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 ?
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