fsid change of ZFS?

[Hiroki Sato]

Hiroki Sato <hrs at freebsd.org> wrote
  in <20110819.002046.908756241495481148.hrs at allbsd.org>:

hr> Hi,
hr>
hr>  I have experienced "Stale NFS file handle" issue when switching
hr>  between oldnfs and newnfs on a CURRENT box (NFS server exporting ZFS
hr>  mountpoints).  The cause was that fsid was changed in the following
hr>  conditions and not in the NFS subsystem itself, but I am wondering if
hr>  these are expected behavior...
hr>
hr>  First, I tried the following configurations of NFS and ZFS, and saw
hr>  if fsid of the same mountpoint (a mounted ZFS dataset) changed or
hr>  not by using statfs(2):
hr>
hr>  compile opts		kld module	fsid[0:1]		kld loaded by
hr>  ----------------------------------------------------------------------------
hr>  NFSSERVER+NFSCLIENT	zfs		865798fa:8346ef02	loader
hr>
hr>  NFSSERVER+NFSCLIENT	zfs		865798fa:8346ef07	kldload(8)
hr>
hr>  NFSSERVER+NFSCLIENT+
hr>  NFSD+NFSCL		zfs		865798fa:8346ef03	loader
hr>
hr>  NFSSERVER+NFSCLIENT+
hr>  NFSD+NFSCL		zfs		865798fa:8346ef08	kldload(8)
hr>
hr>  NFSSERVER+NFSCLIENT	nfsd+nfscl+zfs	865798fa:8346ef08	loader
hr>  ----------------------------------------------------------------------------

 Ah, I found why this happened:

   /*
    * The fsid is 64 bits, composed of an 8-bit fs type, which
    * separates our fsid from any other filesystem types, and a
    * 56-bit objset unique ID.  The objset unique ID is unique to
    * all objsets open on this system, provided by unique_create().
    * The 8-bit fs type must be put in the low bits of fsid[1]
    * because that's where other Solaris filesystems put it.
    */
   fsid_guid = dmu_objset_fsid_guid(zfsvfs->z_os);
   ASSERT((fsid_guid & ~((1ULL<<56)-1)) == 0);
   vfsp->vfs_fsid.val[0] = fsid_guid;
   vfsp->vfs_fsid.val[1] = ((fsid_guid>>32) << 8) |
       vfsp->mnt_vfc->vfc_typenum & 0xFF;

 Since the vfc_typenum variable is incremented every time a new vfs is
 installed, loading order of modules that call vfs_register() affects
 ZFS's fsid.

 Anyway, possibility of fsid change is troublesome especially for an
 NFS server with a lot of clients running.  Can zeroing or setting a
 fixed value to the lowest 8-bit of vfs_fsid.val[1] be harmful?

-- Hiroki
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