fsck talk

[Pav Lucistnik]

LI Xin píše v st 25. 10. 2006 v 12:07 +0800:
> Pav Lucistnik wrote:
> > I was poking around /etc/rc.d/fsck today, and I figured out a few
> > interesting things.
> > 
> > Think three large partitions, for example:
> > /dev/aacd0s1e   /a
> > /dev/aacd1s1e   /b
> > /dev/aacd2s1e   /c
> > 
> > Now, disable background fsck and enable -y flag in rc.conf:
> > background_fsck="NO"
> > fsck_y_enable="YES"
> > 
> > Now imagine a power failure. On reboot, fsck -p manages to mark /a
> > and /b clean, but fails on /c.
> > 
> > Next, fsck -y is executed, which goes over /a again, then over /b and
> > then finally over /c.
> > 
> > Two problems here:
> > 
> > 1) fsck -y does not skip filesystems already cleaned in preen run,
> > because fsck_ffs only skips clean filesystems in preen mode.
> > 
> > 2) fsck -y runs forever, because fsck does execute the jobs in parallel
> > only in preen mode.
> > 
> > Proposed solutions to problem 1:
> > 
> > - change the behaviour of fsck_ffs when executed on clean filesystem,
> >   like this:
> > 
> >     fsck -y /a
> > 
> >   to skip it with message 'already clean'.  There is a -f flag for
> >   people who really want to do a fsck.
> > - add a new flag to fsck_ffs that will skip clean filesystems in
> >   non-preen mode
> 
> I think it is reasonable not to check clean file system, since -f means
> "force", by not specifying the option, it's supposed that clean file
> systems are not being checked, so I vote for the first proposed solution.
> 
> > Plus
> > 
> > - when user sets fsck_y_enable, we can just skip the preen run entirely,
> >   under the condition that one of the above was implemented. That's
> >   another speedup.
> > 
> > Proposed solutions to problem 2:
> > 
> > - change fsck to parallelize jobs in non-preen mode by default
> > - add a flag to fsck to parallelize jobs in non-preen mode
> 
> For very large volumes it can be very nasty to do parallelized check,
> because one volume can cost a lot of memory, which can in turn cause
> fsck to fail.  Of course, I think a flag to parallelize the check would
> be a nice feature and users can optionally enable it through a rc.conf knob.

Thank you for your feedback.

I already have patches to implement majority of this at
http://people.freebsd.org/~pav/fsck/

Feel free to look at them.

-- 
Pav Lucistnik <pav at oook.cz>
              <pav at FreeBSD.org>

Pilot to Co-Pilot: Boo.
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