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Last week I introduced the basics of RAID and described RAID levels 0, 1, 5 and 6. Today I will talk about RAID levels 0+1, 1+0, 5+0 and 5+1 – also known as RAID-01, RAID-10, RAID-50 and RAID-51.

These four RAID levels are examples of nested RAID levels. These RAID configurations are created by combining two simple RAID configurations like for example RAID-0 and RAID-1 into a more complex set-up which offers better performance and better redundancy than each of the single RAID levels. The general approach to creating a nested RAID is to divide the disks into sets of the same size. Each set is then used to create a RAID array. These arrays are usually called nested arrays. Then another RAID is created from the nested arrays. Usually nested RAID levels are created by combining two RAID levels which complement each other’s strengths and weaknesses. Note: regardless of which RAID level you are using remember that raid is not backup. For a short list of risks from which RAID cannot protect you see my previous post.

Let’s start with RAID-01. This RAID level is constructed from an even number of disks, usually four. It is set up by first creating two RAID-0 configurations and then a RAID-1 on top of them. The RAID-1 uses the RAID-0 nested arrays as its components instead of just plain disks. RAID-01 combines features of RAID-0 and RAID-1. The effective size of your storage in an N disk RAID-01 is going to be equal (N/2) times the size of the smallest disk in RAID. The read and write performance is two times that of RAID-1 created with 2 disks. If any disk fails in RAID-01 the whole RAID-0 nested array will be unavailable but your data will be still safe on the second RAID-0 nested array. RAID-01 can survive failure of the second disk from the same RAID-0 nested array but since it won’t be used at this point anyway this doesn’t really offer any additional redundancy compared with RAID-1. After any disk in RAID-01 fails the data is still available but you need to replace it as soon as possible otherwise when any disk in the other RAID-0 nested array fails you’ll have to restore all your data from the backups. You do have backups, don’t you?

Example: RAID-01 with four disks: A, B, C and D.
Disks A and B are combined into one RAID-0 nested array, disks C and D into another.
The whole RAID-01 array can survive simultaneous failure of two disks without any data loss if the failing disks are: A+B or C+D.
If A+C, A+D, B+C or B+D fail you lose your data.

RAID-10 uses a similar approach as RAID-01 but offers better redundancy. RAID-10 also requires an even number of disks, minimum four. It is set up by creating two RAID-1 configurations and then combining them with RAID-0. The RAID-0 uses the RAID-1 arrays as its components. The effective storage size is the same as for RAID-01 and is equal (N/2) times the size of the smallest disk. The read and write performance is the same as of RAID-01. The advantage of RAID-10 is that it can deal with two simultaneous disk failures as long as both failures are in different RAID-1 nested arrays. For a four disk RAID-10 array the probability that the second disk failure will be in a different RAID-1 pair than the first is 66.6% which is pretty good. After any disk in RAID-10 fails the data is still available but you need to replace it as soon as possible otherwise when another disk in the same RAID-1 nested array fails you’ll have to restore all your data from the backups. You do have backups, don’t you?

Example: RAID-10 with four disks: A, B, C and D.
Disks A and B are combined into one RAID-1 nested array, disks C and D into another.
The whole RAID-10 array can survive simultaneous failure of two disks without any data loss if the failing disks are: A+C, A+D, B+C or B+D.
If A+B or C+D fail you lose your data.

RAID-50 is similar to RAID-10 but instead of composing RAID-0 out of two RAID-1 arrays it uses RAID-5 arrays as its building blocks. RAID-50 requires an even number of disks, at least six. The effective storage size of RAID-50 is equal [(N/2)-1] times the size of the smallest disk. The read and write performance is two times that of RAID-5 created with half the amount of disks. RAID-50 can deal with two simultaneous disk failures (similarly as RAID-10) as long as the failed disks are in different RAID-5 nested arrays. For a six disk RAID-50 array the probability that the second disk failure will be in a different RAID-5 nested array than the first is 60%. After any disk in RAID-50 fails the data is still available but you need to replace it as soon as possible otherwise when another disk in the same RAID-5 nested array fails you’ll have to restore all your data from the backups. You do have backups, don’t you?

Example: RAID-50 with six disks: A, B, C, D, E and F.
Disks A, B and C are combined into one RAID-5 nested array, disks D, E and F into another.
The whole RAID-50 array can survive simultaneous failure of two disks without any data loss if the failing disks are: A+D, A+E, A+F, B+D, B+E, B+F, C+D, C+E or C+F.
If A+B, A+C, B+C, D+E, D+F, or E+F fail you lose your data.

RAID-51 is a very rare configuration and sometimes is called “RAID for very very paranoid”. It’s similar to RAID-01 but instead of building a RAID-1 out of RAID-0 arrays it uses RAID-5 arrays as its components. RAID-51 requires an even number of disks, at least six. The effective storage size of RAID-51 is the same as of RAID-50 and equal [(N/2)-1] times the size of the smallest disk. The read performance is equal to RAID-5 with N/2 disks – it can be double if the reads are using both RAID-1 mirrors in parallel. The write performance is equal to RAID-5 with N/2 disks and cannot be improved. RAID-51 can handle three simultaneous disk failures regardless of which disks failed. RAID-50 can survive even more simultaneous disk failures as long as at least one of the RAID-5 nested arrays has only one failed disk. After three disks in RAID-50 fail the data is still available but you need to replace at least one of them as soon as possible otherwise when another disk fails you’ll have to restore all your data from the backups. You do have backups, don’t you?

Example: RAID-51 with six disks: A, B, C, D, E and F.
Disks A, B and C are combined into one RAID-5 nested array, disks D, E and F into another.
The whole RAID-51 array can survive simultaneous failure of more than three disks without any data loss if at least two disks in either of the two nested arrays are OK.
If both nested arrays had two disk failures you would lose your data.

This concludes the RAID mini series. If you want to find out more about RAID I recommend the reference guide at storagereview.com website.

Any questions about RAID? Write a comment and I’ll get back to you.