On Thu, Sep 03, 2015 at 02:45:44PM -0400, Jeff Layton wrote:
> On Thu, 3 Sep 2015 14:31:03 -0400
> "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
>
> > On Thu, Sep 03, 2015 at 01:36:25PM -0400, Jeff Layton wrote:
> > > This is the userland companion patch for the patch to add a "nowcc"
> > > option to knfsd. This just adds the necessary code to allow userspace
> > > to set that option.
> > >
> > > Signed-off-by: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
> > > ---
> > > support/include/nfs/export.h | 3 ++-
> > > support/nfs/exports.c | 5 +++++
> > > utils/exportfs/exportfs.c | 2 ++
> > > utils/exportfs/exports.man | 14 ++++++++++++++
> > > 4 files changed, 23 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/support/include/nfs/export.h b/support/include/nfs/export.h
> > > index 1194255899bd..68bee5c2f305 100644
> > > --- a/support/include/nfs/export.h
> > > +++ b/support/include/nfs/export.h
> > > @@ -26,7 +26,8 @@
> > > #define NFSEXP_CROSSMOUNT 0x4000
> > > #define NFSEXP_NOACL 0x8000 /* reserved for possible ACL related use */
> > > #define NFSEXP_V4ROOT 0x10000
> > > -#define NFSEXP_PNFS 0x20000
> > > +#define NFSEXP_PNFS 0x20000
> > > +#define NFSEXP_NOWCC 0x40000
> > > /*
> > > * All flags supported by the kernel before addition of the
> > > * export_features interface:
> > > diff --git a/support/nfs/exports.c b/support/nfs/exports.c
> > > index 0aea6f154f09..791b5f756c9c 100644
> > > --- a/support/nfs/exports.c
> > > +++ b/support/nfs/exports.c
> > > @@ -276,6 +276,7 @@ putexportent(struct exportent *ep)
> > > if (ep->e_flags & NFSEXP_NOREADDIRPLUS)
> > > fprintf(fp, "nordirplus,");
> > > fprintf(fp, "%spnfs,", (ep->e_flags & NFSEXP_PNFS)? "" : "no_");
> > > + fprintf(fp, "%swcc,", (ep->e_flags & NFSEXP_NOWCC) ? "no" : "");
> > > if (ep->e_flags & NFSEXP_FSID) {
> > > fprintf(fp, "fsid=%d,", ep->e_fsid);
> > > }
> > > @@ -586,6 +587,10 @@ parseopts(char *cp, struct exportent *ep, int warn, int *had_subtree_opt_ptr)
> > > setflags(NFSEXP_PNFS, active, ep);
> > > else if (!strcmp(opt, "no_pnfs"))
> > > clearflags(NFSEXP_PNFS, active, ep);
> > > + else if (!strcmp(opt, "wcc"))
> > > + clearflags(NFSEXP_NOWCC, active, ep);
> > > + else if (!strcmp(opt, "nowcc"))
> > > + setflags(NFSEXP_NOWCC, active, ep);
> > > else if (strncmp(opt, "anonuid=", 8) == 0) {
> > > char *oe;
> > > ep->e_anonuid = strtol(opt+8, &oe, 10);
> > > diff --git a/utils/exportfs/exportfs.c b/utils/exportfs/exportfs.c
> > > index 87582316b086..a2afc80ee6ea 100644
> > > --- a/utils/exportfs/exportfs.c
> > > +++ b/utils/exportfs/exportfs.c
> > > @@ -823,6 +823,8 @@ dump(int verbose, int export_format)
> > > c = dumpopt(c, "no_acl");
> > > if (ep->e_flags & NFSEXP_PNFS)
> > > c = dumpopt(c, "pnfs");
> > > + if (ep->e_flags & NFSEXP_NOWCC)
> > > + c = dumpopt(c, "nowcc");
> > > if (ep->e_flags & NFSEXP_FSID)
> > > c = dumpopt(c, "fsid=%d", ep->e_fsid);
> > > if (ep->e_uuid)
> > > diff --git a/utils/exportfs/exports.man b/utils/exportfs/exports.man
> > > index 93092463153b..9213a228bb04 100644
> > > --- a/utils/exportfs/exports.man
> > > +++ b/utils/exportfs/exports.man
> > > @@ -417,6 +417,20 @@ devices. The default can be explicitly requested with the
> > > .I no_pnfs
> > > option.
> > >
> > > +.TP
> > > +.IR nowcc
> > > +This option disables the collection and reporting of WCC (weak cache
> > > +consistency) data in NFSv3 requests. RFC 1813 recommends that all
> > > +servers report this information to the clients as it allows the client
> > > +to avoid some extra round trips to the server. In some underlying
> > > +filesystems however, collecting this data can be cost prohibitive and
> > > +atomicity is difficult to guarantee.
> >
> > Which filesystems?
> >
>
> NFS is the main one that comes to mind, but this is also the case with
> many clustered filesystems (GFS2, ceph, OCFS2, etc...). You generally
> need to do some network activity to ensure that your getattr is
> consistent and that can be quite expensive.
>
> > > Also, in some client use-cases WCC
> > > +data is ignored or not terribly useful. The default is to always report
> > > +weak cache consistency data to the client in NFSv3 requests, and can be
> > > +explicitly requested with the
> > > +.I wcc
> > > +option.
> > > +
> >
> > This is a little vague. Can we give users any better guidelines?
> >
>
> Sure, I guess we could mention that if you're primarily exporting to
> clients that are doing direct I/O or are setting up a DS for flexfiles
> then you might want to enable this option. Would that clarify things?
Yes, and maybe mention the distributed filesystem case (I assume the
getattr's close to free on most disk filesystems).
--b.
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