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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.

enter image description here

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What do you want? RAID 1 or RAID 0 or just to concatenate the space? – bmike Feb 24 at 15:00
Raid 0 to hopefully double performance. – TWilly Feb 24 at 15:26
@TWilly RAID0 won't double performance, it may result in increases performance over a single disk, but it's certainly not double (and in some systems, depending on the quality of the RAID controller, it may not even be that good)... and if you're mixing an SSD with a HDD (as it appears you are based on the disk capacity you listed above), you'll get at best HDD performance. It can only perform as well as it's lowest common denominator. – SnakeDoc Feb 24 at 18:13
@SnakeDoc I'm using two SSD's now and getting double the read performance compared to just one SSD. I'm getting 900 MB/s read speeds which is a huge improvement. My understanding is that Raid 0 writes and reads to two disks simultaneously resulting in faster read and write speeds. – TWilly Feb 25 at 15:24
@TWilly Faster, yes, double, no. There's overhead and other things involved make doubling performance essentially never happen outside theory. I don't know the quality of the tool you're using to measure, but there's a lot going on that can skew performance tests, such as OS caching and other "magic" it does to improve read times on it's own (since writes are asynchronous usually, and waiting to read something is the more common case as you open programs and what-not). – SnakeDoc Feb 25 at 15:42
up vote 9 down vote accepted

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.

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Thanks for this info, wish I would have known this before. So my options are to switch out the 256 drive, or remove the raid. – TWilly Feb 24 at 16:12
AFAIR you can create a 510 GB RAID with 2 ~255 GB partitions on disk 0/1 and use the rest of the 512 GB disk (~ 256 GB) for other purposes – klanomath Feb 24 at 16:22
good suggestion on partitions, I'll try and set this up, that will prevent me from having to buy another SSD. – TWilly Feb 24 at 18:33
@immibis You can't add a second partition to a (Apple Soft-)RAID already using the first partition of the same disk (tested in Yosemite). – klanomath Feb 24 at 23:26

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.

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baaah, wish I had known this before. I am getting 500MBs Write and 950MBs Read speeds. Write is the same as before, but reads has doubled. Looks like I should either buy another 512, or switch to a fushion drive. Thanks for the tips. – TWilly Feb 24 at 15:25
I don't agree that software raid is inferior. CPUs are quite capable. Back in the nineties there was a difference. Today hardware raid offers you incredible compatibility problems if your controller breaks and you want to rescue your data in a different computer. – Max Ried Feb 24 at 15:49
@Max-Ried I am getting double read speeds which is pretty fast. Any ideas why my write speeds aren't higher? I'm using the following disk speed tool:… – TWilly Feb 24 at 16:05
Software RAID hasn't been slow for a decade or more. For example, I've got a RAID 6 array on a decidedly anemic Atom CPU, and the CPU still rarely gets above 10% load while accessing the array. RAID 0 on a modern CPU shouldn't even show up on the CPU monitor. – Mark Feb 24 at 22:24
Software RAID is (in general) inferior only in the sense that it can often require software, which (in some cases, though probably not this one) would have the potential of having compatibility issues (which is never a good thing when one is having to access data). It is usually slightly slower (and especially noticeable in write speeds, as compared to hardware RAIDS). While I'm not interested in a flame war, but yes, there's a reason why real server have had hardware RAIDs and not software... (and Apple doesn't fall into the category of real server hardware). – frank ankersly Feb 25 at 0:28

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