Re: btrfs

[Christoph Anton Mitterer <calestyo@scientia.net>]

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On Sat, 2016-06-04 at 11:00 -0600, Chris Murphy wrote:
> SNIA's DDF 2.0 spec Rev 19
> page 18/19 shows 'RAID-1 Simple Mirroring" vs "RAID-1 Multi-
> Mirroring"

And DDF came how many years after the original RAID paper and everyone
understood RAID1 as it was defined there? 1987 vs. ~2003 or so?

Also SINA's "standard definition" seems pretty strange, doesn't it?
They have two RAID1, as you say:
- "simple mirroring" with n=2
- "multi mirrioring" with n=3

I wouldn't see why the n=2 case is "simpler" than the n=3 case, neither
why the n=3 case is multi and the n=2 is not (it's also already
multiple disks).
Also why did they allow n=3 but not n>=3? If n=4 wouldn't make sense,
why would n=3, compared to n=2?

Anyway,...
- the original paper defines it as n mirrored disks
- Wikipedia handles it like that
- the already existing major RAID implementation (MD) in the Linux
  kernel handles it like that
- LVM's native mirroring, allows to set the number of mirrors, i.e. it
  allows for everything >=2 which is IMHO closer to the common meaning
  of RAID1 than to btrfs' two-duplicates

So even if there would be some reasonable competing definition (and I
don't think the rather proprietary DDF is very reasonable here), why
using one that is incomptabible with everything we have in Linux?


Cheers,
Chris.

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