Re: Mirrored Raid 5


Subject: Re: Mirrored Raid 5
From: Mike Tibor (tibor@lib.uaa.alaska.edu)
Date: Tue Nov 04 2003 - 14:12:21 AKST


On Tue, 4 Nov 2003, volz wrote:

>
> Hello,
>
> Does anyone have experience with mirrored raid 5's? Can linux scsi drivers and
> logical volume mangement encompass such a thing? Variously called 15, 5+1.
> Anybody know where that documentation lies? I googled "RAID 5 mirror" and got
> some hits but haven't found the substantive info. There is even a newsgroup
> post by someone using redhat 7.3 and the same hardware I want to use to build a
> 1 tbyte RAID 5. but no specifics
>
> Also is anyone aware of way(s) to put parity on a disk or mirrored disks and
> stripe the data w/o parity across an array? Does this provide any additional
> security benefit. Most importantly, would it be possible to survive the death of
> multiple array disks in this case?
>
> Or more genrally the question is: given 12 scsi disks, a linux kernel, couple of
> fast processors and decent scsi card, how much safe storage can I make? (safe
> --> Can we suffer two disks down?)

I've never tried that on Linux, but I think if I were going to do it I'd
get a raid card of some sort, and do one raid level in hardware and use
Linux software raid to handle the other level. For example, setup two
hardware raid5 arrays, and then mirror them in software. Or you could go
the other way and setup six 2-drive mirrors, and then software raid5 the
mirrors. I would think the latter method would be the better of the two
since it should give you better redundancy as well as (potentially) better
performance.

If all you want is the ability to survive two drive failures, then I would
think an even better route would be to just do a single raid5 array on 11
of the 12 disks, and designate the 12th as a hot spare. That way when the
first drive failure hits, the system would automatically rebuild the array
using the hot spare; then when the second drive failure hits you'd just be
operating in degraded mode.

Mike

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