RAID Questions.

[cyberguru 0]

Im probably going to get a motherboard with RAID 0/1 capabilities and I was readin some of the stuff about it in the mobo Manual. It said in there that the RAID volumes will be the size of the smallest disk and that any space left on a larger would be unusable. I know that it wouldnt be used in the array but what about as a normal partition??

For the sake of an easy life I will be using 2 drives of the same sort in RAID 0.

And also If I put 2 drives together they are used as one disk, but will I still be able to partition that disk into other drive letters? eg C: and D: E:...?

[Bursar 0]

The bit about using the size of the smallest disk refers to RAID 1.

 

RAID 0 will basically cause all of your disks to be added together into one big lump. So a 20GB and 30GB disk will give you a RAID 0 capacity of 50GB. You can still partition them with as many drive letters you like.

 

If one drive in the array fails, you lose all of the data on all of the disks, so if it's important stuff, take backups

 

RAID 1 is mirroring. This is where one disk is an exact copy of the other. That's why uf you use a 20GB disk and a 30GB disk, you'll only get 20GB of useable space. The other 10GB will be "lost". If one disk fails, you can break the mirror and use the other disk on its own until the faulty one is repaired.

[EM 0]

http://www.storagereview.com/welcome.pl/...raid/index.html has a great description of Raid. From what I understand, even Raid 0 uses the size of the smallest drive as the baseline, so if you have a 20 and a 30g drive, with raid 0 you end up with one 40g drive, striped for performance rather than redundancy. JBOD is one option that most raid controllers have, basically drive spanning with no redundancy or performance enhancements...that would give you a 50g drive with the two in the example.

 

The above website has a paragraph that says "While common parlance for calculating storage says to multiply the number of data-containing drives by the capacity of the drives, bear in mind that this assumes that all the drives are of the same size (which is the ideal way to set up a RAID array). In fact, the capacity of any RAID array is constrained by the size of the smallest drive in the array; it is that size that should be used when calculating capacity. So, the capacity of a RAID 5 array containing four drives that are 20 GB, 30 GB, 40 GB and 45 GB in size is 60 GB (calculated as 4-1 times 20 GB, the size of the smallest drive.) If you were to actually set up an array using such drives, 55 GB of space would be lost in what is sometimes called drive waste. If an array has drive waste due to unequal drive sizes, this will reduce the storage efficiency of the array as well, of course: the RAID 5 array given as an example here has a storage efficiency of (60 / 135) = 44%, instead of the 75% that would result if all the drives were the same size. Some controllers will let you use such "waste" as regular, non-RAID storage. See this discussion of drive size for related issues."

 

There is also a table on that webpage under Summary Comparison of RAID Levels that shows the capacity and storage efficiency of each level of RAID and that table show that RAID 0 uses 2x the smallest drive as its capacity.

 

Speaking from experience, I have a SCSI raid setup in Raid 0 and I can tell you that when I used to use two drives of different capacity (both 18.2 but different manufacturers so still slightly different capacities) I only have 2x the smallest capacity drive.

 

Ed