Now, to recap what we’ve done so far: we’ve installed mdadm, became familiar with Gnome Disk Utility, and formatted our drives with new partition tables so we can use them in a new RAID 5 array.
Now, we get to install, the RAID array.
In Disk Utility still, go up to the File menu, down to Create and then select RAID Array. In the Create Raid Array window, under General select the RAID Level:
Distributed Parity (RAID-5)
For Stripe Size the default is fine, but essentially the larger the the stripe size the better the array is for larger files and read speeds. The smaller the stripe size is the better the array is for smaller files and write speeds.
For example if you intend to use the array for storing media files and playing them then the default or even the largest, 1 MiB, will likely serve you best.
However, if you have another use in mind for this array, you may want to adjust accordingly. Don’t bog yourself down with this though, use the default if you’re not sure and move on, there are more important things in life to spend your time on!
Now, under Disks we get to, you guessed it, select our Disks for the RAID array. Find and put a check mark next to the Hard Disks you wish to use in your array (the ones that you previously formatted with the GUID Partition Tables).
Note: Remember, you need three disks at a minimum, but you can use more if they are available, and are the same size. The smallest disk in the array multiplied by the total disks in the array will determine the final available space in your array. This is why I recommend using the same sized disks unless you have a good reason not to.
Once you have the disks check marked you will have the option to adjust the Array Size, unless you have a reason not to, I recommend leaving it at the default maximum size (if you do adjust it, parts of this guide may not work for you, and I cannot support it, so beware).
Now, triple check everything you’ve selected and then hit Create button at the bottom to begin the RAID creation process (you will likely be prompted to put in your user password once or twice).
Now, Disk Utility will create and then take you to the new RAID array underthe, likely new, section of Multi-Disk Devices. When you’re first inspecting your new RAID array don’t fret, it will initially be in a scary Degraded state. This is normal!
The array will begin to distribute parity information across the array, this process can take hours, days, or even longer depending on how many disks, the size and speed of the disks, and whether or not you begin to use the array while it is creating the redundancy information.
That’s right, if you format the array with a file system (Format Volume) you can begin to use the array now, however you will have zero redundancy, and even a single disk failure will cause you to loose ALL of the data on the array.
Using the array will also significantly increase the time it takes to go from a Degraded state to a Running state. I strongly recommend allowing the array to complete building itself before continuing on below with Setup.