NFSv4 —
NFS
Version 4 Protocol
The NFS client and server provides support for the NFSv4 specification; see
Network File System (NFS) Version 4 Protocol RFC 7530
and Network File System (NFS) Version 4 Minor Version
1 Protocol RFC 5661. The protocol is somewhat similar to NFS Version 3,
but differs in significant ways. It uses a single compound RPC that
concatenates operations to-gether. Each of these operations are similar to the
RPCs of NFS Version 3. The operations in the compound are performed in order,
until one of them fails (returns an error) and then the RPC terminates at that
point.
It has integrated locking support, which implies that the server is no longer
stateless. As such, the
NFSv4 server remains in
recovery mode for a grace period (always greater than the lease duration the
server uses) after a reboot. During this grace period, clients may recover
state but not perform other open/lock state changing operations. To provide
for correct recovery semantics, a small file described by
stablerestart(5) is used by the server during the
recovery phase. If this file is missing or empty, there is a backup copy
maintained by
nfsd(8) that will be used. If
either file is missing, they will be created by the
nfsd(8). If both the file and the backup copy are
empty, it will result in the server starting without providing a grace period
for recovery. Note that recovery only occurs when the server machine is
rebooted, not when the
nfsd(8) are just
restarted.
It provides several optional features not present in NFS Version 3:
- NFS Version 4 ACLs
- Referrals, which redirect subtrees to other servers
(not yet implemented)
- Delegations, which allow a client to operate on a file locally
- pNFS, where I/O operations are separated from Metadata operations
The
NFSv4 protocol does not use a separate mount
protocol and assumes that the server provides a single file system tree
structure, rooted at the point in the local file system tree specified by one
or more
V4: <rootdir> [-sec=secflavors] [host(s) or net]
line(s) in the
exports(5) file. (See
exports(5) for details.) The
nfsd(8) allows a limited subset of operations to
be performed on non-exported subtrees of the local file system, so that
traversal of the tree to the exported subtrees is possible. As such, the
``<rootdir>'' can be in a non-exported file system. The exception is
ZFS, which checks exports and, as such, all ZFS file systems below the
``<rootdir>'' must be exported. However, the entire tree that is rooted
at that point must be in local file systems that are of types that can be NFS
exported. Since the
NFSv4 file system is rooted
at ``<rootdir>'', setting this to anything other than ``/'' will result
in clients being required to use different mount paths for
NFSv4 than for NFS Version 2 or 3. Unlike NFS
Version 2 and 3, Version 4 allows a client mount to span across multiple
server file systems, although not all clients are capable of doing this.
NFSv4 uses strings for users and groups instead of
numbers. On the wire, these strings can either have the numbers in the string
or take the form:
where ``<dns.domain>'' is not the same as the DNS domain used for host
name lookups, but is usually set to the same string. Most systems set this
``<dns.domain>'' to the domain name part of the machine's
hostname(1) by default. However, this can
normally be overridden by a command line option or configuration file for the
daemon used to do the name<->number mapping. Under FreeBSD, the mapping
daemon is called
nfsuserd(8) and has a command
line option that overrides the domain component of the machine's hostname. For
use of this form of string on
NFSv4, either
client or server, this daemon must be running.
The form where the numbers are in the strings can only be used for AUTH_SYS. To
configure your systems this way, the
nfsuserd(8)
daemon does not need to be running on the server, but the following sysctls
need to be set to 1 on the server.
vfs.nfs.enable_uidtostring
vfs.nfsd.enable_stringtouid
On the client, the sysctl
vfs.nfs.enable_uidtostring
must be set to 1 and the
nfsuserd(8) daemon does
not need to be running.
If these strings are not configured correctly, ``ls -l'' will typically report a
lot of ``nobody'' and ``nogroup'' ownerships.
Although uid/gid numbers are no longer used in the
NFSv4 protocol except optionally in the above
strings, they will still be in the RPC authentication fields when using
AUTH_SYS (sec=sys), which is the default. As such, in this case both the
user/group name and number spaces must be consistent between the client and
server.
However, if you run
NFSv4 with RPCSEC_GSS
(sec=krb5, krb5i, krb5p), only names and KerberosV tickets will go on the
wire.
