Re: data loss on jffs2 filesystem on dataflash

[jscottkasten]

Actually, 3 years ago, I researched this in my
company's lab.  There is about a 1 in 180 probability
of the power to the read/write head failing before the
sector write is complete.

The end result is a sector that fails ECC check.  The
linux kernel reports this and your I/O operation
returns with an error.

Not easy, but if you force a rewrite of the sector,
the problem cures itself.  Not easy to do through the
VFS though.  I was only able to make it happen by
using dd if=/dev/zero on the unmounted device node. 
After that, the sector read just fine.

-Scott Kasten-

--- "linux-os (Dick Johnson)" <linux-os@xxxxxxxxxxxx>
wrote:

> 
> On Thu, 22 Sep 2005, Artem B. Bityutskiy wrote:
> 
> > Valdis.Kletnieks@xxxxxx wrote:
> >> On Thu, 22 Sep 2005 14:48:39 +0400, "Artem B.
> Bityutskiy" said:
> >>
> >>> Joern meant that if HDD starts a block write
> operation, it will
> >>> accomplish it even if power-fail happens
> (probably there are some
> >>> capacitors there). So, it is impossible, say,
> that HDD has written one
> >>> half of a sector and has not written the other
> half.
> >>
> >> Hard drives contain capacitors to prevent writing
> of runt sectors on
> >> a powerfail?  Didn't we go around this a while
> ago and decide it's mostly
> >> urban legend, and that plenty of people have seen
> runt/bad sectors?
> >
> > No idea. But theoretically it should be so, at
> least "good" drives
> > should. May be a competent person will comment on
> this, that's quite
> > interesting.
> >
> > --
> > Best Regards,
> > Artem B. Bityuckiy,
> > St.-Petersburg, Russia.
> 
> The only significant energy storage that hard disks
> contain
> is the inertia of the rotating disk assembly. Since
> the platter
> motor is not a generator it doesn't help. Those tiny
> bypass
> capacitors you see can't store enough energy to do
> anything
> useful during a power failure.
> 
> BUT... The PC/AT power supplies store a lot of
> energy and
> they run for many milliseconds after a power fail.
> 2-100 uF in series = 50 uF @ 300 v.
> J = 1/2 CV^2
> 
> J = 50uF * 300^2 / 2 =  2.25 joules (lots of
> energy).
> 
> If the power-fail line is properly connected and if
> the
> power fail line operates at the correct time, the
> CPU
> will be halted while there is still enough energy
> available
> to complete any write that has gotten to the
> disk-drives sector
> buffer. This does not protect data, but it should
> certainly
> protect the sectors which might now contain header,
> good data
> or junk, and a proper CRC. IOW a good sector.
> 
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.13 on an i686 machine
> (5589.55 BogoMips).
> Warning : 98.36% of all statistics are fiction.
> 
>
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