Is there a tool to visualize a filesystem allocation map on Linux?

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http://unix.stackexchange.com – May you have ever used filesystem defragmentation tools (like Norton SpeedDisk or Piriform Defraggler) on Windows, you have probably seen such a diagram: It displays a filesystem sectors map, painting (as for this particular example) sectors (sets of sectors actually, to fit the whole partition in the screen) occupied by non-fragmented (contiguous) files in blue, the opposite in red and free sectors in white (and some more colours for some more particular cases which can happen to be of interest). You can click on a "sector" and see what particular files "live" there. Is there such a visua (HowTos)