Re: migrating from RAID5 to RAID10

[Drew <drew.kay@gmail.com>]

>> No, it is not degraded, it's a clean 2-drive RAID5. I know it doesn't
>> make much sense as it is ;-)  the intent was to grow the array later,
>> relying on mdadm's grow feature. Right now, I'm guessing that it
>> operates like a RAID1 for all practical purposes.
>
> Actually No. If it is RAID-5 as you claim, then currently it is
> effectively a RAID-0.
>
> RAID-5 is Striping with Distributed Parity and requires a *minimum* of
> 3 disks. Two disks, regardless of how you got there, is considered a
> degraded state.

Actually, I need to retract those statements. A clean two disk RAID-5
array can be created and is actually very similar to a RAID-1 array.

From this thread: http://ubuntuforums.org/archive/index.php/t-566803.html
---
If you're using just 2 disks in a non-degraded setup, then you'll have
the following structure
Drive1 Drive2
Block1: A1 P1
Block2: P2 B1
Block3: C1 P3
Block4: P4 D1

In this case, you're parity blocks will actually be identical to your
non-parity blocks. This is because P1 is calculated as P1 = A1 XOR ??
Since there is no other block to XOR your bits with, md falls back to
?? = 0, and we all know that the XOR of 0 is the identity. That is, P1
= A1 XOR 0 == A1
---

I stand corrected. :-)

-- 
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie
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