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Don't worry, this isn't typical "du vs. df" question. :-) I mounted a squashfs image (via loop) and used du to reported the size: du --apparent-size -lsh image1/ which reported 215M. Then I used rsync ("rsync -aS") to copy all files to a real partition (ext4). To my surprise du reports for this directory now: 302M. Where do the additional 87MB come from? Without --apparent-size the values are 242M and 676M, respectively.

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Run du -ab | sort -k2 on both sides and see what files' sizes differ. – Gilles Mar 1 '12 at 1:35
Maybe hard links get expanded, that is each hard link is copied as file. Try the -H option in rsync. – vasquez Mar 1 '12 at 6:19
@vasquez The -l option for du should have taken care of that but thanks for the suggestion. – antje-m Mar 7 '12 at 6:53

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Gilles had the right idea. Comparing the two outputs showed that squashfs reported different sizes for the directories than ext4 which (probably) explains the difference. Thanks to all who responded.

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So is there actually a nice way to compare disk usage without being confused by the different directory sizes? – drfrogsplat Nov 19 '12 at 1:15

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