Determine which sectors of a hard disk a given file occupies?

Question

Is there a way I can see which sectors of the hard disk a given file occupies?

The filesystem is ext3 or ext4 on Linux.

Of course there is a way but it is not exactly what one would call "easy". What are you trying to accomplish?
I'm performing an operation that requires reading a large amount of input data, and I want to pinpoint the cause of some anomalies in the performance of the reading. I want to know whether there is a correlation between the read performance and the physical layout of the input files on disk. This requires knowing what the physical layout of the input files is on the disk.

Slartibartfast · Answer 1 · 2010-06-22 05:22:45Z

up vote 2 down vote accepted

If you are looking for blocks for file /tmp/squiggle/smurf.tgz, in the /tmp/ filesystem, mounted from /dev/sdg4, you would type:

# debugfs /dev/sdg4
> bmap /squiggle/smurf.tgz 0
> bmap /squiggle/smurf.tgz 1
> bmap /squiggle/smurf.tgz 2
> bmap /squiggle/smurf.tgz 3
....

Is that what you're looking for?

answered Jun 22 '10 at 5:22

	Yes, this seems to be what I'm looking for. I have a couple of follow-up questions: 1) Is there a way to do this without root privileges? 2) Can this be scripted, or can these numbers be queried programmatically? – HighCommander4 Jun 22 '10 at 7:23
	It should be scriptable. I'd say piping your commands in on stdin would probably do the trick. Be very careful with that though, as you can do a LOT of damage to a filesystem with debugfs. As for doing this without root priv's, I don't know, but I suspect not. The filesystem is intended to be an abstraction that hides those details from the user. – Slartibartfast Jun 23 '10 at 2:44

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