Transaction Log File Considerations when using Change Data Capture

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Content Provided by: Paul King, Microsoft

One of the most important things to watch out for with change data capture is the transaction log I/O subsystem. As stated earlier, log file I/O significantly grows when change data capture is enabled in a database. In addition to that, log records stay active until change data capture has processed them. This means that especially in environments where a large latency builds up, the log file can grow significantly because the log space cannot be reused as long as the change data capture scan job has not processed the log records, even in simple recovery model, or even after a log backup in full recovery model.

It should be noted that change data capture works with all recovery models. But when change data capture is enabled, operations that would normally be minimally logged in simple or bulk-logged recovery models are fully logged to enable change data capture to capture all changes.

Be aware also that when a log disk becomes full, you cannot shrink the log file by backing it up and manually shrinking it until change data capture has processed all transactions. But change data capture cannot process the transactions when the log disk is full, because change data capture writes to change tables are logged operations. In this case, the easiest way to recover from this situation is to temporarily add another log file on a different disk.

Posted by: Chris Skorlinski, Microsoft SQL Server Escalation Services

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