RAID questions (multiple arrays, global hot spares, controllers)

[Elliot Wilen]

Hello, these may be newb questions. Please be gentle.

I could use some help configuring a RHEL server to run 
Zimbra. I'm focusing on using a PowerEdge 2950. Zimbra 
generally recommends having at least one RAID 1 or RAID 10 
array, preferably with fast drives, although slower arrays 
(RAID 5 and/or slower drives) are acceptable for storing 
backups and older data. One example config would be RAID 1 
using a pair of 146 GB SAS drives, and a RAID 5 or RAID 10 
built from several inexpensive, large SATA drives. Or the 
second array could also be a RAID 1 if the drives are big 
enough.

I've been using Dell's configuration pages to explore my 
options, but I suspect there are some things I can do that 
just aren't offered pre-configured.

• I'd like to provide hot spares for the arrays. One 
attractive strategy would be to have a global hot spare; 
failing that, one spare per array. Dell's website doesn't 
mention hot spares at all or help out here in terms of 
requirements (drive size matching, controller(s), etc.). 
Assuming that a global hot spare could be configured, is 
it then necessary for all the drives in the system to 
match? Or can e.g. a 500 GB 7200 RPM SATA drive be a hot 
spare for a RAID 0 array made up of 146 GB 10,000 RPM SAS 
drives? Obviously there'd be wasted space in the event the 
spare was activated, and it would slow down the array; but 
it would at least offer protection until the 
malfunctioning drive was replaced.

• The website also doesn't give the option of configuring 
more than two RAID sets. Given the size of my 
installation, this may not be necessary, but it'd be good 
to know if e.g. I could somehow have up four RAID 1s.

• Is there any reason to use either mdadm or a SAS 6/iR, 
especially if a PERC 6/i is installed? E.g. to support 
more than two arrays, or to get some performance/feature 
benefit for certain types of array, or to do RAID 10 by 
mirroring in hardware while striping in software? I assume 
though that a global hot spare couldn't be shared between 
two different hw/sw controllers.

Thanks for any help.

Sincerely,

Elliot Wilen