Messages in this thread | | | Date | Wed, 26 Aug 2009 02:20:45 +0200 | From | Pavel Machek <> | Subject | Re: [patch] document flash/RAID dangers |
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>>>> --- >>>> There are storage devices that high highly undesirable properties >>>> when they are disconnected or suffer power failures while writes are >>>> in progress; such devices include flash devices and MD RAID 4/5/6 >>>> arrays. These devices have the property of potentially >>>> corrupting blocks being written at the time of the power failure, and >>>> worse yet, amplifying the region where blocks are corrupted such that >>>> additional sectors are also damaged during the power failure. >>> >>> I would strike the entire mention of MD devices since it is your >>> assertion, not a proven fact. You will cause more data loss from common >> >> That actually is a fact. That's how MD RAID 5 is designed. And btw >> those are originaly Ted's words. > > Ted did not design MD RAID5.
So what? He clearly knows how it works.
Instead of arguing he's wrong, will you simply label everything as unproven?
>>> events (single sector errors, complete drive failure) by steering people >>> away from more reliable storage configurations because of a really rare >>> edge case (power failure during split write to two raid members while >>> doing a RAID rebuild). >> >> I'm not sure what's rare about power failures. Unlike single sector >> errors, my machine actually has a button that produces exactly that >> event. Running degraded raid5 arrays for extended periods may be >> slightly unusual configuration, but I suspect people should just do >> that for testing. (And from the discussion, people seem to think that >> degraded raid5 is equivalent to raid0). > > Power failures after a full drive failure with a split write during a rebuild?
Look, I don't need full drive failure for this to happen. I can just remove one disk from array. I don't need power failure, I can just press the power button. I don't even need to rebuild anything, I can just write to degraded array.
Given that all events are under my control, statistics make little sense here. Pavel
-- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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