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Patrick O'Callaghan wrote:

Just installed Azureus and tested it. It behaves exactly the same as
Ktorrent, both with and without preallocation.

This is hardly surprising as we're talking about how the filesystem
works, not about about a specific application. As someone else pointed
out, a inconsistency between the results of 'du' and 'df' can occur when
a running process has an open file which has been unlinked, meaning the
space occupied by the file has not yet been reclaimed even though the
file no longer has a name. However, any inconsistency arising from this
would be in the *opposite* direction to what the OP is seeing, i.e. less
actual free space than accounted for by the filesystem size less the
space occupied by (visible) files.

In all of this I've been assuming that preallocation means what it has
always meant traditionally in Unix/Linux: write garbage into the file to
make sure disk space is allocated. It turns out the system can now do
this for you, with the fallocate() call. However this is a very recent
addition and people were still arguing about its semantics less than a
year ago -- see http://lwn.net/Articles/240571. One detail stands out:
with the correct parameter, the apparent size of the file (as reported
by the stat() call) does not change, even if space is allocated beyond
its end. The effect of this on 'df' and 'du' doesn't seem to be
documented anywhere. Furthermore, although it's not explicitly stated,
one presumes that the unused space is reclaimed when the file is closed,
so the OP's question still stands: how can 'du' report more space than
is being used, even if no processes have open files? For that matter,
how can the system preallocate more space than the size of the
filesystem? Doesn't make sense.

BTW, it might be worth knowing what filesystem the OP is using. In my
case it's ext3.
FWIW, the OP is probably happy to have his problem solved.  Chances are he 
is running ext3 as most people take the default.
What would be more interesting would be to know what file system you are 
running.
Never say Never.... :-)


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