GHOST Drives
 

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.

Thanks

What RAID level are you looking to do? RAID 1 would only require two drives, anything higher will need 3 or more.

I am unsure of the RAID level that is set in place. I had asked but no response. Is that the only reason you would need 3 or more is dependant on the RAID level?

EDIT - I am assuming its just RAID 1 because all we are doing is mirroring it just in case something fails... simple yet works well. Would you suggest something more like RAID 5 or?

RAID
 

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.

Or
 

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.