To set up the NFS server that supports
NFSv4, you
will need to set the variables in
rc.conf(5) as
follows:
nfs_server_enable="YES"
nfsv4_server_enable="YES"
plus
if the server is using the ``<user>@<domain>'' form of user/group
strings or is using the ``-manage-gids'' option for
nfsuserd(8).
You will also need to add at least one ``V4:'' line to the
exports(5) file for
NFSv4 to work.
If the file systems you are exporting are only being accessed via
NFSv4 there are a couple of
sysctl(8) variables that you can change, which
might improve performance.
- vfs.nfsd.issue_delegations
- when set non-zero, allows the server to issue Open
Delegations to clients. These delegations permit the client to manipulate
the file locally on the client. Unfortunately, at this time, client use of
delegations is limited, so performance gains may not be observed. This can
only be enabled when the file systems being exported to
NFSv4 clients are not being accessed locally
on the server and, if being accessed via NFS Version 2 or 3 clients, these
clients cannot be using the NLM.
- vfs.nfsd.enable_locallocks
- can be set to 0 to disable acquisition of local byte range
locks. Disabling local locking can only be done if neither local accesses
to the exported file systems nor the NLM is operating on them.
Note that Samba server access would be considered ``local access'' for the above
discussion.
To build a kernel with the NFS server that supports
NFSv4 linked into it, the
must be specified in the kernel's
config(5) file.
To do an
NFSv4 mount, specify the ``nfsv4'' option
on the
mount_nfs(8) command line. This will force
use of the client that supports
NFSv4 plus set
``tcp'' and
NFSv4.
The
nfsuserd(8) must be running if
name<->uid/gid mapping is being used, as above. Also, since an
NFSv4 mount uses the host uuid to identify the
client uniquely to the server, you cannot safely do an
NFSv4 mount when
is set in
rc.conf(5).
If the
NFSv4 server that is being mounted on
supports delegations, you can start the
nfscbd(8)
daemon to handle client side callbacks. This will occur if
nfsuserd_enable="YES" <-- If name<->uid/gid mapping is being used.
nfscbd_enable="YES"
are set in
rc.conf(5).
Without a functioning callback path, a server will never issue Delegations to a
client.
For NFSv4.0, by default, the callback address will be set to the IP address
acquired via rtalloc() in the kernel and port# 7745. To override the default
port#, a command line option for
nfscbd(8) can be
used.
To get callbacks to work when behind a NAT gateway, a port for the callback
service will need to be set up on the NAT gateway and then the address of the
NAT gateway (host IP plus port#) will need to be set by assigning the
sysctl(8) variable vfs.nfs.callback_addr to a
string of the form:
N.N.N.N.N.N
where the first 4 Ns are the host IP address and the last two are the port# in
network byte order (all decimal #s in the range 0-255).
For NFSv4.1, the callback path (called a backchannel) uses the same TCP
connection as the mount, so none of the above applies and should work through
gateways without any issues.
To build a kernel with the client that supports
NFSv4 linked into it, the option
must be specified in the kernel's
config(5) file.
Options can be specified for the
nfsuserd(8) and
nfscbd(8) daemons at boot time via the
``nfsuserd_flags'' and ``nfscbd_flags''
rc.conf(5) variables.
NFSv4 mount(s) against exported volume(s) on the same host are not recommended,
since this can result in a hung NFS server. It occurs when an nfsd thread
tries to do an NFSv4 VOP_RECLAIM()/Close RPC as part of acquiring a new vnode.
If all other nfsd threads are blocked waiting for lock(s) held by this nfsd
thread, then there isn't an nfsd thread to service the Close RPC.
- /var/db/nfs-stablerestart
- NFS V4 stable restart file
- /var/db/nfs-stablerestart.bak
- backup copy of the file
stablerestart(5),
mountd(8),
nfscbd(8),
nfsd(8),
nfsdumpstate(8),
nfsrevoke(8),
nfsuserd(8)
At this time, there is no recall of delegations for local file system
operations. As such, delegations should only be enabled for file systems that
are being used solely as NFS export volumes and are not being accessed via
local system calls nor services such as Samba.