fcm_num_buffers - Number of FCM buffers configuration parameter

You can use this parameter to specify the number of 4KB buffers that are used for internal communications, referred to as messages, both among and within database servers.

Configuration type
Database manager
Applies to
  • Database server with local and remote clients
  • Database server with local clients
  • Partitioned database server or Db2® pureScale® database server with local and remote clients
Parameter type
Configurable Online
Propagation class
Default [range]
32-bit platforms
AUTOMATIC [895 - 65300]
64-bit platforms
AUTOMATIC [895 - 524288]
  • Database server with local and remote clients: 1024
  • Database with local clients: 895
  • Partitioned database server or Db2 pureScale database server with local and remote clients: 4096

Fast communication manager (FCM) buffers are used for both inter-member and intra-member communications by default.

Important: The default value of the fcm_num_buffers parameter is subject to change by the Db2 Configuration Advisor after initial database creation.

You can set both an initial value and the AUTOMATIC value for the fcm_num_buffers configuration parameter. When you set the parameter to AUTOMATIC, FCM monitors resource usage and can increase or decrease resources if they are not used within 30 minutes. The amount that resources are increased or decreased depends on the operating system. On Linux® operating systems, the number of buffers can be increased only 25% more than the starting value. If the database manager attempts to start an instance and cannot allocate the specified number of buffers, it decreases the number until it can start the instance.

If you want to set the fcm_num_buffers parameter to both a specific value and AUTOMATIC, and you do not want the system controller thread to adjust resources lower than the specified value, set the FCM_CFG_BASE_AS_FLOOR option of the DB2_FCM_SETTINGS registry variable to YES or TRUE. The DB2_FCM_SETTINGS registry variable value is adjusted dynamically.

If you are using multiple logical nodes, one pool of fcm_num_buffers buffers is shared by all the logical nodes on the same machine. You can determine the size of the pool by multiplying the value of the fcm_num_buffers parameter by the number of logical nodes on the physical machine. Examine the value that you are using; consider how many FCM buffers are allocated on a machine or machines with multiple logical nodes. If you have multiple logical nodes on the same machine, you might have to increase the value of the fcm_num_buffers parameter. The number of users on the system, the number of database partition servers on the system, or the complexity of the applications can cause a system to run out of message buffers.