df -h - Used space + Avail Free space is less than the Total size of /home

Question

As the output of df -h shows here, something is eating up 5GB of free space. So, it's not available to use.

I'm also noticing sometimes that the hard disk gets filled up to 100% sometimes. So, I had to restart the machine or delete some unncessary files. Only noticed these in /home partition. Don't know whether these two are related, but appreciate if anyone can put some insight into this.

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda8             100G   92G  2.7G  98% /home

Answer 1

By default, ext2/3/4 filesystems reserve 5% of the space to be useable only by root. This is to avoid a normal user completely filling the disk which would then cause system components to fail whenever they next needed to write to the disk.

You can see the number of reserved blocks (and lots of other information about the filesystem) by doing:

sudo tune2fs -l /dev/sda8

For a /home partition, it is probably safe to set the reserved fraction to zero:

sudo tune2fs -m 0 /dev/sda8

Which should make an additional ~5GB available.

Answer 2

This question is really about interpreting the output of the df command, which is famously confusing and if you Google it, you'll see many questions about df.

How to interpret df's output

Size in df's results are inclusive of the 5% reserved space @chronitis told you about. This is the total absolute space. This number will approximate the size shown in the results of fdisk -l for the partition which you're reviewing in the df results.

Used+Avail provides the total effective space and excludes the 5% reserved space. This is the space you can actually work with.

So nothing is missing. The disparity between Size and Used+Avail totals in df's output represents the 5% reserved space.

Free space not accounted for in df's output

If you're scratching your head thinking "But I deleted that file(s), why can't I see the additional space in df?!?", here's why:

If you delete a large file(s) but the process for it hasn't been terminated, this "free" space won't be reflected in the results of df.

sudo lsof +L1 will identify deleted files still showing a PID.

Answer 3

By default, ext2/3/4 filesystems reserve 5% of the space to be useable only by the superuser, root.

There is also some space reserved for metadata, that is necessary for the file system, for examples inodes and the journal.

You can find some details in the manual

man mkfs.ext4