About GAB run-time or dynamic tunable parameters


About GAB run-time or dynamic tunable parameters
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You can change the GAB dynamic tunable parameters while GAB is configured and while the cluster is running. The changes take effect immediately on running the gabconfig command. Note that some of these parameters also control how GAB behaves when it encounters a fault or a failure condition. Some of these conditions can trigger a PANIC which is aimed at preventing data corruption.

You can display the default values using the gabconfig -l command. To make changes to these values persistent across reboots, you can append the appropriate command options to the /etc/gabtab file along with any existing options. For example, you can add the -k option to an existing /etc/gabtab file that might read as follows:

gabconfig -c -n4

After adding the option, the /etc/gabtab file looks similar to the following:

gabconfig -c -n4 -k

Table: GAB dynamic tunable parameters describes the GAB dynamic tunable parameters as seen with the gabconfig -l command, and specifies the command to modify them.

Table: GAB dynamic tunable parameters

GAB parameter	Description and command
Control port seed	This option defines the minimum number of nodes that can form the cluster. This option controls the forming of the cluster. If the number of nodes in the cluster is less than the number specified in the `gabtab` file, then the cluster will not form. For example: if you type gabconfig -c -n4, then the cluster will not form until all four nodes join the cluster. If this option is enabled using the gabconfig -x command then the node will join the cluster even if the other nodes in the cluster are not yet part of the membership. Use the following command to set the number of nodes that can form the cluster: gabconfig -n count Use the following command to enable control port seed. Node can form the cluster without waiting for other nodes for membership: gabconfig -x
Halt on process death	Default: Disabled This option controls GAB's ability to halt (panic) the system on user process death. If _had and _hashadow are killed using kill -9, the system can potentially lose high availability. If you enable this option, then the GAB will PANIC the system on detecting the death of the client process. The default behavior is to disable this option. Use the following command to enable halt system on process death: gabconfig -p Use the following command to disable halt system on process death: gabconfig -P
Missed heartbeat halt	Default: Disabled If this option is enabled then the system will panic on missing the first heartbeat from the VCS engine or the vxconfigd daemon in a CVM environment. The default option is to disable the immediate panic. This GAB option controls whether GAB can panic the node or not when the VCS engine or the vxconfigd daemon miss to heartbeat with GAB. If the VCS engine experiences a hang and is unable to heartbeat with GAB, then GAB will NOT PANIC the system immediately. GAB will first try to abort the process by sending SIGABRT (kill_ntries - default value 5 times) times after an interval of "iofence_timeout" (default value 15 seconds). If this fails, then GAB will wait for the "isolate timeout" period which is controlled by a global tunable called isolate_time (default value 2 minutes). If the process is still alive, then GAB will PANIC the system. If this option is enabled GAB will immediately HALT the system in case of missed heartbeat from client. Use the following command to enable system halt when process heartbeat fails: gabconfig -b Use the following command to disable system halt when process heartbeat fails: gabconfig -B
Halt on rejoin	Default: Disabled This option allows the user to configure the behavior of the VCS engine or any other user process when one or more nodes rejoin a cluster after a network partition. By default GAB will not PANIC the node running the VCS engine. GAB kills the userland process (the VCS engine or the vxconfigd process). This recycles the user port (port h in case of the VCS engine) and clears up messages with the old generation number programmatically. Restart of the process, if required, must be handled outside of GAB control, e.g., for hashadow process restarts _had. When GAB has kernel clients (such as fencing, VxVM, or VxFS), then the node will always PANIC when it rejoins the cluster after a network partition. The PANIC is mandatory since this is the only way GAB can clear ports and remove old messages. Use the following command to enable system halt on rejoin: gabconfig -j Use the following command to disable system halt on rejoin: gabconfig -J
Keep on killing	Default: Disabled If this option is enabled, then GAB prevents the system from PANICKING when the VCS engine or the vxconfigd process fail to heartbeat with GAB and GAB fails to kill the VCS engine or the vxconfigd process. GAB will try to continuously kill the VCS engine and will not panic if the kill fails. Repeat attempts to kill process if it does not die gabconfig -k
Quorum flag	Default: Disabled This is an option in GAB which allows a node to IOFENCE (resulting in a PANIC) if the new membership set is < 50% of the old membership set. This option is typically disabled and is used when integrating with other products Enable iofence quorum gabconfig -q Disable iofence quorum gabconfig -d
GAB queue limit	Default: Send queue limit: 128 Default: Recv queue limit: 128 GAB queue limit option controls the number of pending message before which GAB sets flow. Send queue limit controls the number of pending message in GAB send queue. Once GAB reaches this limit it will set flow control for the sender process of the GAB client. GAB receive queue limit controls the number of pending message in GAB receive queue before GAB send flow control for the receive side. Set the send queue limit to specified value gabconfig -Q sendq:value Set the receive queue limit to specified value gabconfig -Q recvq:value
IOFENCE timeout	Default: 15000(ms) This parameter specifies the timeout (in milliseconds) for which GAB will wait for the clients to respond to an IOFENCE message before taking next action. Based on the value of kill_ntries , GAB will attempt to kill client process by sending SIGABRT signal. If the client process is still registered after GAB attempted to kill client process for the value of kill_ntries times, GAB will halt the system after waiting for additional isolate_timeout value. Set the iofence timeout value to specified value in milliseconds. gabconfig -f value
Stable timeout	Default: 5000(ms) Specifies the time GAB waits to reconfigure membership after the last report from LLT of a change in the state of local node connections for a given port. Any change in the state of connections will restart GAB waiting period. Set the stable timeout to specified value gabconfig -t stable
Isolate timeout	Default: 120000(ms) This tunable specifies the timeout value for which GAB will wait for client process to unregister in response to GAB sending SIGKILL signal. If the process still exists after isolate timeout GAB will halt the system gabconfig -S isolate_time:value
Kill_ntries	Default: 5 This tunable specifies the number of attempts GAB will make to kill the process by sending SIGABRT signal. gabconfig -S kill_ntries:value
Driver state	This parameter shows whether GAB is configured. GAB may not have seeded and formed any membership yet.
Partition arbitration	This parameter shows whether GAB is asked to specifically ignore jeopardy. See the gabconfig (1M) manual page for details on the -s flag.

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