"df" available space output

Hello,

I'm administering some RH 7.3 boxes and noticed that the "Available
Space" output column is not a direct subtraction. For example, the output:

"""
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/cciss/c0d0p1 248911 98858 137202 42% /
"""

Calculating "1k-blocks - Used": 248911 - 98858 = 150053

Why doesn't it match the available space? I have been administering unix
based servers for a while, but this was something I never paid close
attention to because it was trivial. I'm assuming each file system has
some data structure that's accounted for in the Available space.

Thanks.

--
Mike

On 2006-05-25, Mike wrote:
> Hello,
>
> I'm administering some RH 7.3 boxes and noticed that the "Available
> Space" output column is not a direct subtraction. For example, the output:
>
> """
> Filesystem 1k-blocks Used Available Use% Mounted on
> /dev/cciss/c0d0p1 248911 98858 137202 42% /
> """
>
> Calculating "1k-blocks - Used": 248911 - 98858 = 150053
>
> Why doesn't it match the available space? I have been administering unix
> based servers for a while, but this was something I never paid close
> attention to because it was trivial. I'm assuming each file system has
> some data structure that's accounted for in the Available space.

On most systems, a portion of every file system is reserved for use
by the superuser, and not included in the available space.

--
Chris F.A. Johnson, author <http://cfaj.freeshell.org>
Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress)
===== My code in this post, if any, assumes the POSIX locale
===== and is released under the GNU General Public Licence

Chris F.A. Johnson wrote:
> On 2006-05-25, Mike wrote:
>> Hello,
>>
>> I'm administering some RH 7.3 boxes and noticed that the "Available
>> Space" output column is not a direct subtraction. For example, the output:
>>
>> """
>> Filesystem 1k-blocks Used Available Use% Mounted on
>> /dev/cciss/c0d0p1 248911 98858 137202 42% /
>> """
>>
>> Calculating "1k-blocks - Used": 248911 - 98858 = 150053
>>
>> Why doesn't it match the available space? I have been administering unix
>> based servers for a while, but this was something I never paid close
>> attention to because it was trivial. I'm assuming each file system has
>> some data structure that's accounted for in the Available space.
>
> On most systems, a portion of every file system is reserved for use
> by the superuser, and not included in the available space.
>

That would be a good thing for root to login :)

Thanks, Davide/Chris.

--
Mike