SU+J: negative used diskspace (for a while)

[Peter Holm]

On Thu, Jun 23, 2011 at 04:45:38PM +0200, Hans Ottevanger wrote:
> On Tue, Jun 21, 2011 at 06:46:45PM +1000, Peter Jeremy wrote:
> > On 2011-Jun-17 11:42:26 -0700, Kirk McKusick <mckusick at mckusick.com> wrote:
> > >> Date: Fri, 17 Jun 2011 17:34:15 +0200
> > >> From: Hans Ottevanger <hans at beastielabs.net>
> > >> So it takes more than a minute before the disk space is back to "normal"
> > >> values.
> > 
> > >We used to account for deleted blocks at the instant that they were
> > >removed. This accounting was rather complex, so as part of doing
> > >SU+J, Jeff simplified it. Under the simplification, the removal is
> > >not accounted for until part way through the removal process. The
> > >result is that you now get these false negative block counts until
> > >the blocks have been partially reclaimed. If this behavior causes
> > >enough trouble, Jeff might be convinced that the more accurate block
> > >accounting is necessary.
> > 
> > Negative values may also impact NFS clients - though just limiting the
> > reported used space to 0 should avoid them getting too upset.
> > 
> 
> It appears that negative values occur only when the filesystem is almost
> empty (like in my tests). Otherwise the values are just way too low for
> a while.
> 
> > That said, whilst I haven't seen negative used values, ZFS and
> > Solaris UFS also take an extended period before 'df' reports
> > correct values (several minutes for Solaris UFS).  In the case
> > of ZFS, even 'du' can report incorrect information for a while.
> > 
> 
> If I remember well, in "the old days", when Soft Updates was fairly new,
> it also had such issues in the first release, but it was repaired soon
> afterwards.
> 
> Of course it is a real advantage to see the results immediately when you
> are cleaning out filesystems, especially when you are in a hurry.
> 
> I just verified that the current issue was quite recently introduced with
> commit r222958. Before that 'df' reported changes promptly and correctly,
> even with Journaling enabled.
> 
> As an answer to my original message to -current@ Peter Holm has assured me
> (http://lists.freebsd.org/pipermail/freebsd-current/2011-June/025318.html)
> that this is a known issue that is on Jeff's TODO list.
> 

Ah, well my exact wording was "This is a known issue and I believe
that it is on Jeff's to-do list". So I'm afraid I can only assure
you that the problem is known :)

- Peter