Sysinstall automatic filesystem size generation.

[Don Lewis]

On 29 Aug, Matthias Buelow wrote:
> Mark Kirkwood wrote:
> 
>>>>FreeBSD's filesystems are very robust should you lose power.
>>>This sentence is completely bogus (or at best: wishful thinking)
>>>and should be deleted.
>>It's probably correct if you have hw.ata.wc=0 (and are using IDE drives 
>>obviously).
> 
> I'd like to stress the "probably". I've already seen unrepairable
> filesystem corruption with softupdates enabled in the past with
> "good" scsi disks at power loss.

Did you remember to disable write caching by setting the WCE mode page
bit to zero?  At least with SCSI, it doesn't seem to affect performance
under most workloads.

> In addition, with
> softupdates there seems to be a much higher probability of losing
> files, as I have observed.. that is, getting them zero-truncated,
> or even deleted (which shouldn't happen in that scenario, I'm sure
> I've seen the results of a bug), than without.

I've seen this when doing compile, run, panic experiments.  The
executable that I just compiled would end up with a size of zero after
the reboot because it was still cached in RAM and executing from RAM
when the machine paniced.  The executable was scheduled to be written to
disk about 30 seconds after it was compiled and linked, but the machine
paniced before the 30 seconds was up.

Softupdates only tries to guarantee that the on-disk file system is in a
consistent state at all times, with the possible exception that not all
space may be accounted for.