October 7th, 2008
To start this post off I thought that I’d initially go through the core differences between RAID 1 & RAID 5. I haven’t mentioned arrays without RAID because if you in any way value your data RAID is essential.
RAID: Redundant Array of Independent Disks
Array: A number of disks, normally connected to the same RAID Controller
RAID Controller: A dedicated piece of hardware which has the task of replicating data
Capacity – How does my RAID choice affect storage capacity?
Disks have to be set-up in identical sized pairs so x2 1TB drives will give you 1TB of space. If you want more you have to add additional pairs.
A minimum of 3 drives are required and the total capacity is the sum of all the drives, less the size of one of the drives. I.e. x3 1TB drives = 3TB – 1TB = 2TB useable.
Redundancy – Is one more redundant than the other?
RAID 1 offers redundancy in pairs where as within a RAID 5 configuration where any one drives can fail. In the event that a drive fails and is replaced in a RAID 1 array it the file system will run at around half speed until the all the data is recovered. Within a RAID 5 array as there are more drives to share the load the file system will run slower but not as slow as it would in the RAID 1 situation and by adding more drives to an array this performance reduction is reduced.
Performance – Which one is better?
RAID 1 limits the performance to the two drives which are in the array whereas with RAID 5 the load can be shared over a number of disks. So rather than just having two drives which are both writing for each operation it is better to be able to write/read to/from a number of drives.
While the gains do decrease as the number of drives increases a substantial difference in performance would be noticed between an array with 3 drives and say 8. There are cost implications to bear in mind however if it is performance is critical then these can become justifiable.
Isn’t RAID 5 slow?
It used to be the case that RAID 5 was slow however over the years RAID controllers along with everything else has evolved substantially and multi-drive RAID 5 arrays are now plainly faster than RAID 1 arrays.
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