fsck talk

[Pav Lucistnik]

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

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

Comments?

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

The hottest spot in the solar system is neither Mercury, Venus,
nor St. Louis in the summer.
  -- WUSTL Press Release
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