debugfs Question

Aravindan Raghuveer aravindan_raghu at
Fri Apr 20 06:19:41 UTC 2007


I had recently posted a question on this list about
getting LBAs for a file on ext3. Andreas Dilger had
provided a very handy solution to solve the above
problem [Message attached along].

My goal is to be able to read files on a ext3
filesystem through the scsi generic (sg) driver. 
I first use debugfs to extract the block addresses for
every file. I then multiply the block address by 8
[4K/ 512] to obtain the LBAs. I then use a progam
sg_dd in the sg toolkit to read the data stored in the
LBA obtained above. 

However, the data I read through the sg driver does
not match the data in the file. Through some tinkering
of my code, I found that when I offset the original
LBA [obtained as block_address*8] by 63 sectors
[63*512 bytes], the data perfectly matches. That is
LBA' = LBA + 63 corresponds to the actual address used
for the file. I have tested this well when the file is
not fragmented. Initial testing on fragmented files
also shows similar behavior.

I googled around but could not find a convincing
answer as to where this offset crops up from. Has
anyone in the group done a similar experiment? I would
really appreciate any pointers on this issue. 

thanks again, 



--- Andreas Dilger <adilger at> wrote:

> On Apr 16, 2007  11:17 -0700, Aravindan Raghuveer
> wrote:
> > I need to write a user space tool that can dump
> > logical block addresses used by every file in a
> ext3
> > file system. For example, if file foo uses LBAs
> 2,3
> > and file bar uses LBAs 100,102,156, then the
> ouptut
> > should read:
> > 
> > foo         2, 3
> > bar         100,102,156
> > 
> > Is there a tool that exists that can do this? If
> not,
> > what would be a good strategy to write this tool.
> I am
> > learning filesystem programming and would really
> > appreciate any pointers.
> "debugfs -c -R 'stat /path/to/file' /dev/XXX"
> reports, among other things
> the blocks used by that file.
> Cheers, Andreas
> --
> Andreas Dilger
> Principal Software Engineer
> Cluster File Systems, Inc.

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