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What would trigger a RAID 1 Rebuild?

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April 30, 2010 12:43:38 AM

Hello folks,

I'm confused about an event that occurred today, and am hoping someone could shed a little light. I'm running an HP Pavilion m9040n quad-core PC with 2 320GB drives in a RAID 1 configuration.

After an unexpected loss of power my system booted into Windows 7 normally, but with abysmal performance. I could tell there was significant disk activity, but none of the standard tools provided any indication of what was causing the IO.

So I rebooted again and entered the storage configuration utility, where I found that the RAID 1 array was in a "rebuilding" state. There was also a statement that the array would "be rebuilt within the operating system". So I rebooted yet again, waited a while, and the array did eventually rebuild (and the status subsequently returned to "normal").

I then ran an error checking process against the array (the one you initiate from Windows and requires a reboot), and it revealed no problems.

So here's my question - could the power outage have triggered this array rebuild process? Do I need to be concerned that one of the drives might be failing?

Many thanks for any thoughts.

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a c 327 G Storage
April 30, 2010 3:58:39 AM
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Yes, a power outage could have caused this. In a RAID1 system, the same data is written to BOTH disk units, but not at exactly the same time - just really close together. If the power outage happened during ONE of the write operations, the two disk units would have different data on them in a very few places. The RAID management system would have detected this in its automatic checking of how the two disks' data matched or did not, and it would have gone to work to fix that. Its first step would have been to determine which disk had all-good data. Then it would figure out the best way to copy the known-good data to the disk with data flaws as a "background" task which would slow your performance down. When it was finished, it would re-set itself to normal operation of the array and your system would behave normally. At that point any tests of your RAID1 array should show there is NO problem and no mismatch of data versions.

If you know that the whole sequence was triggered by a power outage that probably caused bad data on one disk only, you have little reason to fear that the disks themselves have a permanent flaw developing. The faulty data had a known cause and was fixed.
April 30, 2010 12:51:51 PM

Best answer selected by markr.
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April 30, 2010 12:54:21 PM

Many thanks for the explanation, Paperdoc. It was helpful.
Anonymous
a b G Storage
October 26, 2010 4:57:17 PM

Paperdoc said:
Yes, a power outage could have caused this. In a RAID1 system, the same data is written to BOTH disk units, but not at exactly the same time - just really close together. If the power outage happened during ONE of the write operations, the two disk units would have different data on them in a very few places. The RAID management system would have detected this in its automatic checking of how the two disks' data matched or did not, and it would have gone to work to fix that. Its first step would have been to determine which disk had all-good data. Then it would figure out the best way to copy the known-good data to the disk with data flaws as a "background" task which would slow your performance down. When it was finished, it would re-set itself to normal operation of the array and your system would behave normally. At that point any tests of your RAID1 array should show there is NO problem and no mismatch of data versions.

If you know that the whole sequence was triggered by a power outage that probably caused bad data on one disk only, you have little reason to fear that the disks themselves have a permanent flaw developing. The faulty data had a known cause and was fixed.

Hi,
How can the "end of sync" be detected, if it is a background task ? My drives apparently are entering REBUILD state pretty often: I am downloading some large files then after some time I shut-down the PC and after reset, indeed the RAID utility reports the volume to be in rebuild state and in Win XP it appears the drives ar in use (I play a movie and it keeps halting, althoug the priority of the player is above normal).

Thanks,
Adrian
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