/usr/sbin/sm-notify [-dfn] [-m minutes] [-v name] [-p notify-port] [-P
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 protocol
(or 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 system
A daemon that listens for reboot notifications from other hosts,
and manages the list of hosts to be notified when the local sys-
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
NSM OPERATION IN DETAIL
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 rpc.statd.
rpc.statd records information about each monitored NFS peer on persis-
tent 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, in
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
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
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
string as 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.
If rpc.statd 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 num-
ber, a 32-bit integer that is bumped after each reboot by the sm-notify
command. rpc.statd uses this number to distinguish between actual
reboots and replayed notifications.
Part of NFS lock recovery is rediscovering which peers need to be moni-
tored again. The sm-notify command clears the monitor list on persis-
tent storage after each reboot.
-d Keeps sm-notify attached to its controlling terminal and running
in the foreground so that notification progress may be monitored
-f Send notifications even if sm-notify has already run since the
last system reboot.
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 min-
utes. Specifying a value of 0 causes sm-notify to continue
sending notifications to unresponsive peers until it is manually
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
-n Prevents sm-notify from updating the local system's NSM state
Specifies the source port number sm-notify should use when send-
ing reboot notifications. If this option is not specified, a
randomly chosen ephemeral port is used.
-v ipaddr | hostname
Specifies the network address from which to send reboot notifi-
cations, 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
The sm-notify command must be started as root to acquire privileges
needed to access the state information database. It drops root privi-
leges as soon as it starts up to reduce the risk of a privilege escala-
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 rpc.statd chooses, simply use chown(1) to set the owner
of the state directory.
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 quali-
fied domain names
The forward and reverse DNS mapping of the UTS nodenames should
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 moni-
toring 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 opera-
datagram transport protocols.
/var/lib/nfs/sm directory containing monitor list
/var/lib/nfs/sm.bak directory containing notify list
/var/lib/nfs/state NSM state number for this host
kernel's copy of the NSM state number
rpc.statd(8), nfs(5), uname(2), hostname(7)
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 <email@example.com>
Chuck Lever <firstname.lastname@example.org>
1 November 2009 SM-NOTIFY(8)
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