Last night I made a test for making a comparison between ext2, ext3 and reiser file systems about missing (!) disk spaces.

Basically I created 3 disk partitions with 1 gb size and formatted them with ext2, ext3 and reiser file system types. Then mounted them to folders same with filesystem names. The result df -h output is as below:

Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb1            1004M   20K  953M   1% /ext2_fs
/dev/sdb2            1004M   17M  937M   2% /ext3_fs
/dev/sdb3            1020M   33M  988M   4% /reiser_fs

As you can see both ext2 and ext3 file systems are reserved 50 megabytes at 1 gigabyte disk partition.
This means 5% of disk is missing!!!

Calm down, here is the reason: (man tells everything to us )

This  reserved disk area avoids fragmentation,  and  allows  root-owned  daemons, such  as  syslog,  to continue to function correctly after on-privileged processes are prevented from writing to the filesystem.  The default percentage is 5%.
This means if you format a 300GB disk with ext2 or ext3, you will lost about 15GB for root owned daemons as default.

But you can modify this ratio by using tune2fs command.

For instance, after umounting the partition, you may run tune2fs command as below for decreasing reserved block at /dev/sdb2 partition to 1%:

tune2fs -m 1 /dev/sdb2

You may sea the result bye typing

tune2fs -l /dev/sdb2

and comparing ” block count” and “reserved block count” values.

I want to remind a little point about tune2fs. tune2fs tool can be used just for ext2 and ext3 file systems.
If you want to use a tool like tune2fs for resier file system you have to use reiserfstune tool.
But you have to consider that there is not a parameter for setting reserved disk percentage for reiserfs.