software RAID 1 w/3 Drives

[der.hans]

Am 24. Jul, 2007 schwätzte Harold Michels so:

> The question is, on a three drive software RAID set up will the third
> drive be a hot spare, or will it be a mirror.
>
> On the web pages mentioned earlier in this thread, on HOW-TO 4 or 5 I
> think, reference indicate that the RAID setups are software
> configurable. The drives can be set up with a hot spare, or with all of
> the drives except the first one as duplicate mirror drives. This is all
> under RAID 1.

Cool, I didn't realize you could specify that the 3rd drive also be a
mirror.

> The matter is written with some ambiguity on the first page, but is
> spelled out later.
>
> I am puzzled as to what benefit would be derived from having an extra
> drive hot and spinning all the time wearing out. If you have a failure
> wouldn't you be better off adding a new in at that time and rebuilding
> the data from there?

Double your redundancy double your fun?

I could see where it would be valuable, but then I'd think they're
reaching for a more redundant RAID level to begin with.

I could see the benefit of updating a copy of the mirror onto the hot
spare once in a while when drive load is down. If you do it when load is
down you're not slowing down the main system. Then, if you do need to
move to the hot spare you might reduce the time it takes to rebuild the
array as much of the data is already there. Depends on how the disks are
synced. Whether or not it's just faster to throw down a new copy rather
than having to slow for reads and comparisions would also affect the
usefulness of syncing the hot spare once in a while.

If you have a system that requires you to always have two copies of
whatever, you might need 3 copies in case one of the copies goes away.

For instance, there's the 'break the RAID' method of backup. Put in two
drives in RAID 1, when you need to do a backup remove one of the drives
from the RAID set and back it up, then reconnect it and let it sync. If
you could have 3 drives in the RAID 1 you could maintain your hot mirror
even when you've broken one drive out of the RAID.

Nope, I wouldn't know of any large technology company that was going to
use that mechanism to backup large databases for a proprietary database
instance. Nope, not at all :).

ciao,

der.hans
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