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I am not sure exactly how to word this but I will try...
I was ordering hard drives today and HP told me that I am ordering one short for all my linux systems.. I am ordering two each so they can mirror each other but HP told me I need to order a third as a "ghost drive" and they claim this is standard for these RHEL systems..
I guess that confused me and could not see why... HP didnt offer any suggestions why I need it, so do any of you guys have ideas/suggestions about why that would be needed? Maybe its something obvious or simple.. but im lost.
There are now about 20 different configurations of RAID commonly used. Simple mirroring only requires an even number of drives (two works fine). Raid level 5 is better and faster, and requires at least three drives. Raid level 6 is even more reliable (you can lose two drives without the system shutting down) but requires at least 4 drives.
I am betting that whoever you spoke with assumed Raid-5. I consider it one of the best options.
RAID 5 is faster, but I would hesitate to call it better. As far as reliability goes, RAID 1 is about as good as it gets. No other RAID system lets you lose half of your drives and still be fine except for a 4-drive RAID 6 (RAID 10 will, but only if they're the "correct" two drives that fail).
I either use RAID 1, RAID 10, or RAID 6 for my systems, depending on the space and speed required. For a simple low-cost low-capacity redundant solution, RAID 1 is THE choice, and only requires two drives.
Maybe the HP rep wanted you to have a regular boot/system drive and then a mirrored array for everything else, which would require 3 drives total.
Last edited by suicidaleggroll; 01-17-2012 at 06:38 PM.
Possibly the RH Enterprise standard is that if you mirror the boot drives you need a HOT SPARE to swap in case of failure to maintain the pair without downtime.
You CAN run a three-way mirror, so that if one drive fails you still have a quorum. I would have to ask the RH support people to find out for sure, and if you have a current license you can do that yourself.