there's 2 approaches:
1) RAID 1+Z where you mirror the individual drives across trays and
then RAID-Z the whole thing
2) RAID Z+1 where you RAIDZ each tray and then mirror them
I would argue that you can lose the most drives in configuration 1
and stay alive:
With a simple mirrored stripe you lose if you lose 1 drives in each
tray.
With configuration 2 this takes it 2 drives in each tray.
With configuration 1 you have to lose both sides of a 2 mirrored sets
to fail.
so it's not a space or performance model .. simply an availability
model with failing disk
Jonathan
On Oct 24, 2006, at 12:46, Richard Elling - PAE wrote:
Pedantic question, what would this gain us other than better data
retention?
Space and (especially?) performance would be worse with RAID-Z+1
than 2-way mirrors.
-- richard
Frank Cusack wrote:
On October 24, 2006 9:19:07 AM -0700 "Anton B. Rang"
<[EMAIL PROTECTED]> wrote:
Our thinking is that if you want more redundancy than RAID-Z,
you should
use RAID-Z with double parity, which provides more reliability
and more
usable storage than a mirror of RAID-Zs would.
This is only true if the drives have either independent or identical
failure modes, I think. Consider two boxes, each containing ten
drives.
Creating RAID-Z within each box protects against single-drive
failures.
Mirroring the boxes together protects against single-box failures.
But mirroring also protects against single-drive failures.
-frank
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