The default options for ext3 formatting is to reserve 5% of the partition's space for usage by root, so that the admin can always log in. But 5% on today's storage devices is a fscking big space nevertheless, so it's a good practice setting the reserved space to a lower percentage of the partition space.
DO THIS ONLY ON A UNMOUNTED FILESYSTEM.
For example, to set an arbitrary percentage for reserved space on an existing ext3 filesystem:
# tune2fs -m ${percentage_number} /dev/$partition_device
Or, you could set an arbitrary value of reserved blocks using the option -r instead of -m.
Assuming a default of 4 Kilobytes blocks, every 256 blocks are a reserved Megabyte. Setting 10 Megabytes of reserved space would be like this:
# tune2fs -r 2560 /dev/partition_device
Or... you could read the manual of mke2fs before creating a new filesystem.
