Rollback Records and the Last-Chance Threshold

Rollback records are logged whenever a transaction is rolled back. Servers save enough space to log a rollback record for every update belonging to an open transaction.

If a transaction completes successfully, no rollback records are logged, and the space reserved for them is released.

In long-running transactions, rollback records can reserve large amounts of space.

To check the space used by the syslogs, run sp_spaceused:
sp_spaceused syslogs

name            total_pages     free_pages      used_pages      reserved_pages 
--------------- --------------- --------------- -------------- --------------
syslogs                5632            1179            3783            670 
dbcc checktable(syslogs) produces similar output:
Checking syslogs: Logical pagesize is 2048 bytes 
The total number of data pages in this table is 3761. 
*** NOTICE: Space used on the log segment is 3783 pages (7.39 Mbytes), 67.17%.
*** NOTICE: Space reserved on the log segment is 670 pages (1.31 Mbytes), 11.90%.
*** NOTICE: Space free on the log segment is 1179 pages (2.30 Mbytes), 20.93%.

If the last chance threshold for the transaction log fires when it seems to have sufficient space, it may be the space reserved for rollbacks that is causing the problem.

Related tasks
Determining the Current Space for Rollback Records