Download VCS Communications: GAB and LLT

VCS Communications: GAB and LLT
Communications within a VCS environment are conducted by the Group Atomic Broadcast mechanism (GAB) and
the Low Latency Transport mechanism (LLT). These kernel components are used only by VCS, and replace the
functions of TCP/IP for VCS private network communications.
How GAB Operates
GAB performs three major functions:
Manages cluster memberships.
Monitors heartbeat communication on disk or Ethernet.
Distributes information throughout the cluster.
Managing Cluster Memberships
Because GAB is a global mechanism, all systems within the cluster are immediately notified of changes in resource
status, cluster membership, and configuration. GAB is also atomic, meaning that it continuously maintains a
synchronized state in the cluster membership and configuration files of all cluster systems. If a failover occurs while
transmitting status changes, GAB’s atomicity ensures that, upon recovery, all systems will have the same
information regarding the status of any monitored resource in the cluster.
Monitoring Heartbeats
GAB also monitors heartbeat communication between systems. Heartbeats are signals that are sent periodically from
one system to another to verify that the systems are active. You may manually configure the heartbeat interval and
specify the number of consecutive heartbeats that a system can miss before it determines that another system has
failed.
When a system suspects that another system has failed, the system in question is probed by other systems in the
cluster to verify the failure. If the system remains unresponsive, it is marked DOWN and excluded from the cluster.
Its applications are then migrated to the other systems. GAB ensures when this process begins, all remaining
systems in the cluster have the same information regarding the status of the failed system and the migration of the
applications. Note that GAB may kill the VCS engine when the engine is unresponsive or when the systems
previously disconnected are reconnected.
Distributing Information
GAB distributes information to all systems throughout the cluster regarding system loads, agent reports, and
administrative commands. GAB can also be configured to track and distribute additional information. For a listing
of GAB commands, please see VERITAS TechDoc 232090.
How LLT Operates
LLT provides kernel-to-kernel communications and monitors network communications. LLT can be configured to:
Set system IDs within a cluster.
Set cluster IDs for multiple clusters.
Tune network parameters such as heartbeat frequency.
LLT runs directly on top of the Data Link Protocol Interface (DLPI) layer on UNIX, and the Network Driver
Interface Specifications (NDIS) on Windows NT. This ensures that events such as state changes are reflected more
quickly, which in turn enables faster responses. You may configure LLT to run as “low priority.” This prevents VCS
communication on the public network until the public network is the final link, thereby reducing the rate of heartbeat
broadcasts. For a listing of LLT commands, please see VERITAS TechDoc 232090.