Attachment 17535 Details for Bug 31821 – du-vs-df

du-vs-df

du-vs-df (text/plain; charset=us-ascii), 3.32 KB, created by Michael W Lucas on 2001-11-07 16:10:34 UTC

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Creator: Michael W Lucas

Created: 2001-11-07 16:10:34 UTC

Size: 3.32 KB

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obsolete

>*** 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 11:09:10 2001
>***************
>*** 5869,5874 ****
>--- 5869,5929 ----
>        </qandaentry>
>  
>        <qandaentry>
>+         <question id="du-vs-df">
>+           <para>The <command>du</command> and <command>df</command>
>+             commands show different amounts of disk space available.
>+             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>
>+             goes through the directory tree, measures how large each
>+             file is, and presents the totals.  <command>df</command>
>+             just asks the filesystem how much space it has left.  They
>+             seem to be the same thing, but a file without a directory
>+             entry will affect <command>df</command> but not
>+             <command>du</command>.</para>
>+ 
>+           <para>When a program is using a file, and you delete the
>+             file, the file isn't really removed from the filesystem
>+             until the program stops using it.  The file is immediately
>+             deleted from the directory listing, however.  You can see
>+             this easily enough with a program such as
>+             <command>more</command>.  Assume you have a file large
>+             enough that its presence affects the output of
>+             <command>du</command> and <command>df</command>.  (Since
>+             disks can be so large today, this might be a
>+             <emphasis>very</emphasis> large file!)  If you delete this
>+             file while using <command>more</command> on it,
>+             <command>more</command> doesn't immediately choke and
>+             complain that it cannot view the file.  The entry is
>+             simply removed from the directory so no other program or
>+             user can access it.  <command>du</command> shows that it
>+             is gone -- it has walked the directory tree and the file
>+             isn't listed.  <command>df</command> shows that it is
>+             still there, as the filesystem knows that
>+             <command>more</command> is still using that space.  Once
>+             you end the <command>more</command> session,
>+             <command>du</command> and <command>df</command> will
>+             agree.</para>
>+ 
>+           <para>Note that softupdates can delay the freeing of disk
>+             space; you might need to wait up to 30 seconds for the
>+             change to be visible!</para>
>+ 
>+           <para>This situation is common on web servers.  Many people
>+             set up a FreeBSD web server and forget to rotate the log
>+             files.  The access log fills up <filename>/var</filename>.
>+             The new administrator deletes the file, but the system
>+             still complains that the partition is full.  Stopping and
>+             restarting the web server program would free the file,
>+             allowing the system to release the disk space.  To prevent
>+             this from happening, set up &man.newsyslog.8;.</para>
>+         </answer>
>+       </qandaentry>
>+ 
>+       <qandaentry>
>          <question id="add-swap-space">
>            <para>How can I add more swap space?</para>
>          </question>

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Attachments on bug 31821: 17534 | 17535