linux-kernel - Re: single bit errors on files stored on USB-HDDs via USB2/usb

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Date:	Sat, 09 Dec 2006 21:16:11 +1100
From:	Ben Nizette <ben.nizette@...et.net.au>
To:	Oliver Neukum <oliver@...kum.org>
CC:	Matthias Schniedermeyer <ms@...d.de>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Robert Hancock <hancockr@...w.ca>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	DervishD <lkml@...vishd.net>
Subject: Re: single bit errors on files stored on USB-HDDs via USB2/usb_storage

Oliver Neukum wrote:
> Am Samstag, 9. Dezember 2006 07:11 schrieb Ben Nizette:
>>>>> Also, you mentioned that the corruption occurs systematically on certain
>>>>> byte patterns. Therefore it's certainly not related to the cables.
>>>> It'd guess that too, but who can that say for sure. :-|
>>> You may have a bit pattern that stresses the controllers and suddenly
>>> a marginal cable may matter.
>> The errors occur in strings of 0xFFs.  From the USB standard:
>>
>> a “1” is represented by no change in level and a “0” is represented by a 
>> change in level
> 
> Yes, plus added stuffing bits.
> 
>> so this error-infested bytes are effectively long, quiet times on the 
>> wire.  I would have thought this would be the _least_ stressful time for 
>> the controllers but maybe they are also more susceptible to noise during 
>> this period.
> 
> The longer you don't change the voltage the likelier are reciever and
> transmitter to get out of sync.

Yes, hence the bit-stuffing, you're right :).  And hence this period 
isn't really too stressful for the controller as the stuffed bits come 
relatively often.

We're hoping that any wire-errors get picked up by the CRC anyway so a 
marginal cable under any circumstances shouldn't silently corrupt data. 
  I love that word 'shouldn't' ;)

Regards,
	Ben.
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