Monitors the space available in a file and on the media hosting the file. The space available on the media hosting the files is only included as a part of the space available if autogrowth is enabled for this file.
Unhealthy state is caused by low space available for this file. Low free space can be caused by:
Inadequate file configuration settings (size, max size, autogrowth, etc.)
Inadequate space left on the media
Use the following link to view the performance data: DB File Free Space Performance Data
For a file that has no autogrowth, free space would be the difference between the initial size for a file and the used space.
With autogrowth enabled, the available disk space is included as a part of the over free 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 a file or all files within a Filegroup if autogrowth is disabled
Increasing the max size value for a file if autogrowth is enabled
Enabling autogrowth for at least one file
Moving files to another drive with more free space if growth space is limited
Modifying the thresholds for this monitor to suit the workload
Alternatively, if Filegroup free space is not a concern for the database:
Disable this monitor for this specific Filegroup or all Filegroups
See SQL Server Books Online: Files and Filegroups Architecture