Silly question regarding "du" and "df".
marius at pov.lt
Fri Jan 26 13:39:47 UTC 2007
On Fri, Jan 26, 2007 at 05:45:48PM +0800, Michael T. Richter wrote:
> Being who I am, I also run df and get this:
> Filesystem Size Used Avail Use% Mounted on
> /dev/sda1 79G 38G 38G 50% /media/Repository80GB
> Now I cannot for the life of me make the numbers add up.
That's because by default 5% of disk space in ext3 partitions is
reserved and not included in the Avail column. 79G * 5% ~= 4G. 38G +
38G + 4G = 80G. The 1G difference is just rounding error, run df without
-h to see the exact block counts.
There are two reasons for reserving 5%:
* It reduces disk fragmentation.
* If a user fills up the partition, the superuser can still create
> Absolutely the
> only thing on the disk is this (partial) mirror. Yet "du" reports that
> only 35GB is used up while df reports that 38GB is used up.
That's a bit more interesting. Here's the du report I snipped above:
> 22G pool/main
> 2.8G pool/multiverse
> 591M pool/restricted
> 9.2G pool/universe
> 35G total
The difference can consist of:
* files/directories that are siblings of 'pool' (there may be hidden
ones), but I think that's unlikely
* deleted files that are still opened by some application (their space
will be freed when all applications close them).
* filesystem metadata (block usage bitmaps, inodes, the ext3 journal).
* errors in accounting if the machine crashed and didn't unmount the
filesystem, or if there's corruption of some sort.
I'm pretty certain that in your case those 3 gigs are in filesystem
> Now perhaps
> I can see slack space making up the difference (although almost 10%
> slack space is pretty odd for such large files).
du includes slack space in its calculations. Compare the files sizes
printed by du with those printed by ls -l.
> In case there's some kind of "quota only for root" thing going on (I
> know I can do this in other operating systems so I assume that even
> UNIXen can do it), I su -i and try again. Same numbers all around.
Being root does not change the numbers shown by df. You can see the
number of reserved blocks in a filesystem with tune2fs -l /dev/hdaX.
Midnight Commander also shows the whole available space, IIRC.
> So why do "du" and "df" report different amounts used? And why do the
> numbers reported by "df" not add up? (And why does System
> Monitor->Devices report for that drive a size of 73.4GB instead of 79
> but reports the same amount used as "df"?)
That may be a difference in units. 1G can mean 1000000000 bytes, or it
can mean 1073741824 bytes. 73.4 metric gigabytes = 78.8 binary
gigabytes (or "gibibytes", see http://en.wikipedia.org/wiki/Gibibyte).
> Is this a case of the man
> with two watches never knowing what time it is?
No, just historical legacy.
The memory management on the PowerPC can be used to frighten small children.
-- Linus Torvalds
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