odd output from df command on a SLES

Question

I know I'm not very good at math...but check out this output:

SERVER:~ # df -h /ORACLE/LOGS/b
Filesystem              Size    Used    Avail   Use%    Mounted on
/dev/mapper/LOGSdatavg-LOGSb_lv 96G 82G 9.0G    91% /ORACLE/LOGS/b
SERVER:~ # 
SERVER:~ # df -i /ORACLE/LOGS/b
Filesystem              Inodes      IUsed   IFree       IUse%   Mounted on
/dev/mapper/LOGSdatavg-LOGSb_lv 12746752    30  12746722    1%  /ORACLE/LOGS/b
SERVER:~ # mount | grep -i /ORACLE/LOGS/b
/dev/mapper/LOGSdatavg-LOGSb_lv on /ORACLE/LOGS/b type ext3 (rw)
/dev/mapper/LOGSdatavg-LOGSblog_lv on /ORACLE/LOGS/b/log type ext3 (rw)
SERVER:~ # 
SERVER:~ # uname -a
Linux SERVER 2.6.16.60-0.91.1-smp #1 SMP Tue Oct 18 21:40:25 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux
SERVER:~ # cat /etc/SuSE-release 
SUSE Linux Enterprise Server 10 (x86_64)
VERSION = 10
PATCHLEVEL = 4
SERVER:~ # 

Does anybody know why is 82G+9G=96G? Because I think it's 91G.

Answer 1

It is probably the reserved space for root user. Check it through:

sudo tune2fs -l /dev/mapper/LOGSdatavg-LOGSb_lv | grep -i reser

It is usually 5% of the volume size, exclusively reserved for use by the root user.

From the manual page:

reserved-blocks-percentage

[The] percentage of the filesystem which may only be allocated by privileged processes. Reserving some number of filesystem blocks for use by privileged processes is done to avoid filesystem fragmentation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem. Normally, the default percentage of reserved blocks is 5%.

Answer 2

A certain percentage of your disc is reserved to root and is not shown by df as available. Usually this is 5% - approx 5GiB in your 96GiB disc - and this explains the difference you see.

You can change this value with tune2fs with ext[2|3|4] filesystems.