By default, an ext3 filesystem will reserve 5% of its capacity for special processes that can continue to run when diskspace is running low. Here's the explanation according to the manual.
Specify the percentage of the filesystem blocks reserved for the super-user. This avoids fragmentation, and allows root-owned daemons, such as syslogd(8), to continue to function correctly after non-privi-leged processes are prevented from writing to the filesystem. The default percentage is 5%.
On large partitions (say 8TB+), losing 5% for only those processes is quite a lot. You can check your current value with tune2fs.
# tune2fs -l /dev/partition | grep 'Reserved' Reserved block count: 120542571
In thise case, it means we've 'lost' the ability to use 120542571 blocks on the partition.
You can lower the 5% barrier if you like, but I only suggest doing this for larger partitions. Since the default is 5%, I'll lower it to 2% here.
# tune2fs -m2 /dev/partition tune2fs 1.39 (29-May-2006) Setting reserved blocks percentage to 2% (48223887 blocks)
With the '-m' parameter you can specify a percentage amount of blocks to reserve for that purpose.