missing free space

On 12/31/06, Roberto C. Sanchez wrote:
> On Sun, Dec 31, 2006 at 01:13:30AM +0200, ccostin wrote:
> > On a partition mounted as /tmp on /dev/hda2 df report
> >
> > df -h | grep -E "Size|hda2"
> > Filesystem Size Used Avail Use% Mounted on
> > /dev/hda2 479M 454M 0 100% /tmp
> >
> > and total space used by /tmp directory is
> >
> > du -sh /tmp/
> > 1.4M /tmp/
> >
> > Why this differences between used space reported df and du appear ?
> >
> The way that du measure disk utilization is to check for all existing
> files and directories and see how many disk blocks they use and add them
> all up. OTOH, df checks with the filesystem to see how many blocks are
> not used. You are asking why the sum of blocks used by existing files
> and directories and the unused blocks is not the whole of the blocks in
> the filesystem. The reason is that there are nonexistent files.
>
> What that means is that a program needs a temporary file and so it calls
> a function like tmpfile, which creates a temporary file. In order to
> minimize the vulnerability of information leaking from a temporary file,
> some programs call unlink() right after the file is created. As long as
> the program retains the open file descriptor, the file can be read from
> and written to, but no one else can access it, since it is not
> addressable via the filesystem. Once the program closes, the file
> descriptor is reclaimed by the OS and all the blocks are again marked as
> free.
>
> I believe that qemu does this as I often see the exact same thing you
> are talking about after laeving qemu running for long periods of time.
>

For this moment I don't run any qemu instace. But is somenthing similar.
When I test an example from php5-imagick package
(/usr/share/doc/php5-imagick/examples/resize.php)
apache2 "eat" a lot of disk space (a bug ?) from tmp
Output from lsof

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
apache2 10673 apache 11u REG 3,2 280080000
0 6161 /tmp/magick-XXuayBEU (deleted)

ls /tmp/magick-XXuayBEU
ls: /tmp/magick-XXuayBEU: No such file or directory

After restarting apache2 all things get back to normal, occupied
space is released.

df -h | grep -E "Size|tmp"
Filesystem Size Used Avail Use% Mounted on
/dev/hda2 479M 9.9M 444M 3% /tmp

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