Compound RAID Levels

The primary reason for combining multiple RAID architectures would be to get either a very large, or a very fast, logical disk.

The list below contains a few examples. It is not the limit of what can be done.

RAID-1+0

RAID Level 1+0 (also called RAID-10) is the result of RAID-0 applied to multiple RAID-1 arrays. This will create a very fast, stable array. In this array, it is possible to have multiple disk failures, without loosing any data, and with a minimum performance impact.

To recover from a failed disk, it is necessary to replace the failed disk, and rebuild that disk from its mirror.

For two-drive failures, the probability of survival is 66% for a 4-disk array, and approaches 100% as the number of disks in the array increases.

RAID-0+1

RAID Level 0+1 is the result of RAID-1 applied to multiple RAID-0 arrays. This will create a very fast array. If the RAID-0 controllers (hardware or software) are capable of returning an error for data requests to failed drives, then this array has all the abilities of RAID-10. If an entire RAID-0 array is disabled when one drive fails, this becomes only slightly more reliable then RAID-0.

To recover from a failed disk, it is necessary to replace the failed disk, and rebuild the entire RAID-0 array from its mirror. This requires much more disk I/O than is required to recover from a disk failure in RAID-10. It should be noted that some enterprise-level RAID controllers are capable of tracking which drives in a RAID-0 array have failed, and only rebuilding that drive. These controllers are very expensive.

For two-drive failures, the probability of survival is 33% for a 4-disk array, and approaches 50% as the number of disks in the array increases.

This RAID level is significantly less reliable than RAID-1+0. This is because the structure is inherently less reliable in a multi-disk failure, combined with the longer time to reconstruct after a failure (due to a larger amount of data needing to be copied). The longer time increases the probability of a second disk failing before the first disk has been completely rebuilt.

RAID-3+0

RAID Level 3+0 is the result of RAID-0 applied to multiple RAID-3 arrays. This will improve the performance of a RAID-3 array, and allow multiple RAID-3 arrays to be dealt with as a single logical device. RAID-3+0 has a reliability similar to RAID-3, with improved performance.

This type of array is most commonly found when combining multiple hardware RAID devices into a single logical device.

RAID-5+0

RAID Level 5+0 (also called RAID-53 for some unknown reason) is the result of RAID-0 applied to multiple RAID-5 arrays. This will improve the performance of a RAID-5 array, and allow multiple RAID-5 arrays to be dealt with as a single logical device. The reliability of this type of array is similar to that of a RAID-1+0 array, but it has the performance impacts of RAID-5.

This type of array is most commonly found when combining multiple hardware RAID devices into a single logical device.

RAID-3+3+0

I saw this in 1992. This extreme measure was done to achieve a very high transfer rate on the swap partition of a Cray supercomputer. Using the RAID configuration below, the 4GB of memory could be transferred to disk in under 11 seconds. I include it here as an example of what can be done.

The individual disk drives (they were Seagate ST82368K) had a rudimentary RAID-3 controller built into them, where the individual data bits were routed to different heads, and the parity bit was routed to yet another head. Since the drive had eighteen physical heads, it was configured for two logical heads. The disk was capable of correcting for a bad head, but was not able to rebuild the data after the failed head was repaired (usually, a failure on a head amplifier card). This drive was capable of transferring 24MB/S sustained, which was excellent for the time (a fast SCSI drive could rarely sustain more than 3MB/S).

The individual drives were combined in a RAID-3 4+1 array, using a hardware controller. This was a full implementation of RAID-3, and could rebuild a drive, if necessary. This allowed a failed drive to be removed, and repaired, without damaging the array. This portion of the array was capable of transferring 96MB/S sustained.

The RAID-3 arrays were combined in a RAID-0 4-wide array, in software. This gave a sustained transfer rate of 384MB/S to the array.

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