Free Disk Space by Reducing Reserved Blocks March 12, 2009

I use a one terabyte external hard drive for backing up data. The drive came formatted with a FAT filesystem, which I immediately replaced with the more reliable ext3. After formatting the drive as ext3 I noticed that the storage capacity had dropped considerably.

Why did this happen? Linux filesystems such as ext3 reserve a percentage of their capacity for privileged processes. In the case that the filesystem fills up, important processes with be able to continue functioning and writing to the disk.

By default 5% of a filesystem will be reserved. For modern high-capacity disks, this is much higher than necessary. On my 1 TB disk, this 5% works out to be 50 GB! If you’ve got a large filesystem, or a filesystem which does not store your Linux system files, it’s safe to reduce the percentage of reserved blocks to free up that disk space.

Use the tune2fs utility to do this. The command below would set the percentage of reserved blocks on the partition /dev/sdf1 to 1%:

sudo tune2fs -m 1 /dev/sdf1

Be sure to replace /dev/sdf1 with your own path; you can use the df command to get a listing of your filesystem device paths and their mount points.

I freed 37 GB of space by changing the reserved blocks to 1% on my TB drive.