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 part of the space available if auto grow is enabled for this file.
An unhealthy state is caused by having low available for this file. Low available space can be caused by:
Inadequate file configuration settings (size, max size, auto grow, etc.)
Inadequate space left on media
Use the following link to view the performance data: DB File Free Space Performance Data
No Auto Grow
For a file that has no auto grow, available space would be the difference between the initial size for a file and the used space.
Auto Grow Enabled
With auto grow enabled, the available disk space is included as part of the over available space. In addition to the difference between the file size and the used space, the available space for files with auto grow 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 available space for database files that have auto grow could mean that the file is approaching the limits of the hosting logical drive. For files with auto grow enabled with a max size, low available space could also mean that the file is approaching the max size specified for a file.
The available 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 part of the available 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 file group if auto grow is disabled
Increasing the max size value for a file if auto grow is enabled
Enabling auto grow 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 file group available space is not a concern for the database:
Disable this monitor for this specific file group or all file groups
See SQL Server Books Online: Files and Filegroups Architecture