RAID 0 setup does not contain full disk space from slices

I just setup a striped RAID 0 on my 2012 MacBook Pro with two SSDs. One drive is 256 GB and the other is 500 GB, but my RAID disk is only 510 GB. Disk2 is my RAID disk, with disk0 and disk1 being slices.

Does anyone know what's going on and how to fix this?

I erased all disks prior to setting this up. See attached image from diskutil list output.

I used the OS X Yosemite Disk Utility to setup the RAID and now I am running El Capitan.

Solution 1:

You can't fix this. The size of a RAID is always a multiple of the smallest volume used in the setup.

The chunk sizes (stripe sizes, interlace sizes) as well as the number of chunks have to be equal on all used volumes. Usually the chunk size is 64 kb. These requirements are determined by the size of the smallest disk and limit the size of the resulting RAID volume.

Solution 2:

For your purposes, RAID 0 is not going to really get you faster or larger, and is not going to accomplish your goal (which I"m assuming is a blazing fast Mac with a large HDD, as that's why most folks do it)..

1. RAID 0 (on disk utility) is software RAID. in other words it requires the system to maintain. and that's never really good. nor is it fast.

2. RAID 0 software is going to use the smallest size HDD as the basis for every raid build.. so it has assumed that ALL your drives are 256gb, because the smallest is 256.. therefore 256 + 256 = 512.... the remaining data space on your 512gb drive is utterly useless.

3. Your MBP has a 6gb data bus. use it. get a faster drive and you'll be happier. if you can swing it, Samsung EVO 1tb or 2tb - quite snappy performance....

4. you can still target the 2nd drive within the OS, and data-share across them... For instance, you could put the 256 in the main bay, run the OS and your Apps on it, and have all your users and user data on the 512 drive.

This is often better, because you've reduced reads/writes for each drive, so the system is more efficient.