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I ran du -hs to find out how big my /home/user directory was and it reported as 18G. I cleared out almost 4G of data and ran du -hs again to find out how much space I actually saved and it still reports as 18G. --apparent-size makes it report as 19G. What am I doing wrong?

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Did you "clear out" the filesystem by unlinking files that were in use at the time? –  David Schwartz Jun 18 '12 at 20:40

3 Answers 3

up vote 3 down vote accepted

If the files are still in use, du will not update.

Here's a post with an exercise so you can see this in action : (for BSD, but it's equivalent in Linux)

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df will do this but I don't think that du will follow this behavior. –  mdpc Jun 18 '12 at 21:44
    
Goodness! What does this user have running to hold 4GB of files open? Wow! That's a lot of wedged zombies. (Ewww, mental image...) –  lornix Jun 28 '12 at 5:11
    
@lornix: a DVD image perhaps, that was being played with VLC, mplayer etc. –  Cristian Ciupitu Jun 29 '12 at 14:51

Another possibility that can cause this is filesystem compression and utilities that don't know about it. I saw this with ZFS a few years ago on Solaris but the theory should be the same for Ubuntu and any other filesystem that supports compression.

I created a file with mkfile 10g test. After this, ls -lh reported it as 10GB and du -h reported it as 0.

If the 4GB you cleared out was found with ls -lh then it might not have been taking up nearly as much space as it seemed.

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Something similar can also happen with sparse files. dd if=/dev/zero of=test bs=10G seek=1 count=0 creates a file that ls -lh reports as having 10GBs. –  Cristian Ciupitu Jun 18 '12 at 23:07

This could be caused by hard links which means that the files you deleted still exist under other names. To find them, run find -type f -links +1.

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