Is df giving wrong numbers?

stf92 · 09-10-2011, 12:59 AM

Kernel 2.6.21.5, slackware 12.0
du belongs to coreutils.
coreutils 6.9

Hi:
How can this be?

Code:

semoi@darkstar:~/bin$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda3              18G   14G  3.3G  81% /
/dev/hda1              20G  9.2G   11G  47% /xp
/dev/hdc              1.6G  1.6G     0 100% /mnt
semoi@darkstar:~/bin$

I don't understand the Use = 81% datum. 14 / 18 = -0.77 < 0.81. If you think its a rounding error,

Code:

semoi@darkstar:~/bin$ df   
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda3             18097492  13804204   3373972  81% /
/dev/hda1             20472816   9608416  10864400  47% /xp
/dev/hdc               1670664   1670664         0 100% /mnt
semoi@darkstar:~/bin$

13804204 / 18097492 = ~0.76, still well below 0.81 whereas
9608416 / 20472816 = ~0.47, the right figure. I guess du is right, but what is the cause of the discrepancy?

NoStressHQ · 09-10-2011, 01:09 AM

The % is about the effective used space...
Try (18-3.3) / 18
Or (18097492 - 3373972) / 18097492

and you get the right "used percentage"...

Edit: On the disk, the files spend "round blocks" so many tiny files lose some space...

ponce · 09-10-2011, 01:16 AM

ext* filesystems reserve 5% of the space by default: this is done to let daemons run to allow you, for example, a n ssh login into the host also with a 100% full filesystem.

stf92 · 09-10-2011, 02:19 AM

I see. Size - (Used + Avail) = 919316. And 919316 / Size = 0.051, and sure enough, if I do it in bytes, I get 0.050! Fine arithmetics, thanks.

rg3 · 09-11-2011, 12:29 PM

More specifically, the space is not reserved for any daemon but for the superuser. See the -m option explanation in "man mke2fs".