Short answer:
Of course you can hot plug disks in a mirroring configuration.
Long answer:
These misunderstandings usually come from the fact that the System i has
a longer history than PC servers, and terms are used to interchangeably
these days that it's hard to tell what is what.
RAID is a generic term, meaning Redundant Array of Independent Disks.
There's nothing inherently describing in this term how the redundancy is
achieved, but when System i folks talk about RAID, they usually mean
RAID5. Since IBM has introduced controllers that support RAID6, they've
started to adapt their configuration.
Now, there's a RAID configuration called "RAID1" - which just mirrors
two disks.
However, Mirroring in i5/OS can do much more than a single RAID
controller that mirrors two disks - the System i can achieve disk
redundancy across controllers, busses, and even towers.
Now, both RAID5 and Mirroring in i5/OS have come a long way, and both
required downtime for disk replacements at the dawn of time (long before
I was born). But this isn't the case nowadays.
What the System i can't do, and enterprise SANs usually CAN do is IO
Multipathing, essentially multiple paths that lead to the same disk, it
seems that the System i folks thought that mirroring is enough.
Based on the information i've found with a few quick googles searches,
even a 595's Storage System is _NOT_ redundant if you're not using
Mirroring. Mirroring can protect you from Bus, Controller, Tower
failure, if designed correctly. However, with Mirroring your only disk
configuration option is RAID1, which means you have to buy almost twice
as many disks to as with RAID5/RAID6.
And on the Small Business Side: Mirroring is the only thing you can
afford with two 70GB spindles.
-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[
mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of rob@xxxxxxxxx
Sent: Monday, June 04, 2007 10:30 PM
To: Midrange Systems Technical Discussion
Subject: RE: CACHE BATT EXP 90 DAYS???
Honestly I've lost several disk drives under RAID. However, I've
experienced no downtime because of it. The drives were hot replaced and
the users didn't notice a thing. Just trying to find out what the big
advantage is to mirroring. I thought with mirroring when you lost a
drive, you still didn't lose data, but you did have to drop the system
to
replace the drive. I hope I am misinformed because I fail to see the
advantage in that. Perhaps that's just how it used to be?
Rob Berendt
midrange.com
