sm-notify - send reboot notifications to NFS peers
/usr/sbin/sm-notify [-dfn] [-m minutes] [-v name]
[-p notify-port] [-P path]
File locks are not part of persistent file system state. Lock state is thus lost
when a host reboots.
Network file systems must also detect when lock state is lost because a remote
host has rebooted. After an NFS client reboots, an NFS server must release all
file locks held by applications that were running on that client. After a
server reboots, a client must remind the server of file locks held by
applications running on that client.
For NFS version 2 and version 3, the Network Status Monitor
NSM for short) is used to notify NFS peers of reboots. On Linux, two separate
user-space components constitute the NSM service:
- A helper program that notifies NFS peers after the local
- A daemon that listens for reboot notifications from other
hosts, and manages the list of hosts to be notified when the local system
The local NFS lock manager alerts its local rpc.statd
of each remote peer
that should be monitored. When the local system reboots, the sm-notify
command notifies the NSM service on monitored peers of the reboot. When a
remote reboots, that peer notifies the local rpc.statd
, which in turn
passes the reboot notification back to the local NFS lock manager.
The first file locking interaction between an NFS client and server causes the
NFS lock managers on both peers to contact their local NSM service to store
information about the opposite peer. On Linux, the local lock manager contacts
records information about each monitored NFS peer on persistent
storage. This information describes how to contact a remote peer in case the
local system reboots, how to recognize which monitored peer is reporting a
reboot, and how to notify the local lock manager when a monitored peer
indicates it has rebooted.
An NFS client sends a hostname, known as the client's caller_name
each file lock request. An NFS server can use this hostname to send
asynchronous GRANT calls to a client, or to notify the client it has rebooted.
The Linux NFS server can provide the client's caller_name
or the client's
network address to rpc.statd
. For the purposes of the NSM protocol,
this name or address is known as the monitored peer's mon_name
addition, the local lock manager tells rpc.statd
what it thinks its own
hostname is. For the purposes of the NSM protocol, this hostname is known as
There is no equivalent interaction between an NFS server and a client to inform
the client of the server's caller_name
. Therefore NFS clients do not
actually know what mon_name
an NFS server might use in an SM_NOTIFY
request. The Linux NFS client records the server's hostname used on the mount
command to identify rebooting NFS servers.
When the local system reboots, the sm-notify
command reads the list of
monitored peers from persistent storage and sends an SM_NOTIFY request to the
NSM service on each listed remote peer. It uses the mon_name
the destination. To identify which host has rebooted, the sm-notify
command normally sends my_name
string recorded when that remote was
monitored. The remote rpc.statd
matches incoming SM_NOTIFY requests
using this string, or the caller's network address, to one or more peers on
its own monitor list.
does not find a peer on its monitor list that matches an
incoming SM_NOTIFY request, the notification is not forwarded to the local
lock manager. In addition, each peer has its own NSM state number
32-bit integer that is bumped after each reboot by the sm-notify
uses this number to distinguish between actual
reboots and replayed notifications.
Part of NFS lock recovery is rediscovering which peers need to be monitored
again. The sm-notify
command clears the monitor list on persistent
storage after each reboot.
- Keeps sm-notify attached to its controlling terminal
and running in the foreground so that notification progress may be
- Send notifications even if sm-notify has already run
since the last system reboot.
- -m retry-time
- Specifies the length of time, in minutes, to continue
retrying notifications to unresponsive hosts. If this option is not
specified, sm-notify attempts to send notifications for 15 minutes.
Specifying a value of 0 causes sm-notify to continue sending
notifications to unresponsive peers until it is manually killed.
- Notifications are retried if sending fails, the remote does
not respond, the remote's NSM service is not registered, or if there is a
DNS failure which prevents the remote's mon_name from being
resolved to an address.
- Hosts are not removed from the notification list until a
valid reply has been received. However, the SM_NOTIFY procedure has a void
result. There is no way for sm-notify to tell if the remote
recognized the sender and has started appropriate lock recovery.
- Prevents sm-notify from updating the local system's
NSM state number.
- -p port
- Specifies the source port number sm-notify should
use when sending reboot notifications. If this option is not specified, a
randomly chosen ephemeral port is used.
- This option can be used to traverse a firewall between
client and server.
- -P, --state-directory-path
- Specifies the pathname of the parent directory where NSM
state information resides. If this option is not specified,
sm-notify uses /var/lib/nfs by default.
- After starting, sm-notify attempts to set its
effective UID and GID to the owner and group of this directory.
- -v ipaddr | hostname
- Specifies the network address from which to send reboot
notifications, and the mon_name argument to use when sending
SM_NOTIFY requests. If this option is not specified, sm-notify uses
a wildcard address as the transport bind address, and uses the
my_name recorded when the remote was monitored as the
mon_name argument when sending SM_NOTIFY requests.
- The ipaddr form can be expressed as either an IPv4
or an IPv6 presentation address. If the ipaddr form is used, the
sm-notify command converts this address to a hostname for use as
the mon_name argument when sending SM_NOTIFY requests.
- This option can be useful in multi-homed configurations
where the remote requires notification from a specific network
command must be started as root to acquire privileges
needed to access the state information database. It drops root privileges as
soon as it starts up to reduce the risk of a privilege escalation attack.
During normal operation, the effective user ID it chooses is the owner of the
state directory. This allows it to continue to access files in that directory
after it has dropped its root privileges. To control which user ID
chooses, simply use chown
(1) to set the owner of the
Lock recovery after a reboot is critical to maintaining data integrity and
preventing unnecessary application hangs.
To help rpc.statd
match SM_NOTIFY requests to NLM requests, a number of
best practices should be observed, including:
- The UTS nodename of your systems should match the DNS names
that NFS peers use to contact them
- The UTS nodenames of your systems should always be fully
qualified domain names
- The forward and reverse DNS mapping of the UTS nodenames
should be consistent
- The hostname the client uses to mount the server should
match the server's mon_name in SM_NOTIFY requests it sends
Unmounting an NFS file system does not necessarily stop either the NFS client or
server from monitoring each other. Both may continue monitoring each other for
a time in case subsequent NFS traffic between the two results in fresh mounts
and additional file locking.
On Linux, if the lockd
kernel module is unloaded during normal operation,
all remote NFS peers are unmonitored. This can happen on an NFS client, for
example, if an automounter removes all NFS mount points due to inactivity.
TI-RPC is a pre-requisite for supporting NFS on IPv6. If TI-RPC support is built
into the sm-notify
command ,it will choose an appropriate IPv4 or IPv6
transport based on the network address returned by DNS for each remote peer.
It should be fully compatible with remote systems that do not support TI-RPC
Currently, the sm-notify
command supports sending notification only via
datagram transport protocols.
- directory containing monitor list
- directory containing notify list
- NSM state number for this host
- kernel's copy of the NSM state number
RFC 1094 - "NFS: Network File System Protocol Specification"
RFC 1813 - "NFS Version 3 Protocol Specification"
OpenGroup Protocols for Interworking: XNFS, Version 3W - Chapter 11
Olaf Kirch <firstname.lastname@example.org>
Chuck Lever <email@example.com>