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SubjectRe: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Theodore Tso wrote:
> On Wed, Jan 16, 2008 at 09:02:50PM -0500, Daniel Phillips wrote:
>> Have you observed that in the wild? A former engineer of a disk drive
>> company suggests to me that the capacitors on the board provide enough
>> power to complete the last sector, even to park the head.
>>

Even if true (which I doubt), this is not implemented.

A modern drive can have 16-32 MB of write cache. Worst case, those
sectors are not sequential which implies lots of head movement.

>
> The problem isn't with the disk drive; it's from the DRAM, which tend
> to be much more voltage sensitive than the hard drives --- so it's
> quite likely that you could end up DMA'ing garbage from the memory.
> In fact the fact that the disk drives lasts longer due to capacitors
> on the board, rotational inertia of the platters, etc., is part of the
> problem.

I can tell you directly that when you drop power to a drive, you will
lose write cache data if the write cache is enabled. With barriers
enabled, our testing shows that file systems survive power failures
which routinely caused corruption without them ;-)

ric




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