Systems Engineering and RDBMS

We have covered RAID levels before in our posts. You can read about the different RAID levels here and the I/O characteristics here.  While building up a DR (Disaster Recovery) environment for one of our clients, one of the questions asked by the client was: “How is RAID 1+0 different than RAID 0+1?”.  Both RAID 0+1 and RAID 1+0 are multiple RAID levels which means that they are created by taking a number of disks and then dividing them up into sets. And within each of these sets, a single RAID level is applied to it in order to form the arrays.  Then, the second RAID level is applied at the top of it to form the nested array.  RAID 1+0 is also called as a stripe of mirrors and RAID 0+1 is also called as a mirror of stripes based on the nomenclature used for RAID 1 (mirroring) and RAID 0 (striping).  Let’s follow this up with an example:

Suppose that we have 20 disks to form the RAID 1+0 or RAID 0+1 array of 20 disks.

a) If we chose to do RAID 1+0 (RAID 1 first and then RAID 0), then we would divide those 20 disks into 10 sets of two.  Then we would turn each set into a RAID 1 array and then stripe it across the 10 mirrored sets.

b) If on the other hand, we choose to do RAID 0+1 (i.e. RAID 0 first and then RAID 1), we would divide the 20 disks into 2 sets of 10 each.  Then, we would turn each set into a RAID 0 array containing 10 disks each and then we would mirror those two arrays.

So, is there a difference at all?  The storage is the same, the drive requirements are the same and based on the testing also, there is not much difference in performance either.  The difference is actually in the fault tolerance.  Let’s look at the two steps that we mentioned above in more detail:

RAID 1+0:
Drives 1+2     = RAID 1 (Mirror Set A)
Drives 3+4     = RAID 1 (Mirror Set B)
Drives 5+6     = RAID 1 (Mirror Set C)
Drives 7+8     = RAID 1 (Mirror Set D)
Drives 9+10     = RAID 1 (Mirror Set E)
Drives 11+12     = RAID 1 (Mirror Set F)
Drives 13+14     = RAID 1 (Mirror Set G)
Drives 15+16     = RAID 1 (Mirror Set H)
Drives 17+18     = RAID 1 (Mirror Set I)
Drives 19+20     = RAID 1 (Mirror Set J)

Now, we do a RAID 0 stripe across sets A through J.  If drive 5 fails, then only the mirror set C is affected.  It still has drive 6 so it will continue to function and the entire RAID 1+0 array will keep functioning.  Now, suppose that while the drive 5 was being replaced, drive 17 fails, then also the array is fine because drive 17 is in a different mirror set.  So, bottom line is that in the above configuration atmost 10 drives can fail without effecting the array as long as they are all in different mirror sets.

Now, let’s look at what happens in RAID 0+1:

RAID 0+1:
Drives 1+2+3+4+5+6+7+8+9+10        = RAID 0 (Stripe Set A)
Drives 11+12+13+14+15+16+17+18+19+20    = RAID 0 (Stripe Set B)

Now, these two stripe sets are mirrored.  If one of the drives, say drive 5 fails, the entire set A fails.  The RAID 0+1 is still fine since we have the stripe set B.  If say drive 17 also goes down, you are down.  One can argue that that is not always the case and it depends upon the type of controller that you have.  Say that you had a smart controller that would continue to stripe to the other 9 drives in the stripe set A when the drive 5 fails and if later on, drive 17 fails, it can use drive 7 since it would have the same data.  If that can be done by the controller, then theoretically speaking, RAID 0+1 would be as fault tolerant as RAID 1+0.  Most of the controllers do not do that though.