RAID1 or RAID0+1 have 50% redundancy. In other words, all data is written onto two disks. RAID5 and RAID4 depend on the number of disks in the array.
The percentage of redundancy is trivial to calculate - it is 1/#_of_disks_in_array.
3 disks is the minimum in RAID4 or RAID5
3 disks in array = 33% redundancy
4 disks in array = 25% redundancy
5 disks in array = 20% redundancy
There can be many disks in an array if you desire however, the reliability decreases as the number of disks increases and the time to loss of data also increases as the number of disks increases. (Second disk failure prior to a parity rebuild being done on the array.)
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This was first published in August 2002