VFAT i_pos value

[Kai Meyer]

On 12/01/2011 07:38 AM, OGAWA Hirofumi wrote:
> Kai Meyer<kai at gnukai.com>  writes:
>
>> I'm getting this error:
>> FAT: Filesystem error (dev sblsnap0)
>>       fat_get_cluster: invalid cluster chain (i_pos 523791)
>>
>> I'm wondering if there was a way to figure out what sector is causing
>> the error? I would like to try and track down what is changing that
>> sector and fix the problem. Is there a straight forward way to convert
>> i_pos to a sector value? I've been staring at the fat.c and fat.h code
>> all morning, and I'm having trouble grok'ing the flow.
> The i_pos means directory entry (contains inode information in unix-fs)
> position,
>
>      block number == i_pos / (logical-blocksize / 32)
>      offset       == i_pos&  (logical-blocksize / 32)
>
> the above position's directory entry contains information for
> problematic file. This is how to use i_pos information.
>
> FWIW, in this error case, the cluster chain in FAT table which is
> pointed by that entry, it has invalid cluster value.
>
> Thanks.

If you would verify my math for me, I would appreciate it.

In this case, my logical block size is 4096, because byte 13 of the 8Gb 
file system is 8, and I take that to be 8 * 512, which is 4096. So:

block_number = 523791 / (4096 / 32) = 4092
offset = 523791 % (4096 / 32) = 15  // I assume you meant modulo in your 
original post, and not binary AND.

So if the block_number is 4092, I would multiply that by 8 (sectors per 
logical block) to get the sector number:
32736

Does the error indicate that sector contains the corrupted data? Or is 
it the sector that contains the information that points to the corrupted 
data? Or is it something entirely different?

-Kai Meyer