power cutoff and raid

M

mista ting

Gawd
Joined
Apr 13, 2006
Messages
742
i am little bit confused about RAID failures.......when they say data loss and data corruption if the power is cut off they just means the windows or w/e files is lost right? not the hard drive it self is damaged permanently?
 
You lose data (drive itself isn't damaged) if you have write caching enabled either through the RAID controller or your OS as it is first stored in RAM. This is why many RAID controllers offer battery backups units.
 
damn you. I was really looking forward to messing with the noobie. Now I have to just concur instead of making him crap his pants. ...take all my fun...
 
This is actually a valid question. Consider the following; a home PC, no UPS, some number of internal disks running RAID, and then, suddenly, the power is cut.

At that very instance the PS voltages immediately began their to descent to zero. The system on the other hand has no idea this is happening and will continue with its normal operation. As the 5V to the disk drive(s) decrease, it will reach a point where the PCB logic will no longer operate. Now, if at exactly that precise point in time the disk was in the middle of writing to a sector (regardless of whether the data is in the cache) that operation will be truncated, resulting in a partial write but more importantly, no ECC having been generated. That particular sector, and/or file will now exhibit a nonrecoverable error, and when read by the the RAID controller, the drive will be flagged as bad and a rebuild will be initiated.
 
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