RAID 0 not logically equalling HDD capacity

Question

I just received a HP Microserver. I've bought 2 2TB drives and I'm trying to RAID 0 them with the 250GB drive that the microserver comes with. When I RAID the 2 2TB drives together I get 4TB, but if I try and RAID all three, I get 750GB. This doesn't seem logical to me. Could someone explain this for me? Thanks

Answer 1

The RAID 0 will only take the maximum capacity of the smaller of the disks :

2TB (considered as 250GB) + 2TB (considered as 250GB) + 250GB = 750 GB

Why ? Because the RAID 0 needs to split each data in 3 and give it to each disk at the same time, increasing the overal I/O speed . Thus, if you were writing the data over 750 GB, 2 chunk of the data could be written on the first 2 disks of 2TB, but there would be no space on the 250GB left for the system to write on it. And No, RAID 0 doesn't handle a multiple size array given if disk space is available (like, from 0 to 750GB, it would write 3x faster, and then, between 750GB and 4250GB it would write 2x faster.

The solution is

• to buy another 2TB (or more, but as this case, if you buy a 3TB, only it's first 2TB would be used in that array ...) to replace the 250GB
• keep the current 250GB if you want a x3 performance on the raid and don't want to buy another disk
• let the raid with 2TB and 2 disks to have x2 faster performances and good size without buying a new disk

(x2, x3 are just theoritical IO perf increase, but it's around that in general)