Re: Why is one sync() not enough?

[Denis Vlasenko]

On Wednesday 15 June 2005 11:17, Helge Hafting wrote:
> Nico Schottelius wrote:
> 
> >Hello again!
> >
> >When my system shuts down and init calls sync() and after that
> >umount and then reboot, the filesystem is left in an unclean state.
> >
> >If I do sync() two times (one before umount, one after umount) it
> >seems to work.

sync before umount is superfluous.

> >Can someboy explain that to me?
> >  
> >
> You shouldn't need those syncs, as umount does its own
> syncing.  There may be other explanations:
> 
> * Your reboot actually powers down (or resets) the disk.
>    IDE disks are known for caching stuff, they may indicate
>    that data is written slightly before it actually happens.
>    (The same applies to scsi - if you enable caching there for
>    the little extra performance it buys.)
>  
>    Rebooting really quickly after umount in such a case can cut
>    power to the disk before it finishes writing.  If this is the case,
>    then a few seconds of sleep after umount before reboot
>    will work just as well as that sync.  I don't recommend this
>    as a solution, but it is an easy diagnostic!
> 
> * Your startup script accidentally mounted the fs twice.
>   (Yes - linux support that, and the first umount won't undo
>    both mounts.)  This simply means the fs isn't umounted
>   when you reboot, but an extra sync and you might get lucky.

My reboot script is checking (/proc/mounts) for stray rw mounts on reboot,
prints a warning and waits for a keypress. This helps spot such things.
--
vda

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