Inconsistencies in df

[Tim Lastine]

Hi,

We are wondering why df gives such peculiar outputs on large disk
drives?  The used space and the available space do not add up the
reported size of the partition.  Is this a problem, or is it just the
way it is?  We are using a 250 Gig SATA (ad4) drive and a 1000 Gig
SATA Raid 0(da0) drive.   Incidentally, while partitioning the drives
on install, the disk geometry warning came up for both drives.  We
just used the geometry that the installer chose.


canicula-a# df -h
Filesystem Size Used Avail Capacity Mounted on
/dev/ad4s1a 2.9G 55M 2.6G 2% /
devfs 1.0K 1.0K 0B 100% /dev
/dev/ad4s1f 62G 4.0K 57G 0% /opt
/dev/da0s1d 906G 4.0K 833G 0% /raid
/dev/ad4s1e 3.9G 810M 2.8G 22% /usr
/dev/ad4s1d 2.9G 302K 2.7G 0% /var

canicula-a# df
Filesystem 1K-blocks Used Avail Capacity Mounted on
/dev/ad4s1a 3049102 56150 2749024 2% /
devfs 1 1 0 100% /dev
/dev/ad4s1f 64489864 4 59330672 0% /opt
/dev/da0s1d 949613700 4 873644600 0% /raid
/dev/ad4s1e 4058062 29730 2903688 22% /usr
/dev/ad4s1d 3045006 302 2801104 0% /var


Thank You

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[Greg Barniskis]

Tim Lastine wrote:
> Hi,
>
> We are wondering why df gives such peculiar outputs on large disk
> drives?  

If I'm not mistaken, it's because "Available" is a relative term.
Some space is reserved by the OS for itself. See part 9.25 of

http://www.freebsd.org/doc/en_US.ISO8859-1/books/faq/disks.html

I haven't done the math against your example, but I'd wager it adds
up correctly if you revise your assumptions accordingly.

--
Greg Barniskis, Computer Systems Integrator
South Central Library System (SCLS)
Library Interchange Network (LINK)
<gregb at scls.lib.wi.us>, [phone]
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[Greg Barniskis]

Greg Barniskis wrote:

> Tim Lastine wrote:
>
>> Hi,
>>
>> We are wondering why df gives such peculiar outputs on large disk
>> drives?  
>
>
> If I'm not mistaken, it's because "Available" is a relative term. Some
> space is reserved by the OS for itself. See part 9.25 of
>
> http://www.freebsd.org/doc/en_US.ISO8859-1/books/faq/disks.html
>
> I haven't done the math against your example, but I'd wager it adds up
> correctly if you revise your assumptions accordingly.
>

Sorry, 9.25 and 9.26 both.

--
Greg Barniskis, Computer Systems Integrator
South Central Library System (SCLS)
Library Interchange Network (LINK)
<gregb at scls.lib.wi.us>, [phone]
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[parv-3]

in message <7.0.0.16.0.20051222100521.01ed28d8@...>,
wrote Tim Lastine thusly...
>
> We are wondering why df gives such peculiar outputs on large disk
> drives?  The used space and the available space do not add up the
> reported size of the partition.  Is this a problem, or is it just the
> way it is?  We are using a 250 Gig SATA (ad4) drive and a 1000 Gig
> SATA Raid 0(da0) drive
...
> canicula-a# df -h
> Filesystem   Size   Used   Avail  Capacity  Mounted on
> /dev/ad4s1a  2.9G   55M    2.6G   2%        /
...
> canicula-a# df
> Filesystem  1K-blocks    Used      Avail   Capacity Mounted on
> /dev/ad4s1a   3049102   56150     2749024     2%    /
...
> /dev/ad4s1f  64489864       4    59330672     0%    /opt
> /dev/da0s1d 949613700       4   873644600     0%    /raid
> /dev/ad4s1e   4058062   29730     2903688    22%    /usr
> /dev/ad4s1d   3045006     302     2801104     0%    /var

(Tabs were evil in the above case as df outputs were not lined up
correctly; i have readjusted the above.)

Try to take 8% reserve in account ...

  df -t ufs \
  | tail +2 \
  | awk ' { sum = $3 + $4 +  $2 * 0.08 ;
            printf "%s: space diff: %f\n" , $1 , $2 - sum
          }
        '

... subject to rounding errors.  Here is an example ...

  # df
  Filesystem  1K-blocks     Used   Avail Capacity  Mounted on
  /dev/ad0s2a    138606    51952   75566    41%    /
  devfs               1        1       0   100%    /dev
  /dev/ad0s2d    594926   417042  130290    76%    /usr
  /dev/ad0s2e    475886    95826  341990    22%    /var
  /dev/ad0s2g  16724102 11949534 3436640    78%    /misc
  /dev/ad0s2f   2875822    16170 2629588     1%    /work
  procfs              4        4       0   100%    /proc

  # df -t ufs | tail +2 | awk '{ ... }' # awk program is given above
  /dev/ad0s2a: space diff: -0.480000
  /dev/ad0s2d: space diff: -0.080000
  /dev/ad0s2e: space diff: -0.880000
  /dev/ad0s2g: space diff: -0.160000
  /dev/ad0s2f: space diff: -1.760000


  - Parv

--

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