On Thu, 11 Dec 2014, Robert White wrote:
On 12/11/2014 07:56 PM, Zygo Blaxell wrote:
RAID5 with even parity and two devices should be exactly the same as
RAID1 (i.e. disk1 ^ disk2 == 0, therefore disk1 == disk2, the striping
is irrelevant because there is no difference in disk contents so the
disks are interchangeable), except with different behavior when more
devices are added (RAID1 will mirror chunks on pairs of disks, RAID5
should start writing new chunks with N stripes instead of two).
That's not correct. A RAID5 with three elements presents two
_different_ sectors in each stripe. When one element is lost, it would
still present two different sectors, but the safety is gone.
The above quote is discussing two device RAID5, you are discussing
three device RAID5.
I understand that the XOR collapses into a mirror if only two datum
are involved, but that's a mathematical fact that is irrelevant to the
definition of a RAID5 layout. When you take a wheel off of a tricycle
it doesn't just become a bike. And you can't make a bicycle into a
trike by just welding on a wheel somewhere. The infrastructure of the
two is completely different.
True. A two-device RAID5 is not the same as a degraded three-device
RAID5.
So RAID5 with three media M is
M MM MMM
D1 D2 P(a)
D3 P(b) D4
P(c) D5 D6
If MMM is lost D1, D2, D3, and D5 are intact
D4 and D6 can be recreated via D3^P(b) and P(c)^D5
M MM X
D1 D2 .
D3 P(b) .
P(c) D5 .
So under _no_ circumstances would a two-disk RAID5 be the same as a
RAID1 since a two disk RAID5 functionally implies disk three because
the _minimum_ arity of a RAID5 is 3. A two-disk RAID5 has _zero_ data
protection because the minimum third element is a computational
phantom.
You again seem to be treating a "two disk RAID5" as synonymous with your
degraded three disk RAID5 above. It is not.
RAID5 with two media M would be:
M MM
D1 P(a)
P(b) D2
D3 P(c)
[and each P would be identical to its corresponding D]
In short it is irrational to have a "two disk" RAID5 that is "not
degraded" in the same way you cannot have a two-wheeled tricycle
without scraping some part of something along the asphalt.
There is nothing irrational about it at all, except that it is
exactly equivalent to two disk RAID1.
A RAID1 with two elements presents one sector along the "stripe".
A RAID5 with N elements presents N-1 sectors along the "stripe",
so I'm not sure what the problem is with setting N=2.
I realize that what has been implemented is what you call a two drive
RAID5, and done so by really implementing a RAID1, but it's nonsense.
It's not really, it's merely an argument of semantics if you want
to define it as nonsense.
I mean I understand what you are saying you've done, but it makes no
sense according to the definitions of RAID5. There is no circumstance
where RAID5 falls back to mirroring. Trying to implement RAID5 as an
extension of a mirroring paradigm would involve a fundamental conflict
in definitions. Especially when you reached a failure mode.
I have no idea what you mean by "a fundamental conflict in definition".
This is so fundamental to the design that the "fast" way to assemble a
RAID5 of N-arity (minimum N being 3) is to just connect the first N-1
elements, declare the raid valid-but-degraded using (N-1) of the
media, and then "replacing" the Nth phantom/missing/failed element
with the real disk and triggering a rebuild. This only works if you
don't need the initial contents of the array to have a specific value
like zero. (This involves fewest reads and the array is instantly
available while it builds.)
There is no reason you could not do exactly this with N=2.
As soon as you start writing to the array, the stripes you write
"repair" the extents if the repair process hadn't gotten to them yet.
Its basically impossible to turn a mirror into a RAID5 if you _ever_
expect the code base to to be able to recover an array that's lost an
element.
Again, I'm not really sure what you mean.
Uh, no. A raid 6 with three drives, or even two drives, is also
degraded because the minimum is four.
You're doing your weird semantic dance again. Just because you
define the minimum to be four does not mean that someone talking
about a three device RAID6 is talking about a degraded four device
RAID6, they're not.
As above, a non-degraded three-device RAID6 can be perfectly
sensibly defined. Once again, it has exactly the same failure
properties as a three device RAID1 (any two of the devices can
fail), so it's a bit pointless. But not "impossible"...
A B C D
D1 D2 Pa Qa
D3 Pb Qb D4
Pc Qc D5 D6
Qd D7 D8 Pd
You can lose one or two media but the minimum stripe is again [X1,X2]
for any read (ABCD)(ABC.)(AB..)(A..D) etc.
Minimum arity for RAID6 is 4, maximum lost-but-functional
configuration is arity-minus-two.
A B C
D1 Pa Qa
Pb Qb D2
Qc D3 Pc
D4 Pd Qd
They're only missing if you believe the minimum number of RAID5 disks
is not two and the minimum number of RAID6 disks is not three.
I do believe that, because that's what the terms are universally taken
to mean.
Apparently not universally.
--
David Taylor
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