RAID 51 is a hybrid of RAID 5 and RAID 1, it is a RAID 1 mirrored array where the composite elements are not individual hard disk drives but each is a RAID 5 array. The other way of looking at RAID51 is that we have a RAID5 with the additional security of it being mirrored to another RAID5.
It can be considered as a RAID 1 built from "super-disks" as each individual element is like a disk with build in redundancy and can survive the loss of a single disk.
As with most of the nested type RAID implementations they are found in relatively sophisticated IT environments with high levels of additional data protection such as tape backup and archiving. The RAID level used provides enhanced protection over a standard RAID5 array to maintain data availability as up to three disks may fail before the RAID ceases to be accessible.
As with any RAID there is a point at which the array will cease to operate, in the case of RAID 51 if two disk fail in each of the RAID 5 arrays then the entire RAID will cease to operate and a data recovery operation might be required. As RAID51 is a hybrid of RAID5 and RAID1 the data recovery process is similarly a hybrid of that for each of these RAID levels.
As with other RAID levels the incidence of bad sectors on any individual hard disk drive usually results in access to the failing unit being denied. In either part of a RAID51 array having one disk taken out of the equation will result in that part of the array dropping into degraded operation, depending upon the array set-up it could even could the entire mirror as failed and start working only with the fully operational one, this is entirely down to the RAID hardware.
Under normal circumstances failures can be survived and failing disks swapped out with no system down time nor reduction in data availability. If, however, multiple failures occur simultaneously the entire RAID might fail. The data recovery process must first secure all data from the fully working drives and then deal with the failing ones to secure all available data from them before they fail completely. So long as there is not a precise correspondence between the failed sectors on the drives between each element of the mirror a complete recovery of data will be achieved. Even if some of the failed sectors do match a significant level of data recovery is usually achievable
If, rather than sectors becoming unreadable, there are hardware errors resulting in hard drives becoming inaccessible, and each RAID5 element goes off-line, then the RAID51 will no longer be operational. In most instances it will not be the case that four or more hard drives have crashed beyond hope, it is more often the case that one has failed completely and caused a destabilisation of the bus resulting in an erroneous report that other drives have failed. Even if four or more hard drives have failed the likelihood is that enough of the individual devices will be recoverable for the RAID data recovery to be a 100% success.
