Re: negative diskspace usage

[Andries Brouwer]

On Fri, Jan 21, 2005 at 03:11:06PM +0100, Wichert Akkerman wrote:

> After cleaning up a bit df suddenly showed interesting results:
> 
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/md4             1019M  -64Z  1.1G 101% /tmp
> 
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/md4               1043168 -73786976294838127736   1068904 101% /tmp
> 
> This is on a ext3 filesystem on a 2.6.10-ac10 kernel.

Funny.

The Used column is total-free, so free was 2^66 + 964440.
That 2^66 no doubt was 2^64 in a computation counting 4K-blocks,
and arose at some point where a negative number was considered unsigned.

But having available=1068904 larger than free=964440 is strange.

I assume this was produced by statfs or statfs64 or so.
You can check using "strace -e statfs64 df /dev/md4" that
these really are the values returned by the kernel,
so that we can partition the blame between df and the kernel.

The values are computed by

        buf->f_blocks = es->s_blocks_count - overhead;
        buf->f_bfree = ext3_count_free_blocks (sb);
        buf->f_bavail = buf->f_bfree - es->s_r_blocks_count;

that is: blocks = total - overhead, and available = free - reserved.
strace shows three values, and I expect tune2fs or so will show 2 more.

More available than free sounds like a negative count of reserved blocks.
Are you still able to examine the situation?

Andries

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