Re: Problem with df utility

[Bob Proulx]

Reji K Mathew wrote:
> While I am running a df command the output I am getting have some
> mismatches.
> 
> The total space, available space and used space are not matching.

Uh, I can't resist saying that if they were to match then one of the
columns would be redundant and would not need to be displayed.  Just
joking.  :-)

> I am attaching the df output with this mail.
> 
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/cciss/c0d0p2     3.0G  2.5G  327M  89% /
> /dev/cciss/c0d0p1      38M  3.8M   32M  11% /boot
> /dev/cciss/c0d0p3     4.9G  4.3G  424M  92% /oracle
> /dev/cciss/c0d0p5      24G   21G  2.3G  90% /oracle/data1
> /dev/cciss/c0d0p7      24G   22G  1.0G  96% /oracle/data2
> /dev/cciss/c0d0p13     24G   19G  3.8G  84% /oracle/data3
> /dev/cciss/c0d0p14     24G   21G  2.7G  88% /oracle/data4
> /dev/cciss/c0d0p9      29G   21G  7.2G  74% /oracle/index1
> /dev/cciss/c0d0p11     19G   17G  2.2G  88% /oracle/index2
> /dev/cciss/c0d0p15    6.9G  6.0G  588M  92% /tmp
> 
> Can you give an explanation for this problem.

I don't see any problems with that output.  But a common confusion is
that people forget about the filesystem's "minfree" value.

Here is an example:

  df -l /mnt/a
  Filesystem           1k-blocks      Used Available Use% Mounted on
  /dev/sda8             61156396  31912492  26137340  55% /mnt/a

  echo '31912492+26137340' | bc -ql
  58049832  # non-system space

  echo '(61156396-58049832)' | bc -ql  
  3106564   # system reserved space

  echo '(61156396-58049832)/61156396' | bc -ql
  .05079704173542208079  # percentage of minfree, 5% in this case

I have a 5% minfree reserved for system use.  That will not show up as
available space.

When the filesystem is created 5%-10% of the total disk space will be
reserved by the filesystem as 'minfree'.  This is available only to
root processes and not to non-root processes.  The amount of minfree
is dependent upon the defaults of the particular mkfs.  You probably
have 5% unless you changed it.  Also some disk space will be used by
Inodes in some filesystems.  On others it is dynamic.

Secondly the 'df' command is just returning data from the statfs(2)
call.

    struct statfs {
       long    f_type;     /* type of filesystem (see below) */
       long    f_bsize;    /* optimal transfer block size */
       long    f_blocks;   /* total data blocks in file system */
       long    f_bfree;    /* free blocks in fs */
       long    f_bavail;   /* free blocks avail to non-superuser */
       long    f_files;    /* total file nodes in file system */
       long    f_ffree;    /* free file nodes in fs */
       fsid_t  f_fsid;     /* file system id */
       long    f_namelen;  /* maximum length of filenames */
       long    f_spare[6]; /* spare for later */
    };

So disk is usually total - minfree - inodes - free but the command
does not compute it.  The df command is just reporting what the
filesystem has precomputed for those values.

Also remember that when approaching a completely full disk the
performance of the filesystem can degrade exponentially.  It is then
difficult for the filesystem to find free disk blocks and won't be
able to automatically produce defragmented physical layout.  A small
percentage of minfree is required to ensure reasonable performance.  I
believe this plus ensuring root having a little extra when users
filled the disk were the original reasons for minfree.  It used to be
10% but has been reduced to 5% in recent years because of the much
larger disk capacity.

Bob