SNA Server LU Allocation Favors Active Remote Connections over Available Local Connections

ID: Q236591

The information in this article applies to:

Microsoft SNA Server, versions 4.0, 4.0 SP1, 4.0 SP2

IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SUMMARY

In the following scenario, two SNA Servers are deployed, SNAS1 (primary) and SNAS2 (backup). SNAS1 is geographically local to the client and SNAS2 is geographically remote from the client. Both SNA Servers share the same domain and organization-wide TN3270 LU pool.

The PU on SNAS1 undergoes a scheduled or unscheduled recycle and recovers to return to an active/available status. During this period of downtime, a new client request for a session is serviced by SNAS2, and thereafter, the following behavior may be observed:

If a new user connects to an LU in the pool through SNAS1, they are actually given an LU from SNAS2.

After all pool LUs are consumed on SNAS2, only then will SNAS1 allocate its own LUs that are in the pool.

If a previously "In-Use" LU on SNAS2 is released and becomes available, a new user connecting to SNAS1 will be given this remote LU rather than SNAS1 allocating one of its local LU resources.

Although this behavior is by design, there have been requests to support an option for TN3270 that allocates sessions across restarted/recovered local connections in preference to remote connections.

NOTE: This feature does not provide TN3270 client load balancing across more than one TN3270 server. That function must be provided by the TN3270 client or possibly the TCP/IP name service being used. If this functionality is needed, SNA Server provides transparent client load balancing across SNA Server computers using its native 3270 interface. For additional information about SNA Server load balancing, please see the following articles in the Microsoft Knowledge Base:

Q185446 TN3270 Server LUA Pool Use Should Load Balance Across Servers

Q128244 SNA Server Load Balancing and Hot Backup

For additional information about shared LUA pool use by more than one TN3270 server, please see the following article in the Microsoft Knowledge Base:

Q182139 LUA Pool Cannot Be Assigned to Two TN3270 Servers

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After this update is applied to the SNA Server, this new behavior for TN3270 (LUA/3270 application) resource location is enabled by setting the ResLocFlags registry entry to a DataValue of either 0x8000 or 0x8001 under:

KEY_LOCAL_MACHINE\System\CurrentControlSet\Services \TN3270\Parameters ResLocFlags: REG_DWORD,default = 0

If the DataValue is 0x8000, then all Open(SSCP) messages are first sent to the local SNA Server even if an existing connection is open to a remote server.

If the DataValue is 0x8001, it enables both the "try local first" and the "load balance amongst remote servers" feature as detailed in article Q185446.

In both cases, the SNA Server first tries to use local resources; if a request cannot be satisfied locally, it uses a remote resource. There is no load balancing among local SNA Servers, that is, the TN3270 Server first consumes all resources from one local SNA server before allocating resources from the other local SNA servers.

NOTE: The default behavior is to always use an existing active connection first.

After this update is applied and the ResLocFlags entry is set, the SnaBase service and TN3270 service must be restarted for the change to take effect.

MORE INFORMATION

This feature is available in the latest service pack for SNA Server version 4.0. For additional information, please see the following article in the Microsoft Knowledge Base:

Q215838 How to Obtain the Latest SNA Server Version 4.0 Service Pack

This feature was first included in SNA Server version 4.0 Service Pack 3.

Additional query words:

Keywords : sna4sp3fea sna4 sna4sp1 sna4sp2
Version : WINDOWS:4.0,4.0 SP1,4.0 SP2
Platform : WINDOWS
Issue type : kbinfo