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Rick Koshi
Asked: 2011-09-09 22:24:56 +0800 CST

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

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This is an obscure question, I know. I'm trying to do some performance testing of some disks on a Linux box. I'm getting some inconsistent results, running the same test on the same disk. I know that disks have different performance depending on which part of the disk is being accessed. In particular, reads and writes to the outside of the disk have much higher throughput than reads and writes to the inside part of the disk, due to near-constant data density and constant rotational speed.

I'd like to see if my inconsistencies can be attributed to this geometry-induced variance in throughput. Is it possible, using existing tools, to find out where on the disk a file has been placed?

If not, I suppose I can write something to directly seek, read, and write to the device file itself, bypassing (and destroying) the filesystem, but I'm hoping to avoid that. I'm currently using ext4 on a 3.0 kernel (Arch Linux, if it matters), but I'm interested in techniques for other filesystems as well.

performance linux filesystems debugging

4 Answers

  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
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  2. Best Answer

    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
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  3. 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.

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  4. The question is rather old, but there is another answer that could be useful for those finding this on Google: filefrag (in Debian it is inside package e2fsprogs).

    # filefrag -eX /usr/bin/aptitude
Filesystem type is: ef53
File size of /usr/bin/aptitude is 4261400 (1041 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..     1fa:    15bd805..   15bd9ff:    1fb:            
   1:      1fb..     3f2:    15c6608..   15c67ff:    1f8:    15bda00:
   2:      3f3..     410:    15c8680..   15c869d:     1e:    15c6800: last,eof
/usr/bin/aptitude: 3 extents found

    It has the advantage that it works also for other filesystems (I used it for UDF), which do not appear to be supported by other tools described here.

    The offset presented in the output are meant to be in multiple of the block size written in the second line (4096 here). Beware that logical offsets might not be contiguous, as a file can have holes in it (when supported by the filesystem).

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