In early 2010 we successfully performed what we believe to be the first Xsan data recovery procedure that involved working on only one of the two striped RAID arrays, totally independent of the second member of the array and the Metadata server.
Our client, a London University, had a crashed Xsan storage network that consisted of two Infortrend 16-bay storage servers, each configured as RAID 3, and then striped to RAID 30. The Metadata server was a third Infortrend bay, consisting of 24 hard drives. All were connected using fibre channel and gigabit ethernet.
The Xsan system had suffered from a power failure. The Metadata server and one of the Data servers shut down cleanly. However, the second Data server’s uninterruptible power supply had unknowingly been disconnected, with the result that two drives were severely damaged by the power spike and subsequent failure.
One drive had already failed from excessive bad sectors, but the system failed to flag this correctly and, after copying the drive to the global spare, it then marked the failed disk as a global spare. This meant that the failure of only one more drive would bring the system down – but we already had a situation with multiple drive failure.
Once we’d repaired the two failed drives and the two with bad sectors, we made secure backups of all the drives and started the long process of the file system rebuild. It had to present identically to the rest of the Xsan as it did before the failure – all logical disk and logical volume ID numbers had to match the original configuration, and this we achieved by forensically editing individual bits on the hard drives to force the new IDs.
We then delivered the recovered RAID array member to our client and, with some final on-site configuration, we managed to bring the entire storage network back online with absolutely no data loss whatsoever.
It’s important to bear in mind that the University’s own system suppliers and consultants had deemed the data unrecoverable, as did Infortrend Technical Support, and even Apple Xsan Technical Support.
This proves that, even if you have been told by the highest authority that your data is not recoverable, this is almost never the case, in our experience. In fact, the statements by the above Technical Support consultants could easily have convinced the client to abandon recovery, and re-install from scratch – with complete loss of data. We were told by everyone that nothing could be done. Our expertise with low-level data recovery and forensics techniques proved them all wrong.