Curious discrepancy

Orion

Just noticed this when I was checking disk usage at home.

~$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md1              686G  144G  508G  23% /

System Monitor agrees with it:


But 507+143 is 650 - not 685.

I've a nice little graphical app - Disk Usage Analyzer which tells a different story:


An extra 40Gb free. And these figures add up.

~$ df -H
Filesystem             Size   Used  Avail Use% Mounted on
/dev/md1               737G   155G   545G  23% /

So which is right and why? It's almost as if df gives the total space using 1Gb=1000Mb but free space as 1Gb=1024Mb. Surely not?

AndrewMc

ext2 and ext3 filesystems typically reserve a small amount of the available space for the root user. Normally this amount is 5%, which is exactly the discrepancy you're seeing. Your first two tools are taking this into account, but the third isn't.

From the mke2fs man page:

-m reserved-blocks-percentage

Specify the percentage of the filesystem blocks reserved for the super-user. This avoids fragmentation, and allows root-owned daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem. The default percentage is 5%.

If you're ever really, really stuck for that last bit of space, you can reduce this percentage (to zero, if you like) with tune2fs.

karlr42

Thanks for the info, I always vaguely wondered what was up, my free disk space always seems to be different depending on the app

Orion

Thanks AndrewMc - didn't know that. So the space is free but reserved. Makes sense.