Maximize available disk space on Ext 3 & 4 filesystems

By default, Ext 3 & 4 filesystems reserve 5% of available disk space for special processes (root user, syslog, fragmentation avoidance, etc.). On big partitions, it amount to quite a lot of lost space. This howto describe how to get back this disk space.

This howto is tested on:

• Debian 5.0 Lenny
• Debian 6.0 Squeeze
• Ubuntu 11.04 Natty Narwhal
• Debian 10.0 Buster

Settings

Choose the partition for which reserved disk space will be removed.

partition='/dev/sda5'

Set the new reserved blocks percentage (here, 1% in stead of the default 5%):

reservedBlocksPercentage='1'

Installation

Detect if sudo is available (“command” is used if not):

cmdProxy='command'
command type -f 'sudo' &>'/dev/null' && cmdProxy='sudo'

Detect the filesystem block size:

blockSize="$(${cmdProxy} dumpe2fs "${partition}" 2>'/dev/null' \
  | command grep "Block size" \
  | command sed -e 's/.* //')"

Detect the current number of reserved blocks:

currentReservedBlockCount="$(${cmdProxy} dumpe2fs "${partition}" 2>'/dev/null' \
  | command grep "Reserved block count" \
  | command sed -e 's/.* //')"

Free the reserved blocks:

${cmdProxy} tune2fs -m "${reservedBlocksPercentage}" "${partition}"

Detect the new number of reserved blocks:

newReservedBlockCount="$(${cmdProxy} dumpe2fs "${partition}" 2>'/dev/null' \
   | command grep "Reserved block count" \
   | command sed -e 's/.* //')"

Compute the freed disk space (in Mega bytes):

freedBlockCount="$(( ${currentReservedBlockCount} - ${newReservedBlockCount} ))"
echo "Freed block count: ${freedBlockCount}
Freed disk space:  $(( ( ${freedBlockCount} * ${blockSize} ) / 1000000)) Mb"

Thanks

• Thanks to Termitor on #ubuntu-fr IRC channel for this trick.