Reserved disk space on the root partition in Linux

In Linux by default a certain percentage of space (by default 5%, but you can change that with the -m option to tune2fs) is reserved on a root partition. So if you run df on an almost full filesystem, you may see about 100% used, but still there is a difference between the size and the total used space! This is because the percentage refers to the unreserved space.

# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 193G 183G 288M 100% /
...
Why is some space put aside? Consider this reserved space can only be used by processes running with the root privileges. So the purpose is to allow some of the process to continue running for a while and using more disk space even if some other non-privileged processes gone crazy fill all the non-reserved space. If you have a very big drive, you may want to reduce the percentage of reserved space to avoid waste. E.g. 5% of 1 TB (one terabyte) is about 51 GB, but 5% of 6 TB is a whopping 307 GB, probably too much reserved space, if you have a root partition that big.

If you are in dire straits you may want to unreserve this space so it can be used by non-root processes:

# tune2fs -m 0 /dev/sda3

Then you suddenly have a 5% more free space:

# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 193G 183G 11G 95% /
...

but beware that the filesystem may get fragmented badly without any space reserved. ext4 suffers less from this problem, so it is recommended that you remount the root partition as ext4, if you are still using ext3. This is safe to do and does not require a conversion (of course not all ext4 features will be available). Just replace ext3 with ext4 in /etc/fstab for your root partition (/) and reboot. You can safely revert to ext3 the same way. Anyway, you cannot live long with an almost full partition. You need to make more space available and/or cleanup.