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i had a power outage. before the outage my RAID 5 (4 SATA drives) was fully synced. perhaps I was doing some trivial writes during the poweroutage, but afterwards windows decided it has to do a full resynchronization.

Why does windows have to do that? any way to prevent it? I would expect only the bytes/sectors being written would require re-synchronization, not the entire thing.


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Just go back and think about what you asked. How does it know which sectors were being written as the power was cut? – JamesRyan Apr 30 '12 at 9:17
a bit snarky huh? ;) sure at the disk level, maybe can not. but since i'm talking about windows OS software RAID, I'd think/hope that this is something that'd work with NTFS: marking the file as dirty as step #1 then doing the writes, then marking it back as clean. maybe my wistful thinking eh, though sounds efficient enough to me.... – JasonS Apr 30 '12 at 10:21
The harddrive has no concept of a file, just blocks of data. Writes are already small so multiplying them by 3 isn't going to be very efficient. Strategies like you suggest are done at the filesystem level on top of RAID to prevent corruption. – JamesRyan Apr 30 '12 at 10:43
up vote 4 down vote accepted

perhaps I was doing some trivial writes during the poweroutage

As far as RAID is concerned, there's no such thing as a "trivial write". A write is a write, and if the RAID goes down in an unclean state, where there's a strong possibility that a write didn't get properly written everywhere it needed to, it's going to do a full resync to make sure things are healthy.

To prevent this, use a hardware RAID controller with a battery-backed write cache. Short of changing to a hardware RAID controller, at least get yourself a UPS with a long enough capacity to make it through short power outages. Good quality UPSes can also be configured to automatically shut down your servers gracefully in the event of a power outage. This would also prevent your RAID and filesystem going down hard.

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as mentioned in my reply to JamesRyan's comment, I'd hope that there was better collaboration between windows and the software RAID, but yeah. i already ordered a ups as my half-arsed solution. thanks for the answer. – JasonS Apr 30 '12 at 10:25

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