Re: [PATCH 068/104] virtiofsd: passthrough_ll: control readdirplus

[Miklos Szeredi]

On Fri, Jan 10, 2020 at 4:18 PM Daniel P. Berrangé <address@hidden> wrote:
>
> On Fri, Jan 10, 2020 at 04:13:08PM +0100, Miklos Szeredi wrote:
> > On Fri, Jan 10, 2020 at 4:04 PM Dr. David Alan Gilbert
> > <address@hidden> wrote:
> > >
> > > * Daniel P. Berrangé (address@hidden) wrote:
> > > > On Thu, Dec 12, 2019 at 04:38:28PM +0000, Dr. David Alan Gilbert (git) 
> > > > wrote:
> > > > > From: Miklos Szeredi <address@hidden>
> > > > >
> > > >
> > > > What is readdirplus and what do we need a command line option to
> > > > control it ? What's the user benefit of changing the setting ?
> > >
> > > cc'ing Miklos who understands this better than me.
> > >
> > > My understanding is that readdirplus is a heuristic inherited from NFS
> > > where when you iterate over the directory you also pick up stat() data
> > > for each entry in the directory.  You then cache that stat data
> > > somewhere.
> > > The Plus-ness is that a lot of directory operations involve you stating
> > > each entry (e.g. to figure out if you can access it etc) so rolling it
> > > into one op avoids the separate stat.  The unplus-ness is that it's an
> > > overhead and I think changes some of the caching behaviour.
> >
> > Yeah, so either may give better performance and it's hard to pick a
> > clear winner.  NFS also has an option to control this.
>
> IIUC from the man page, the NFS option for controlling this is a client
> side mount option. This makes sense as only the client really has knowledge
> of whether its workload will benefit.
>
> With this in mind, should the readdirplus control for virtio-fs also be a
> guest mount option instead of a host virtiofsd CLI option ? The guest admin
> seems best placed to know whether their workload will benefit or not.

Definitely.   In fact other options, e.g. ones that control caching,
should probably also be client side (cache=XXX, writeback,
timeout=XXX, etc).

This needs an extension of the INIT message, so options can be passed
to the server.   Added this to our TODO list.

Thanks,
Miklos