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RAID away
Posted by Mark on Oct 16, 2008
Interesting day for RAID recovery work, though emotions were divided with one happy customer and one greatly saddened one.
Like the old London bus joke about “none for ages then three come at once” RAID data recovery is not an everyday requirement but two turned up this morning within ½ and hour of one another. Both were 12 disk DELL units, one reported that 3 drives had failed, the other that two were offline.
Could we recover the data?
Often with RAID when multiple disks are reported as having failed we find that one disk has given up and that the unit has become “confused” and ended up reporting multiple failures. This was the case with the first unit we examined, as one drive could not be accessed whereas all of the others could be read from, to begin with anyhow.
With RAID, just as with any hard disk, our first priority is to secure the data. This is done by taking a sector by sector copy of each disk, this is known as an image. Once an image has been taken of each disk then this can be backed up to tape and then there is no chance of the situation being made worse. It adds a bit of time, but to avoid this step would be irresponsible as if another drive crashed all would be lost.
Imaging all of the disks in parallel meant that the whole process took a little over three hours. During this time the failed disk was examined, it had suffered a severe head crash and all that remained of the recording surface was dust covering the inside of the casing and clogging the final filter.
The remaining disks could all be read apart from a few sectors on two of the drives where read errors were reported.
This was a unit configured as a RAID 5 so there was data and parity information. The parity data is calculated from the user data so that if a sector cannot be read from a disk, its contents can be rebuilt so long as the corresponding sectors can be read from the other disks. Fortunately the failed sectors on the other two drives did not correspond and so a complete rebuild of the RAID5 user data could be made. The file system was intact and the recovery was all over bar the extraction of files.
The other RAID unit?
With this one it was the case that two disks had suffered head crashes. After discussion with the customer it turned out that one disk had gone off line a week earlier and the RAID unit had been operating in degraded mode, they had a replacement disk on order but it had not yet arrived when the other disk failed. The unit had no backup, two disks were crashed beyond hope of recovery so things were not good.
One potential problem with RAID units is that they contain drives that are always running, and all in identical environmental conditions. If the unit is knocked then all of the drives take a bash, if the air conditioning is not up to the job or has to be turned off then all drives go over temperature. The chances of multiple drive failures are not as remote as you might believe.
Without going into the depths of the way the RAIDing is organised, with two drives out of action we could rebuild a file system, but one with some significant gaps in it. Any file that was greater than 64KB was definitely damaged, as it turned out the only intact files were ones that were resident within the Master File Table (MFT) of the NT file system (NTFS).
In either case having an up to date backup would have negated the need for any form of data recovery. One customer was very lucky and the other not.
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