During normal online processing, space is reserved in the transaction log file for possible use during recovery when it is rolling back transactions. The reserved space is used to write Compensation Log Records (CLRs), which describe the work performed during the rollback.

Generally, the logging system reserves approximately as much log file space to perform the rollback as was required to log the original operation. Exceptions are insert and update operations, which require less reserved space than the original log.

In the Log File page in the Performance Monitor window, you can see a close approximation of the log file space required for both normal log writes and for CLRs. Also displayed is the number of log file blocks reserved for use by the recovery system at any point in time. To access the Log File page, you click on the Log Information branch in the Performance Monitor window, and click the Log File tab in the Properties pane.

You can also accomplish these tasks using the sysmod command and the SET LOG_TRACE statement (see Set Log_Trace Statement--Trace Log Writes).