Stupid question about drive capacity

I attached a USB 1TB drive to my 10.2 system. Capacity is reported as 917GB, which I understand. It was formatted as ext3.

However, I don’t understand this meta info reported by the desktop app:

Free: 648.8 GB
Total: 916.9 GB
Used: 221.5 GB

If I do the math: 221.5 + 648.8 = 870.3

So, what happened to the remaining space? Also, is there a console command I can use to query the stats on the drive, to see if I get a different report?


917 * 0.05 = 45,85GB which equates to 5% that is automatically reserved for ext3 root user upon format and establishing the filesystem.

To change this value to 0% (if you do not need reserved space) you can use the tune2fs command ( man tune2fs )

Ah, thank you…that helps.

Second dumb question: why would I need 5% of reserved space on this drive – for what purpose was it intended?

All I use this drive for is a nightly cron/rsync backup of my home directory and a few other important files.

If it is ok to tune away the 5% I’ll try it…I just don’t know if it’s “ok” (?)


This space is reserved for root, so if i.e the disk is “full” and a user cannot work any more, root can still “repair” it.

I would not set it to 0 if it contains a partition with system- or user-data (/home, /var /usr /tmp and of course /), at least leave 1% free then.

For an external “data storage only” disk, setting the reserved space to 0 is OK.

There’s another reason, performance. For ext3, allocation of new space slows down sharply and fragmentation much more likely as free space drops below a certain percentage as the kernel has to do more work looking for free blocks. So to keep performance acceptable, the solution is simply to reserve some free space. 5% isn’t necessarily the appropriate setting for all uses of the filesystem though, e.g. if the filesystem is pretty static then you can use a lower figure, but it may not be worth your time trying to optimise this.

Look at it this way, 50GB of a 1TB disk may seem a lot, but when you are down to 10%, you’ll probably want to look at buying more disk space. So decreasing that reserved percentage only buys you a little more time, however long it takes you to use up that amount of space.