Re: Networked filesystems vs backing_dev_info

[Peter Zijlstra <peterz@infradead.org>]

On Sat, 2007-10-27 at 23:30 +0200, Peter Zijlstra wrote:
> On Sat, 2007-10-27 at 16:02 -0500, Steve French wrote:
> > On 10/27/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > > Hi,
> > >
> > > I had me a little look at bdi usage in networked filesystems.
> > >
> > >  NFS, CIFS, (smbfs), AFS, CODA and NCP
> > >
> > > And of those, NFS is the only one that I could find that creates
> > > backing_dev_info structures. The rest seems to fall back to
> > > default_backing_dev_info.
> > >
> > > With my recent per bdi dirty limit patches the bdi has become more
> > > important than it has been in the past. While falling back to the
> > > default_backing_dev_info isn't wrong per-se, it isn't right either.
> > >
> > > Could I implore the various maintainers to look into this issue for
> > > their respective filesystem. I'll try and come up with some patches to
> > > address this, but feel free to beat me to it.
> > 
> > I would like to understand more about your patches to see what bdi
> > values makes sense for CIFS and how to report possible congestion back
> > to the page manager. 
> 
> So, what my recent patches do is carve up the total writeback cache
> size, or dirty page limit as we call it, proportionally to a BDIs
> writeout speed. So a fast device gets more than a slow device, but will
> not starve it.
> 
> However, for this to work, each device, or remote backing store in the
> case of networked filesystems, need to have a BDI.
> 
> >   I had been thinking about setting bdi->ra_pages
> > so that we do more sensible readahead and writebehind - better
> > matching what is possible over the network and what the server
> > prefers.  
> 
> Well, you'd first have to create backing_dev_info instances before
> setting that value :-)
> 
> >   SMB/CIFS Servers typically allow a maximum of 50 requests
> > in parallel at one time from one client (although this is adjustable
> > for some).
> 
> That seems like a perfect point to set congestion.
> 
> So in short, stick a struct backing_dev_info into whatever represents a
> client, initialize it using bdi_init(), destroy using bdi_destroy().

Oh, and the most important point, make your fresh I_NEW inodes point to
this bdi struct.

> Mark it congested once you have 50 (or more) outstanding requests, clear
> congestion when you drop below 50.
> 
> and you should be set.
>