RE: hot spare disk

OK, I think we're splitting hairs here, and we're actually agreeing. In
RAID6, it's not a "spare" it is indeed part of the array, yes. But it's
a spare as in you can lose one and still be 100% protected.

We're not splitting hairs. There's a distinct difference in what each term means.

"Spare" implicitly means optional. You don't have to have a hot-spare drive in a RAID6 (or RAID5,
RAID1, RAID1E, RAID5E, RAID5EE, RAID10, RAID1E0, RAID50, RAID60) system. Just like you don't have to
have a spare tire. But you can't have a RAID6 system without 2 parity drives.

Here's a nice RAID6 + hot spares scenario:
http://publib.boulder.ibm.com/infocenter/eserver/v1r2/index.jsp?topic=/diricinfo/fqy0_cusehs.html

or http://tinyurl.com/6em3el

In RAID5 you can have a true "spare" in the machine, as it it's not in
use by anything but it's there. However, I would not consider it "hot"
since it's not available at the time of the event (T0). Even if moved
into the array at T0, there is array rebuild time, call it N, such that
you're _unprotected_ from T0 to T0+N, hence not "hot". Warm? Yes. Hot,
no. :)

"hot" meaning the drive is installed, powered up and spinning, but with no data on it. The system can
automatically start reconstructing a failed drive onto it.

"warm" or stand-by meaning the drive is installed but not powered on. The system can automatically
start reconstructing a failed drive onto it.

"cold" would mean the drive is in a closet or desk drawer somewhere and somebody will need to install
it.

The key reason to choose "hot" vs. "warm" is that with a "hot" spare you know the drive is working.
With a "warm" drive, there's always a possibility that it will fail to come up when its needed.
Downside to a "hot" spare is of course that you're putting hours on the drive.

HTH,

Charles Wilt
Software Engineer
CINTAS Corporation - IT 92B
513.701.1307
wiltc@xxxxxxxxxx




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