Re: mkfs.btrfs limits "odd" [and maybe a "failed" phantom device?]

[Robert White <rwhite@pobox.com>]

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.

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...