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

[Miklos Szeredi]

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.

Thanks,
Miklos