Re: du accuracy

[Russell Coker <russell@coker.com.au>]

On Fri, 10 Apr 2015, Liu Bo <bo.li.liu@oracle.com> wrote:
> > Above are some consecutive du runs.  Why does the space used go from 1.2G
> > to  1.1G before going up again?  The file was created by "cat /dev/sde >
> > 2gsd" so it definitely wasn't getting smaller.
> >
> > 
> >
> > What's going on here?
> 
> What's your mount options?  with autodefrag or compression?

Linux server 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt4-2 (2015-01-27) x86_64 
GNU/Linux

Above is the kernel I'm using, one of the recent ones from Debian/Jessie.

UUID=XXXX /big btrfs skip_balance,relatime,nosuid,nodev 0 0

Above is the /etc/fstab line.

/dev/sdb2 /big btrfs 
rw,seclabel,nosuid,nodev,relatime,space_cache,skip_balance 0 0

Above is the /proc/mounts line.  I'm not doing anything noteworthy here.  

# du -h tmp|tail -1 ; sleep 10 ; du -h tmp|tail -1 
1.6G    tmp
1.4G    tmp

I've just had something similar happen when rsyncing a directory full of files 
to the server and running du to check on the progress.  It's probably possible 
that a rename could happen at the wrong time to cause a file to be missed in 
the du count.  So this could be legit.  But the case described in my previous 
message concerned a single file that was being extended.

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