Date: Wed, 7 Nov 2001 07:10:02 -0800 (PST) From: Dima Dorfman <dima@trit.org> To: freebsd-doc@freebsd.org Subject: Re: docs/31821: new FAQ: du/df Message-ID: <200111071510.fA7FA2K14003@freefall.freebsd.org>
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The following reply was made to PR docs/31821; it has been noted by GNATS.
From: Dima Dorfman <dima@trit.org>
To: mwlucas@blackhelicopters.org
Cc: FreeBSD-gnats-submit@freebsd.org
Subject: Re: docs/31821: new FAQ: du/df
Date: Wed, 07 Nov 2001 15:06:27 +0000
mwlucas@blackhelicopters.org wrote:
> *** en_US.ISO8859-1/books/faq/book.sgml-dist Mon Nov 5 10:49:36 2001
> --- en_US.ISO8859-1/books/faq/book.sgml Wed Nov 7 09:05:22 2001
> ***************
> *** 5869,5874 ****
> --- 5869,5921 ----
> </qandaentry>
>
> <qandaentry>
> + <question id="du-vs-df">
> + <para>The <command>du</command> and <command>df</command>
> + commands show different amounts of disk space available?
Is this first sentence really a question (i.e., does the question mark
belong there)?
> + What's going on?</para>
> + </question>
> +
> + <answer>
> + <para>You need to understand what <command>du</command> and
> + <command>df</command> really do. <command>du</command>
> + shows how much of the disk is designated <emphasis>used by
> + files</emphasis>. <command>df</command> shows how much of
> + the disk is designated <emphasis>free</emphasis>. While
> + they seem to be opposite, that isn't exactly true.</para>
I think the distinction isn't what they do, but how they do it; if one
knows how much space is in use and the size of the disk, one can
compute how much space is free. On the other hand, how they do it
matters: du traverses the directory tree and counts how much space is
in use by each file. If a file has no directory entry, it won't be
counted. On the other hand, df just asks the filesystem how much
space it thinks is in use, and the filesystem doesn't care whether
files have directory entires or not, just as long as they take up
space on the disk.
> +
> + <para>When a program has a file open, and you delete the
> + file, it isn't really deleted until the program releases
> + the file. You can see this easily enough with a program
> + such as <command>more</command>. If you delete a file
> + while using <command>more</command> on it,
> + <command>more</command> doesn't immediately choke and
> + complain that it cannot view the file. You cannot access
> + the file from anywhere except that <command>more</command>
> + window.
This is a very good example.
> If the file is large enough,
This shouldn't (and doesn't) matter. People just tend to notice when
they're missing 18M instead of 18K :-).
> + <command>df</command> will show that it is gone. The
> + space the file took up is now designated free.
> + <command>du</command> will show that it is still there, as
> + the space the file uses is still technically in use. Once
> + you end the <command>more</command> session,
> + <command>du</command> and <command>df</command> will
> + agree.</para>
This is backwards. 'du' typically reports less space is in use, not
df (if you wrote this looking at the recent thread on -stable, note
that the originator misnamed the output; what he claimed was from du
was from df, and vice versa). The reason is that 'rm' removes the
file's directory entry, so 'du' never sees it, but 'df' does. See
above where I explained why 'df' seems it.
Here's a small example to demonstrate my point. (Note that I cleaned
up the `script' output a little to make it more readable.)
Script started on Wed Nov 7 14:50:19 2001
root@mirage# mdmfs -s 32m md /mnt
root@mirage# cd /mnt
root@mirage# dd if=/dev/random of=fish bs=1m count=20
20+0 records in
20+0 records out
20971520 bytes transferred in 27.938557 secs (750630 bytes/sec)
root@mirage# cat>open.c
#include <fcntl.h>
#include <unistd.h>
int
main(void)
{
int d = open("fish", O_RDONLY);
pause();
}
^D
root@mirage# make open
cc -O -pipe open.c -o open
root@mirage# ./open &
[1] 20447
root@mirage# df
Filesystem 1K-blocks Used Avail Capacity Mounted on
/dev/ad0s1a 254063 68466 165272 29% /
devfs 1 1 0 100% /dev
/dev/ad0s1f 2554468 600470 1749641 26% /usr
/dev/ad0s1e 1016303 226578 708421 24% /var
/dev/md0c 32476 20520 9360 69% /mnt
root@mirage# du
20520 .
root@mirage# rm fish
root@mirage# df
Filesystem 1K-blocks Used Avail Capacity Mounted on
/dev/ad0s1a 254063 68466 165272 29% /
devfs 1 1 0 100% /dev
/dev/ad0s1f 2554468 600470 1749641 26% /usr
/dev/ad0s1e 1016303 226578 708421 24% /var
/dev/md0c 32476 20520 9360 69% /mnt
root@mirage# du
16 .
root@mirage# killall open
[1] + Terminated ./open
root@mirage# sync
root@mirage# df
Filesystem 1K-blocks Used Avail Capacity Mounted on
/dev/ad0s1a 254063 68466 165272 29% /
devfs 1 1 0 100% /dev
/dev/ad0s1f 2554468 600470 1749641 26% /usr
/dev/ad0s1e 1016303 226578 708421 24% /var
/dev/md0c 32476 16 29864 0% /mnt
root@mirage# du
16 .
root@mirage# exit
Script done on Wed Nov 7 14:52:01 2001
Notes:
- The `open' program just opens the file and keeps it open until it's
killed. I didn't use `more' as you did in your example because I
wouldn't be able to show when I started and killed it (it needs a
tty).
- The `sync' command isn't instantaneous; about 10 seconds passed
between the sync and the subsequent df which shows the space is free.
This is because of softupdates.
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