discrepency between "df" and "du" on tmpfs filesystem?

discrepency between "df" and "du" on tmpfs filesystem?

[Christopher Friesen]

Someone noticed this on one of our machines.  The rootfs is a 256MB 
tmpfs filesystem.  Depending on how you check the size, you get two 
different answers.


root@10.41.50.66:/root> df -hl
Filesystem            Size  Used Avail Use% Mounted on
rootfs                256M  250M  6.4M  98% /
none                   32M  116K   32M   1% /tmp

root@10.41.50.66:/root> df -kl
Filesystem           1K-blocks      Used Available Use% Mounted on
rootfs                  262144    255684      6460  98% /
none                     32768       116     32652   1% /tmp

root@10.41.50.66:/root> du -sxk /
204672  /

root@10.41.50.66:/root> du -sxh /
200M    /

Anyone know what's going on?

Chris

Re: discrepency between "df" and "du" on tmpfs filesystem?

[Hugh Dickins]

On Thu, 1 Dec 2005, Christopher Friesen wrote:
> 
> Someone noticed this on one of our machines.  The rootfs is a 256MB tmpfs
> filesystem.  Depending on how you check the size, you get two different
> answers.
> 
> root@10.41.50.66:/root> df -kl
> rootfs                  262144    255684      6460  98% /
> 
> root@10.41.50.66:/root> du -sxk /
> 204672  /
> 
> Anyone know what's going on?

df tells you what the filesystem says is in use or free, via statfs.
du goes looking at the contents of the filesystem, totalling stats.
Any files unlinked but held open will be counted by df but not by du.
There might also be a discrepancy over indirect blocks, I'm not sure.

Hugh

Re: discrepency between "df" and "du" on tmpfs filesystem?

[Andreas Schwab]

Hugh Dickins <hugh@veritas.com> writes:

> On Thu, 1 Dec 2005, Christopher Friesen wrote:
>> 
>> Someone noticed this on one of our machines.  The rootfs is a 256MB tmpfs
>> filesystem.  Depending on how you check the size, you get two different
>> answers.
>> 
>> root@10.41.50.66:/root> df -kl
>> rootfs                  262144    255684      6460  98% /
>> 
>> root@10.41.50.66:/root> du -sxk /
>> 204672  /
>> 
>> Anyone know what's going on?
>
> df tells you what the filesystem says is in use or free, via statfs.
> du goes looking at the contents of the filesystem, totalling stats.
> Any files unlinked but held open will be counted by df but not by du.
> There might also be a discrepancy over indirect blocks, I'm not sure.

Also an empty filesystem usually does not have zero use, since the
filesystem overhead (inode table, etc) may be accounted in the statfs
counts.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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