Re: [PATCH] improve documentation of snapshot unaware defrag

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

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On Mon, 2015-12-28 at 02:51 +0000, Duncan wrote:
> 1) Btrfs very specifically and deliberately uses *lowercase* raidN
> in part to make that distinction, as the btrfs variants are chunk-
> level (and designed so that at some point in the future they can be
> subvolume and/or file level), not device-level (and at that future
> point, not necessarily filesystem level either).
I guess no "normal" user would expect or understand that lower/upper
case would imply any distinction.


> 2) Regarding btrfs raid1 and raid10's current very specific two-way-
> mirroring in particular, limiting to two-way-mirroring in the 3+
> devices 
> case is well within established definitions and historic usage.  
> Apparently, the N-devices = N-way-mirroring usage is relatively new, 
> arguably first popularized by Linux mdraid, after which various
> hardware 
> raid suppliers also implemented it due to competitive pressure.  But
> only 
> two-way-mirroring is required by the RAID-1 definition.
No, this is not true.

This http://www.eecs.berkeley.edu/Pubs/TechRpts/1987/CSD-87-391.pdf is
the original paper on RAID.
Chapter 7 describes RAID1 and the clearly says "all disks are
duplicated" as well as "Level 1 RAID has only one data disk".

I wouldn't know any single case of a HW RAID controller (and we've had
quite a few of them here at the Tier2) or other software implementation
where RAID1 had another meaning than "N disks, N mirrors".



> Even were that not the case, point #1, btrfs' very specific use of 
> *lowercase* raid1, still covers the two-way-limitation case just as
> well 
> as it covers the chunk-level case.
Hmm wouldn't still change anything, IMHO,... saying "lower case RAID is
something different than upper case RAID" would be just a bit ...
uhm... weird.

Actually, because btrfs doing it at the chunk level (while RAID being
at the device level), proves while my point that "raid" or "RAID" or
any other lower/upper case combination shouldn't be used at all.


Cheers,
Chris.

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