Re: [linux-pm] [RFC] why do we need run disk sync before entering S3

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

On Wednesday 13 May 2009, Alan Stern wrote:
> On Wed, 13 May 2009, Zhang Rui wrote:
> 
> > Hi, all,
> > 
> > I did some S3 tests on an eeepc901, the total suspend time(from issue
> > the suspend command to power down) is about 2.5s~3s.
> > something interesting is that kernel runs disk sync before entering S3
> > state, and this takes about 0.7~1.2s.
> > my question is that, why do we need this for s2ram?
> > can we remove this and run sys_sync for S4 only?
> 
> At the risk of sounding foolish, I'd guess that a system in S3 (or more
> generally, suspend-to-RAM) is a lot more at risk of losing power or
> failing to restore than a normally running system.  (A normally running
> system is trivially not at risk of failing to restore!)  Consequently
> it makes sense to flush the I/O buffers before entering this state, to
> minimize the potential for loss of data.
> 
> When you think about it, a system in S4 is actually _less_ likely to 
> run into trouble than one in S3, since it can't fail because of loss of 
> power.  So if anything, we should remove the disk sync from hibernation 
> and leave it in system suspend.

I generally agree, but I think we may also leave the syncing to the user space,
in both cases.

Thanks,
Rafael