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Until Informix Dynamic Server 10.0 the number of MSC (or Miscellaneous) Virtual Processors was limited to one. With IDS 10.0 multiple MSC VPs can be configured using the VPCLASS onconfig parameter (e.g. VPCLASS msc,num=2), or added dynamically using onmode -p 1 msc.

What does that mean and why would I want to use it?
The MSC VP is a type of AIO VP, executing operating system calls on behalf of other threads. While the AIO VP mainly takes care of filesystem I/O calls the MSC VP executes some network and authentication calls like getpwnam(), gethostbyname(), gethostbyaddr() etc. (It also maintains some statistics like VP CPU usage.) The MSC VP gets used at client connection time for the operating system part of user validation, and in most cases one is sufficient.

Get to the point
In some cases, for example when a connection pool creates a large number of client connections at once, and/or when a PAM (Pluggable Authentication Module) implementation causes the operating system part of user authentication to take longer, the MSC VP can either be very busy serving the new connections or waiting for the OS to respond. When this happens new client connections can take longer to service and users will see a longer delay when connecting for the first time. Situations like this might benefit from having an extra MSC VP. Running onstats like onstat -g iov, onstat -g glo and onstat -g stk <tid> can help show what the MSC VP is doing but keep in mind there are other possible causes when new connections experience delays (such as operating system and network problems) so some troubleshooting (and tech support assistance) would generally be required, but at least dynamically adding a VP is an easy test to make if you suspect this might help.

注意：由于MSC VP会涉及到getpwnam(), gethostbyname(), gethostbyaddr() 调用，因此，对于informix的客户端的主机名解析可能会存在向外部DNS请求查询，而此可能会导致延时，因此，informix服务器上最好是做好各客户端主机名在本地hosts文件中的注册，以便减少因解析主机而带来的延时。同时要注意服务器端与客户端服务器端间的信任关系，否则可能会出现956的报错：

01:00:01  listener-thread: err = -956: oserr = 0: errstr = (informix@hostname): Client host or user (informix@hostname) is not trusted by the server.
11:22:13  listener-thread: err = -956: oserr = 0: errstr = (nobody@hostname): Client host or user (nobody@hostname) is not trusted by the server.
15:34:16  listener-thread: err = -956: oserr = 0: errstr = (nobody@hostname): Client host or user (nobody@hostname) is not trusted by the server.

# finderr 956
/home/informix/bin/finderr: line 79: test: 20020.9: integer expression expected
-956    Client client-name is not in /etc/hosts.equiv on the remote host.

This operation cannot be completed because the specified remote computer
does not recognize the name of the client computer (the local host,
whose name the hostname command returns). Client names are normally
recorded in the configuration file /etc/hosts.equiv. The rhosts file on
the remote computer might also need modification. For more information
on these files and the relation between them, try man rhosts on a UNIX
system. See the remote host administrator to ensure that the client
host name is specified in /etc/hosts.equiv file for the remote host.
This message appears in versions prior to Version 6.0.

-956    Client client-name or user is not trusted by the database server.

The desired database server does not consider the client computer or
your user ID trusted. Ask the system administrator to include the
client computer as a trusted host and/or your user id as a trusted user
in the /etc/hosts.equiv file on the server computer or in the .rhosts
file in your home directory on the server computer. See the UNIX manual
pages on HOSTS.EQUIV(5) for more information.

Alternatively, you can explicitly specify your user name and password
in the USER clause of a CONNECT statement or in the .netrc file in your
home directory on the client computer. See the IBM Informix Guide to SQL:
Syntax for information on the CONNECT statement, and the UNIX manual
pages regarding NETRC(5) for information on the .netrc file. This
message appears with Version 6.0 and later.

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