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the result of "df -h" shows below:

/dev/sda1       5.6G  3.2G  2.2G   60% /
udev            992M  4.0K  992M    1% /dev
tmpfs           401M  752K  401M    1% /run
none            5.0M     0  5.0M    0% /run/lock
none           1002M     0 1002M    0% /run/shm

the result of "du -hs /" shows below:

1.7G    /

Why is that?

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Is that the full output of du -hs /? When I run it on my computer, I do not have permissions to read many directories. – Sparhawk Jun 1 '13 at 3:37
I run it under "sudo su". – Emiya Kiritsugu Jun 1 '13 at 4:03
Perhaps this possible dupe? – Sparhawk Jun 1 '13 at 4:07
I am afraid not. That command is to reduce the reverse space of system. but the reverse space is not large to 3.2-1.7=1.5G – Emiya Kiritsugu Jun 1 '13 at 4:21
Oh yes, sorry, I misread the other question. Also, there the df used space ≈ du output. – Sparhawk Jun 1 '13 at 4:24

df and du measure two different things....

du reports the space used by files and folders--even this is more than the file size. A few quick experiments on my system show that 4K is a minimum file size in terms of disk space.

df reports the space used by the file system. This includes the overhead for journals and inode tables and such.

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