Re: Networked filesystems vs backing_dev_info

[Peter Zijlstra]

On Sat, 2007-10-27 at 16:02 -0500, Steve French wrote:
> On 10/27/07, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> 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().

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

and you should be set.

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