array with no actual redundancy. As in > > mdadm --create md0 --level=r5 --raid-devices=3 /dev/sda missing /dev/sdc > > the resulting array would be the same effective size as a stripe of > the two drives, but when the third was added later it would just > slot in as a replacement for the missing device and the airity-3 > thing would "reestablish" it's redundancy. (this is actually what > mdadm does internally with a normal build, it blesses the first N-1 > drives into an array with a missing member, and adds the Nth drive > as a "spare" and then the spare is immediately adopted as a > replacement for the "missing" drive.) > > The parity computation on a single value is just nutty waste of time > though. "Backing it out" when the array is degraded is double-nuts. > > Maybe everybody just decided it was too crazy to consider for the > CPU time penalty...? > > So yea, semantics... apparently... -- Hugo Mills | There's an infinite number of monkeys outside who hugo@... carfax.org.uk | want to talk to us about this new script for Hamlet http://carfax.org.uk/ | they've worked out! PGP: 65E74AC0 | Arthur Dent
signature.asc
Description: Digital signature
