RAID 5, if applied correctly and against a suitable I/O profile, will provide one of the most cost-effective forms of protection available while affording the volumes it is servicing I/O performance equivalent to required needs.

Through distributing an addressable parity bit across each of the physical disks within the RAID group, the group can recover from the failure of a single disk by reading the parity storage data from each of the remaining disks and applying an algorithm against it to rebuild to a replacement disk.

So, how much overhead do you incur in the implementation of RAID 5 configuration? Simply put, when sizing a RAID 5 group, allow for the total amount of disks you need to suit your capacity needs + 1.

An example is given below:

Total required usable capacity = 750Gb
Size of drives available = 300Gb
Number of drives required for RAID 5 group = 4x300Gb drives (3+1)
Total usable capacity = 900Gb

Note: The total usable disk capacity of a given drive is very rarely equivalent to the total marketed capacity of that drive, due to engineering overheads present on each disk.