Periodically, the database server flushes transactions and data within the buffer pool to disk. Until the transactions and data are flushed to disk, the data and transactions are in a state of flux. Instead of forcing every transaction to disk immediately after a transaction is completed, the database server writes transactions to the logical log. The database server logs the transactions as they occur. In the event of a system failure, the server:
  • Replays the log to redo and restore the transactions.
  • Returns the database to a state consistent with the state of the database system at the time of the failure.

To facilitate the restoration or logical recovery of a database system, the database server generates a consistency point, called a checkpoint. A checkpoint is a point in time in the log when a known and consistent state for the database system is established. Typically, a checkpoint involves recording a certain amount of information so that, if a failure occurs, the database server can restart at that established point.

The purpose of a checkpoint is to periodically move the restart point forward in the logical log. If checkpoints did not exist and a failure occurred, the database server would be required to process all the transactions that were recorded in the logical log since the system restarted.

A checkpoint can occur in one of these situations:

  • When specific events occur. For example, a checkpoint occurs whenever a dbspace is added to the server or a database backup is performed.

    Typically, these types of events trigger checkpoints that block transaction processing. Therefore, these checkpoints are called blocking checkpoints.

  • When resource limitations occur. For example, a checkpoint is required for each span of the logical log space to guarantee that the log has a checkpoint at which to begin fast recovery. The database server triggers a checkpoint when the physical log is 75 percent full to avoid physical log overflow.

    Checkpoints triggered by resource limitations usually do not block transactions. Therefore, these checkpoints are called nonblocking checkpoints.

    However, if the database server begins to run out of resources during checkpoint processing, transaction blocking occurs in the midst of checkpoint processing to make sure that the checkpoint completes before a resource is depleted. If transactions are blocked, the server attempts to trigger checkpoints more frequently to avoid transaction blocking during checkpoint processing. For more information, see Strategy for estimating the size of the physical log.

If failover occurs, and the secondary server becomes the primary server, checkpoint discrepancies between the two servers can affect re-connection attempts. If a checkpoint on the new secondary server does not exist on the new primary server, attempts to connect the secondary server to the primary server fail. The secondary server must be fully restored before it can connect to the primary server.

Automatic checkpoints cause the database server to trigger more frequent checkpoints to avoid transaction blocking. Automatic checkpoints attempt to monitor system activity and resource usage (physical and logical log usage along with how dirty the buffer pools are) to trigger checkpoints in a timely manner so that the processing of the checkpoint can complete before the physical or logical log is depleted. The database server generates at least one automatic checkpoint for each span of the logical-log space. This guarantees the existence of a checkpoint where fast recovery can begin. Use the AUTO_CKPTS configuration parameter to enable or disable automatic checkpoints when the database server starts. (You can dynamically enable or disable automatic checkpoints by using onmode -wm or onmode -wf.)
  • If automatic checkpoints are triggered too frequently because of physical log activity, you can increase the physical log size or use a plogspace to automatically tune the physical log resources.
  • If automatic checkpoints are triggered too frequently because of logical log activity, you can set the AUTO_LLOG parameter in the onconfig file to allow the server to automatically increase the logical log space to reduce checkpoint frequency.

Manual checkpoints are event-based checkpoints that you can initiate. The database server provides two methods for determining how long fast recovery takes in the event of an unplanned outage.

  • Use the CKPTINTVL configuration parameter to specify how frequently the server triggers checkpoints.
  • Use the RTO_SERVER_RESTART configuration parameter to specify how much time fast recovery takes.
    When you use the RTO_SERVER_RESTART configuration parameter:
    • The database server ignores the CKPTINTVL configuration parameter.
    • The database server monitors the physical and logical log usage to estimate the duration of fast recovery. If the server estimates that fast recovery exceeds the time specified in the RTO_SERVER_RESTART configuration parameter, the server automatically triggers a checkpoint.

The RTO_SERVER_RESTART configuration parameter is intended to be a target amount of time, not a guaranteed amount of time. Several factors that can increase restart time can also influence fast recovery time. These factors include rolling back long transactions that were active at the time of an unplanned outage. For more information about the RTO_SERVER_RESTART and AUTO_CKPTS configuration parameters, see the topics on configuration parameters in the HCL OneDB™ Administrator's Reference.