Re: Re: standby to disk transition

["Rafael J. Wysocki" <rjw@sisk.pl>]

Hi,

On Tuesday 14 March 2006 21:33, Pavel Machek wrote:
> > > We need to be able to rollback the state of the filesystem in memory and on 
> > > disk to the point where the last checkpoint was made. Memory would be 
> > > straight forward if we want to do it dumbly and slowly - just reload the 
> > > whole check pointed image. If we want to be more efficient, we'd want to just 
> > > load the pages that had changed (Mark on (first) write?). But filesystems 
> > > seem to be a whole different story. Do any of the commonly used fses have 
> > > support for checkpointing and rollback back at the moment?
> > 
> > I'm not sure if we need a rollback as such.  What we need is to make sure
> > the filesystems state will be consistent before as well as after we have
> > "reloaded" the snapshot.
> 
> Even if you make sure *kernel* is consistent with changed filesystem,
> userland is going to be badly confused. Imagine what will happen with
> memory mapped files, for example. 

Something like what happens when you suspend with a mounted CD
and mmapped files from there, then you replace the CD while suspended
and resume.  Not a wise thing to do, but I think people will do such things
from time to time and we'd better be prepared to handle them nicely.

> I'm not sure how it could work...

IMO memory mapped files are the most difficult problem here,
but the rest seems to be doable in general.

Rafael