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Enabling the Delayed Durability feature activates a modified log sync strategy. Ordinarily, when a transaction commits, c-treeACE does not return from a TRANEND() call until log entries associated with the transaction have been written to persisted storage (e.g., your disk drive). With the new modified log sync strategy enabled only the following is ensured:
Before any data or index image is written to disk, all associated log entries are on disk.
With this modified log sync, in an extreme case, a transaction could be committed without any associated log entries on disk. Or a TRANBEG may be on disk, but not a corresponding TRANEND. This means c-tree cannot guarantee recovery of all transactions that have returned a successful commit.
When Delayed Durability is active, the link between committing a transaction and syncing the transaction log to disk is de-coupled, and the number of log syncs is greatly reduced. Additional logic ensures that before c-tree data or index contents are written to disk, all necessary log entries are on disk. If log information is cached after a commit, the log will not necessarily be synced to persisted storage, leaving a small vulnerable window for data loss potential in case of system failure.
The Delayed Durability time window configuration is an added c-treeACE internal operations thread ensuring transaction log data is periodically synced on a regular basis. This log sync thread coordinates the transaction log sync to disk and guarantees that no more than N seconds can pass without a log sync to persisted storage. This thread wakes periodically and syncs accumulated transaction log data before the DELAYED_DURABILITY value is exceeded. A setting of 1 second is recommended as it provides excellent performance gains with minimal window of data loss in event of system failure.
Note: The transaction log buffer is always flushed when full. This buffer is defined as a 64K buffer. Any number of updates exceeding 64K will always trigger a log buffer sync to disk.
