fsck strangeness

[Chris]

On 20/08/07, Ian Smith <smithi at nimnet.asn.au> wrote:
> Sorry for the repeat post folks, but I goofed last time, leaving out the
> subject line while replying to the digest.  Still curious .. Ian
> =======
>
> On Sat, 18 Aug 2007 21:32:28 +0200 Erik Trulsson <ertr1013 at student.uu.se> wrote:
>  > On Sat, Aug 18, 2007 at 08:21:42PM +0100, Christopher Key wrote:
>  > > Hello,
>  > >
>  > > I'm having some rather strange behaviour with fsck.
>  > >
>  > > When I boot the system, it asserts that all the file systems are clean, but
>  > > subsequently running an fsck on /dev/ad8s1e (mounted as /var) detects
>  > > errors.  Even if this first check is run whilst the file system is mounted,
>  > > and is hence run in NO WRITE mode, a second check doesn't find block
>  > > errors.  If I then unmount the file system and check the disk, it's fine,
>  > > as indeed it is if I unmount, remount, then check.  However, if I then
>  > > reboot, the process repeats, and an fsck immediately after reboot will find
>  > > errors again.  If I bring the system up in single user mode, and run fsck
>  > > either before or after mounting /var, it finds no errors.
>  > >
>  > > I'm running 6.2_RELEASE with a custom kernel based upon generic-smp, but
>  > > with a lot of unecessary bits removed, and geom_mirror compiled in.  I
>  > > don't think it's the drive that's at fault, all the other partitions in the
>  > > slice are fine, it's a fairly new drive, and it passes a self test quite
>  > > happily.  Included below is a transcript that attempt to show what's going
>  > > on in detail, is there anything else relevant?
>  > >
>  > > Can anyone suggest what might be going on and how to fix it, or suggest
>  > > some slightly better diagnostics?  Apologies if this is an RTFM issue, I
>  > > have had a good dig through the handbook, but can't seem to find anything
>  > > that helps.
>
>  > Running fsck on a file system that has been mounted read/write will almost
>  > always report spurious errors and can really screw up the disk if it tries
>  > to 'correct' those errors.
>
> I'm a bit confused by this.  I've been running 'fsck -n' over FreeBSD
> systems since 2.2.6, and modulo seeing the at-the-time inconsistencies
> on those filesystems in /etc/fstab that are mounted, as Chris reported
> and as are expected, I've never had a problem with it, nor seen the sort
> of inconsistent results between runs that Chris is reporting.
>
>  > You should normally not run fsck on a mounted filesystem and you should
>  > *NEVER* run fsck on a filesystem that has been mounted read/write.
>
> This seems to imply that using the -n switch may have different results
> than not using it and having fsck determine 'NO WRITE' itself from the
> fact that it's noticed that the fs is mounted?  Are you suggesting by
> "can really screw up the disk if it tries to 'correct' those errors"
> that fsck might WRITE to a mounted fs that it's showing as 'NO WRITE'?
>
> I've never had any screwups with it, but then I've always specified -n.
>
> Later Bill Moran said:
>
>  > Don't run fsck on mounted filesystems unless they're mounted read-only.
>  >
>  > Although, it's possible I misunderstood your description of the problem.
>
> so I'm still curious, and am wondering if Chris using SMP kernel and/or
> geom_mirror might have anything to do with this?  Or whether his use of
> 'umount -f' might be (or cause) the problem indicated by his results?
>
>  > > # umount -f /var
>  > >
>  > >
>  > > # mount /var
>
> Cheers, Ian
>
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If its bad to run fsck on a mounted read,write then why does
background fsck do it? or you talking about foreground fsck only?

Chris