One of the big questions out there is how do I know if I should use RAID 5, or RAID 10, or something else?
The answer is usually something abstract like “if you have a lot of writes then use RAID 10, otherwise use RAID 5″. We’ll I’ve finely gotten some numbers from someone. These numbers are all unofficial and your mileage may vary.
On a typical RAID array (JBOD, DAS, etc) if your disk will have a higher than 10% change rate then you’ll want to look at a RAID 10 array. Now if you are using an EMC array (keep in mind I got these numbers from Dell/EMC) then you’ve got more leeway. The recommended number to stay below on an EMC CX line of arrays is 30%. So if your data change is less than 30% you should be OK on a RAID 5 array, higher than that and you’ll want to move onto a RAID 10 array.
Now if you want the extra read performance that RAID 5 gives you, but you want more redundancy than RAID 5, take a look at RAID 6. It is only slightly more expensive per Gig (especially when working on larger RAID arrays as their are more disks) as you have double parity. While there is a little more overhead for RAID 6 over RAID 5, this additional overhead is typically only an extra 2% to use RAID 6.
Now obviously these numbers are for when the RAID Array is running at full capacity. The lower the load you are putting on the RAID Array, the higher the percentage of changes you can use and still safely use a RAID 5 (or RAID 6) array.