Checkpoints

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:

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.)
Tip:
  • 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.

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 Informix® Administrator's Reference.


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