RE: [PATCH v1 38/38] nfs: add a Kconfig option for NFS reexporting and documentation

["Frank Filz" <ffilzlnx@mindspring.com>]

Jeff Layton said:
> On Wed, 18 Nov 2015 15:22:20 -0500
> "J. Bruce Fields" <bfields@fieldses.org> wrote:
> 
> > On Tue, Nov 17, 2015 at 06:53:00AM -0500, Jeff Layton wrote:
> > > +Filehandle size:
> > > +----------------
> > > +The maximum filehandle size is governed by the NFS version. Version
> > > +2 used fixed 32 byte filehandles. Version 3 moved to variable
> > > +length filehandles that can be up to 64 bytes in size. NFSv4
> > > +increased that maximum to 128 bytes.
> > > +
> > > +When reexporting an NFS filesystem, the underlying filehandle from
> > > +the server must be embedded inside the filehandles presented to
> clients.
> > > +Thus if the underlying server presents filehandles that are too
> > > +big, the reexporting server can fail to encode them. This can lead
> > > +to NFSERR_OPNOTSUPP errors being returned to clients.
> > > +
> > > +This is not a trivial thing to programatically determine ahead of
> > > +time (and it can vary even within the same server), so some
> > > +foreknowledge of how the underlying server constructs filehandles,
> > > +and their maximum size is a must.
> >
> > This is the trickiest one, since it depends on an undocumented
> > implementation detail of the server.
> >
> 
> Yes, indeed...
> 
> > Do we even know if this works for all the exportable Linux filesystems?
> >
> > If proxying NFSv4.x servers is actually useful, could we add a per-fs
> > maximum-filesystem-size attribute to the protocol?
> >
> 
> Erm, I think you mean maximum-filehandle-size, but I get your point...
> 
> It's tough to do more than a quick survey, but looking at new-style fh:
> 
> The max fsid len seems to be 28 bytes (FSID_UUID16_INUM), though you
> can get that down to 8 bytes if you specify the fsid directly. The fsid
choice is
> weird, because it sort of depends on the filehandle sent by the client
(which
> is used as a template), so I guess we really do need to assume worst-case.
> 
> Once that's done, the encode_fh routines add the fileid part. btrfs has a
> pretty large maximum one: 40 bytes. That brings the max size up to 68
bytes,
> which is already too large for NFSv3, before we ever get to the part where
> we embed that inside another fh. We require another 12 bytes on top of the
> "underlying" filehandle for reexporting.
> 
> So, no this may very well not work for all exportable Linux filesystems,
but it
> sort of depends on the situation (and to some degree, what gets sent by
the
> clients). That's what makes this so hard to figure out programmatically.
> 
> As far as extending the protocol...that's not a bad idea, though that's
> obviously a longer-term solution. I don't think we can reasonably rely on
that
> anyway. Maybe though...

I've been thinking about this kind of thing with Ganesha's proxy server, and
conveniently, you have also provided a good use case for proxy...

One option I was going to give Ganesha is the ability to in export
configuration indicate the upstream server is Ganesha, and expect the export
configuration to be mirrored (easy for a config tool to do across the set of
servers, primary and proxy) so that Ganesha could just pass handles through.
Something similar might be possible for knfsd. With a bit more work, we
could be prepared to deal with other servers (like Ganesha providing for
knfsd or visa versa) to break apart the upstream handle to an "export"
component which can be static, and a "filesystem specific" portion that
needs to be passed through. So Ganesha could break out knfsd's fsid encoding
and map that to an exportid, and just pass through the payload handle (the
portion that comes from the exportfs interface).

Frank


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus