How can I find out where a file is physically located on the disk (block numbers)?

You could use debugfs for this:

debugfs -R "stat ~/myfile" /dev/hda1

Change the hard/partition drive accordingly and make sure the drive is unmounted. You will get a list with all the blocks used:

BLOCKS:
(0):1643532
TOTAL: 1

You can use the FIBMAP ioctl, as exemplified here, or using hdparm:

/ $ sudo /sbin/hdparm --fibmap /etc/X11/xorg.conf

/etc/X11/xorg.conf:
 filesystem blocksize 4096, begins at LBA 0; assuming 512 byte sectors.
 byte_offset  begin_LBA    end_LBA    sectors
           0    1579088    1579095          8

This thread may give you some insight into ext4 file placement algorithm.

debugfs has a bmap function, which seems to give the data you want. You should be able to give it consecutive blocks of a file and get the physical block numbers.

How can I find out where a file is physically located on the disk (block numbers)?

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