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

Peter Jeremy peterjeremy at
Tue Jun 21 10:51:43 UTC 2011

On 2011-Jun-17 11:42:26 -0700, Kirk McKusick <mckusick at> wrote:
>> Date: Fri, 17 Jun 2011 17:34:15 +0200
>> From: Hans Ottevanger <hans at>
>> 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.

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.

Peter Jeremy
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