Freebsd filesystem ( hard reboot )

Peter Schuller peter.schuller at
Thu Dec 6 11:20:15 PST 2007

> Well any number of things, but the most recent was a prolonged power
> outage.

It is important to differentiate between expected fsck activity and 

If you are running without write caching turned on (which is the default), a 
power outtage will constitute a crash from which a file system cannot 
guarantee to recover unless it takes measure to punch through the write cache 
at appropriate moments.

If you machine shutdown softly as a result of the UPS communicating that power 
was running out, you "should" not have to fsck (barring other issues in the 
past). fsck need in these cases would indicate a software bug, or a hardware 

If on the other hand your machine just lost power when the UPS finally died, 
you are relying on luck for recovery if you're on ufs/reiserfs/xfs/etc. Some 
environments will correctly handle this (e.g., ZFS), but most won't. The 
problem being that drive write caching will prevent the file system from 
guaranteeing ordering of certain critical operations that must be ordered in 
order to guarantee successfull recovery to a consistent state.

That said, you are not supposed to need to answer interactive questions on 
boot for all cases of expected inconsistencies. If you are getting prompts as 
a result of unexpected inconsistencies, that indicates *something* is wrong.

/ Peter Schuller

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