SM-NOTIFY(8) manual page
Table of Contents
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 protocol (or NSM for short)
is used to notify NFS peers of reboots. On Linux, two separate user-space
components constitute the NSM service:
- sm-notify
- A helper program that notifies
NFS peers after the local system reboots
- rpc.statd
- A daemon that listens
for reboot notifications from other hosts, and manages the list of hosts
to be notified when the local system reboots
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 rpc.statd.
rpc.statd
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,
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 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. In 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 my_name.
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 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 number, 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 monitored again. The sm-notify command
clears the monitor list on persistent 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 directly.
- -f
- 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.
- -n
- 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 pathname
- 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 address.
The sm-notify 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 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 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 or IPv6.
Currently, the sm-notify
command supports sending notification only via 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
- /proc/sys/fs/nfs/nsm_local_state
- 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 <okir@suse.de>
Chuck Lever <chuck.lever@oracle.com>
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