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

[Nigel Cunningham <ncunningham@crca.org.au>]

Hi.

On Wed, 2009-05-13 at 10:03 +0200, Rafael J. Wysocki wrote:
> 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.

Sorry for coming into this discussion late - I was unavailable most of
last week.

Another point to remember is that syncing from userspace will not be
guaranteed to stop data loss. To be sure that all of userspace's writes
are synced, we need to do the syncing after userspace has stopped
submitting the writes, which implies after userspace has been frozen.

Regards,

Nigel