[RFC] Should vfs.nfsrv.async be implemented for new NFS server?

[Rick Macklem]

Ivan Voras wrote:
> On 9 November 2011 17:09, Rick Macklem <rmacklem at uoguelph.ca> wrote:
> > Ivan Voras wrote:
> > [good stuff snipped for brevity]
> >>
> >> This seems too extreme... doesn't NFSv4 have its own fsync()-like
> >> RPC
> >> that does that manually? If it does, then I don't think there are
> >> any
> >> differences between doing a write() on a local file system with
> >> e.g.
> >> soft-updates enabled and doing a write on a NFS file system - in
> >> both
> >> cases, no data is even remotely guaranteed to survive a crash
> >> unless a
> >> fsync (or equivalent operation) was issued.
> >>
> > I think you have to be a bit careful when an assumption about loss
> > of
> > data after a crash is applied to a file server. If a client crashes,
> > everyone pretty much assumes that recently written data might have
> > been
> > lost (in a Unix-like world, at least) and the crash is visible to
> > users.
> >
> > However, take the following scenario:
> > - User on desktop client edits a document for an hour, then saves
> > it.
> >  --> NFS server crashes/reboots just after the "save".
> > - User on desktop client goes off reading email for an hour, using a
> >  mail system not affected by the NFS server crash/reboot.
> > - User goes back to document and finds an old damaged document file.
> >  --> User is very upset...
> >
> > My point is that a user on a client may not even realize the server
> > crashed/rebooted. The above case can happen if async!=0, but not if
> > async==0.
> 
> Thank you for the excellent explanation! I see the problem but I think
> it is still well within the judgement call of the system
> administrator. It should probably not be the default (though I recall
> from some benchmarks way back that it might be the default on Linux)
> but it should definitely exist as an option.
> 
Yes, agreed. I think the trick is making sure the sysadmins understand
what the option does, so they can make the call.

Unless others jump up and say otherwise, I'll probably commit the patch
to head once I here from Josh that it works.

Have fun with it, rick