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| p.1 #11 · HELP! Both Raid Drives "not readable" |
This is an interesting case - many of us here use RAID-based disk technologies, but how many of us actually understand the underlying technology. How many really know what happens when a drive fails?
First up, any kind of read or write glitch will cause a RAID to drop a drive or enter "degraded" mode. This might be an actual fault with a drive, or it might be that the RAID was powered down whilst data was reading or writing to the array. In any case, if the system believes that the data on different drives in the array is not coherent, the array will automatically be "degraded" until such time as a full rebuild has occurred. During the rebuild all data on the array is checked to be coherent, with the contents of the drive that incurred a failure being checked against those drives known to be good. This is a sensitive time for the array, as another failure during rebuild can leave the array not knowing which (if any) of the drives holds "good" data against which the others can be rebuilt. I'm going to hazard a guess that this is what has happened to the OP - it is mentioned that the array is powered up only when needed, so it is likely that on it's last use the array was shutdown whilst trying to recover from an error (possibly even caused by its previous shutdown), thus leaving it in an indeterminate state.
Because it was a RAID 1, all the data still resided on each drive and so all that was needed to get it off was some software that could read the container format for the RAID partition on the drive and willfully ignore the "degraded" filesystem flag. The OP is lucky he had a RAID 1, as any other RAID level would not offer this possibility.
Luckily this story has a happy ending. However it could easily not have. To the OP (and anyone else who similarly misunderstands RAID), though RAID arrays offer performance and data security benefits (as well as the ability to make several smaller drives into a bigger virtual drive), RAID is primarily an UPTIME solution: it is intended as a way to keep data available in the event of hardware (i.e. disk) failure. It follows from the acronym: "Redundant Array of Inexpensive Disks". Accordingly, RAID arrays are not intended to be turned on and off according to need. They should be left turned on. If they are shut down, it is important that they are done so properly to ensure coherency of the array. You cannot treat a RAID array like a big USB stick if you want it to work properly...