Monitors the space available on the log files and on the media hosting the log files. The space available on the media hosting the log files are only included as a part of the space available if autogrowth is enabled for at least one of the files.
Unhealthy state is caused by low space free space across all log files. Low free space can be caused by:
Inadequate file configuration settings (size, max size, autogrowth, etc.)
Inadequate space left on the media
Lack of periodic regular backups
Log not truncating
Use the following links to view the performance data: DB File Log Free Space Performance Data
This monitor aggregates the space available for each log file within a database depending on each file configuration:
For a file that has no autogrowth, free space would be the difference between the initial size for a file and the used space.
In addition to the difference between the file size and the used space, the free space for files with autogrowth enabled would be the minimum of either the difference between the max size and the file size and the free space left on disk.
Low free space for database files that have autogrowth could mean that the file is approaching the limits of the hosting logical drive. For files with autogrowth enabled with a max size, low free space could also mean that the file is approaching the max size specified for a file.
The free space calculations also take into account that the file can also fail to grow if the growth amount of the file is greater than the amount left on disk, and that the file cannot grow if the difference between the max size and the current file size is less than the growth amount. In these situations, available free space left on disk is not included as a part of the free space, because we cannot grow any longer.
This issue may be resolved by either:
Increasing the allocated size for log files if autogrowth is disabled
Increasing the max size value for a log file if autogrowth is enabled
Enabling autogrowth for at least one file
Moving log files to another drive with more free space if growth space is limited
Performing regular backups of log files for FULL and BULK-LOGGED recovery modes
Investigate why the log is not getting truncated:
Long open running active transactions
Long running backup operations
Replicated transaction open for a long time (Transactional Replication)
Mirror lagging behind principal server (Database Mirroring)
Modifying the thresholds for this monitor to suit the workload
Alternatively, if log file free space is not a concern for the database:
Disable this monitor for this specific database or all databases
See SQL Server Books Online: Files and Filegroups Architecture