Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

[Ric Wheeler <ric@emc.com>]

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