fsck talk

[LI Xin]

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.

Cheers,
-- 
Xin LI <delphij at delphij.net>	http://www.delphij.net/
FreeBSD - The Power to Serve!

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