Re: If I have a single bad sector, how many failed reads should simple dd report?

[Mark Lord <kernel@teksavvy.com>]

On 09/07/10 03:04 PM, Greg Freemyer wrote:
..
>> When I re-ran it, /var/log/messages reported 10 bad logical blocks.
>> And even worse, dd reported 20 bad blocks.  I examined the data dd
>> read and it had 80KB of zero'ed out data.  So that's 160 sectors worth
>> of data lost because of a single bad sector.  At most I was expecting
>> 4KB of zero'ed out data.
..

That's just the standard, undesirable result of the current SCSI EH
when used with libata for (mainly) desktop computers.

I have patches (against older kernels) to fix it, but have yet to
get both myself and James B. interested enough simultaneously to
actually get the kernel fixed.  :)
..
> cmp -bl clean.dd corrupt.dd>  delta.log
>
> Looking at delta.log, the bytes start disagreeing 4 bytes prior to the
> sector boundary.  How can that be?
..

Dunno.  That could be a different problem.
I've seen the odd scattered report of data corruption on ext3/ext4
with large data sets, but don't know what the root causes were.

Cheers