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RAID 1 (Mirroring)


RAID 1 is often used for the boot devices in servers or for critical data where reliability requirements are paramount. Usually 2 hard disk drives are used and any data written to one disk is also written to the other. RAID 1 is also incorporated within some "nested" or hybrid RAID configurations where its disk mirroring is used to add some degree of data redundancy to a non-error correcting RAID such as RAID 0.

In the event of a failure of one drive the system can switch to single drive operation, the failed drive replaced and the data transferred to a replacement drive to rebuild the mirror.

RAID 1 Data Recovery

RAID 1 Data Recovery is not often a requirement as it is not that common for two hard drives to fail at precisely the same moment, and as RAID 1 is mostly used for system volumes and not critical data storage, but when a pair of disks are in the same enclosure, at the same temperature, and connected to the same power supply, they can both fail or begin to develop errors at the same time. With most systems there is a reliance on one or other of the mirrored pair to be working fully, and no ability to "mix-and-match" the good sectors from each.

Unreadable hard drive sectors

Most hard disk drives develop bad sectors at some time, in fact many have them from the time of manufacture but they are mapped out by the hard disk's own defect management system. Once bad sectors manifest themselves within the user data area of a hard disk then problems will arise either with files becoming damaged, or critical elements of the file system being affected so that the volume can no longer be mounted.

With RAID1 all data that is written to the primary disk is also written to a backup disk, this means that as soon as there is a problem with the primary disk that prevents correct operation its use is discontinued and the secondary disk is used. However if both disks develop problems then the entire RAID is no longer accessible.

Data recovery under these circumstances entails the securing of the data from either of the disks and then a process whereby the good sectors from each disk are merged to form a single good disk "image" and then a file system recovery is performed using this data.

Failure following degraded RAID 1 operation

The RAID data recovery process is complicated if the failures have not taken place simultaneously. If one drive fails and the system remains in operation, then the second hard drive fails, the mirror has been "broken" and some elements of the data from the fists drive will not longer match that on the second drive. The data recovery operation can still merge data from both drives, but there is no guarantee that any replacement data used from the primary drive will be suitable as it might be "out-of-date". The recovery process now has to combine the physical merging of sectors with an analysis of the file system in order to determine, where practicable, that only valid data is being used.

Where RAID 0 failures occur it is tempting to take action to try and fix the problem, but this can be fatal. If one of the drives had been off-line as a result of a system instability or intermittent drive problem it is possible that it could come back on line. If this happens it will be reported that the mirrored pair of disks do not match, accepting any option to re-synchronise the mirrored pair could result in the older data overwriting the newer data. Altirium recommend seeking professional advice before taking any action to attempt to rectify any RAID problem.

Last Updated (Wednesday, 22 July 2009 09:13)

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