Hi Hugo, On 03/09/2013 09:31 PM, 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 Even I found a bit more rational the "nCmSpP" format, I think that this is a bit too complex. As you told: > Chris is definitely planning fixed values for mS (so you can ask > for, say, exactly 4 stripes and one parity), and values for nC greater > than 2. As far as I know, there aren't any plans for nC > 1 and pP > 0 > together. I haven't got far enough into the kernel code to work out > whether that's simple or not to implement. On the basis of that we should handle few cases than the full "nCDmSpP" format allow. So I suggest to allow the following "shorts forms": - DUP -> dD (to allow more that 2 copy per disk) - RAID1 -> nC or *C - RAID0 -> mS or *S - RAID10 -> nCmS or *CmS or nC*s - RAID with parity -> mSpP or *SpP or mS*p (it is possible ?) - single -> 1C or 1D or 1S or "single" where d,n,m,p are integers; '*' is the literal '*' and means "how many possible". For example if I have 6 disks: *C2S -> means: 3 copies, 2 stripes ( m*n = 3*2 == num disk == 6) 2C*S -> means: 2 copies, 3 stripes ( m*n = 3*2 == num disk == 6) *S2P -> means: 2 parity, 4 stripes ( p+m = 4+2 == num disk == 6) We could also have some defaults, like: d=2, n=2, m=*, p=1, so some common forms become: D -> means DUP (d==2) S -> means RAID0 (s=*) C -> means RAID1 (n=2) CS -> means RAID10 (n=2, m=*) SP -> means RAID5 (m=*, p=1) SP2 -> means RAID6 (m=*, p=2) It would allowable also more complex form like 5C3D4S2P but I don't think it will be supported ever. I have a request: could we consider to swap letters with the numbers ? For example 3S2P could become S3P2. For my *subjective* point of view, this seems more clear. What do you think ? However I am open also to the following forms dD -> DUP(d), as backward compatibility DUP means d==2 nC -> RAID1(n), as backward compatibility RAID1 means n==2 mS -> RAID0(n), as backward compatibility RAID0 means m==* nCmS -> RAID10(n,m), as backward compatibility RAID10 means n=2, m=* mSpP -> RAIDP(n,m), as backward compatibility RAID6 means p=2, m=* RAID5 means p=1, m=* more verbose but also more familiar to the administrator. What do you think ? -- gpg @keyserver.linux.it: Goffredo Baroncelli (kreijackATinwind.it> Key fingerprint BBF5 1610 0B64 DAC6 5F7D 17B2 0EDA 9B37 8B82 E0B5 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
