The transaction log is a circular log. It can’t reuse space if any active transaction holds onto a VLFL (virtual log file) even if it’s old.
SELECT last_user_seek, last_user_scan, modifications FROM sys.dm_db_index_usage_stats WHERE database_id = DB_ID('SalesDB') AND object_id = OBJECT_ID('Orders'); The result: last_user_seek was yesterday. modifications was over 50,000. Guru Guide To Sql Server Architecture And Internals.pdf
UPDATE STATISTICS Orders; The plan switched to an index seek. The ETL dropped to 12 minutes. Good, but not great. Why not 8 minutes? Alex dug deeper. During the ETL, he monitored: The transaction log is a circular log
Alex killed the orphaned transaction (after confirming with the dev), shrunk the log safely, and set up alerting for long-running open transactions. modifications was over 50,000
He ran:
He looked at sys.dm_tran_database_transactions during the ETL. One transaction had an old database_transaction_begin_time from 3 hours ago—an open transaction from a developer’s BEGIN TRAN in SSMS that was never committed or rolled back.
SELECT * FROM sys.dm_os_buffer_descriptors WHERE database_id = DB_ID('SalesDB'); He saw that 40 GB of the buffer pool was filled with old data from a morning report. The ETL’s needed pages (the clustered index of Orders ) were being paged in from disk— couldn’t save it because the scan had already caused random I/O earlier.