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Re: [K12OSN] Linux "Software RAID"



Almquist Burke wrote:
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On Aug 9, 2008, at 8:19 AM, David Hopkins wrote:

I"ve enjoyed reading this thread, but have a question: I thought one
of the reasons to continue using RAID 5 vs RAID 1 was speed.  Tests on
my systems show that the RAID 5 I/O is at least 3x the speed of RAID
1.

How many disks in the RAID 5? And are they all the same size/speed? I'm actually fond of RAID 10 (Striping across multiple mirrors) on SATA drives. The downside is that you still loose half your disk space, as you would in any RAID 1 array (although SATA drives are so big now I don't worry about that). On the upside, it's easy to do in software, and gives you better redundancy than 4 disks in RAID 5. IMHO having 4 or 6 7200 SATA disks in RAID 10 should be pretty fast and costs less and stores more than 4 SCSI disks in RAID 5. RAID 5 is more efficient with disk space, and that's the only reason to prefer it to RAID 1 or 10, I don't worry about RAID 5 with SATA drives, but SCSI drives of the same size are so much more expensive, so if you are using those fast SCSI drives then a RAID 5 makes sense to preserve your (relatively) expensive disk space.

I use RAID 5 in 14-disk arrays at work. We find the speed excellent, and the savings in disk space is enormous. Say you have fourteen 500GB disks, be they SATA or SCSI. With RAID 10, your storage would be 14 disks * 500GB / 2, which is 3.5TB. With RAID 5, you'd have (14 disks - 1 for parity) * 500GB = 13 * 500GB = 6.5TB. I'll take the extra 3TB any day!

--TP


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