How to monitor Windows disk space usage on a mounted drive without assigned letter

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Published on - last updated on April 4th 2024 - Listed in Windows Monitoring Icinga Nagios

Monitoring a drive on Windows is pretty easy, as a drive usually has a drive-letter assigned (for example C:).

In the following example I'm using NSClient++ running as agent on the Windows host while on the monitoring server I use check_nt to query the agent:

nagios@monitoring:~# /usr/lib/nagios/plugins/check_nt -H sqldevserver -p 1248 -v USEDDISKSPACE -l "D" -w 100G -c 99
D:\ - total: 99.87 Gb - used: 10.93 Gb (11%) - free 88.94 Gb (89%) | 'D:\ Used Space'=10.93Gb;99.87;98.87;0.00;99.87

But what about drives that appear in the disk management but are not assigned with a drive letter but are rather mounted as a folder?

In this example we have the classical C: drive for the Windows OS and an additional D: as data partition. But as you can see in the Disk Management UI, Disk 2 (named SQL_Data_DEV001) and Disk 3 (named SQL_Log_DEV001) have no drive letter assigned.

Windows Disk Management Drives without letter

Instead they're mounted as a sub-folder inside D:

  • Drive 2 is mounted on D:\SQL_Data
  • Drive 3 is mounted on D:\SQL_Log
Windows drives mounted as subfolder

Unfortunately when using NSClient, check_nt and the USEDDISKSPACE variable, this won't work out because a drive letter is a must. From the check_nt manpage:

  Size and percentage of disk use.
  Request a -l parameter containing the drive letter only.

But NSClient++ also speaks NRPE and its internal checks are even newer. To check a drive with NSClient as agent and check_nrpe from the monitoring server:

nagios@monitoring:~# /usr/lib/nagios/plugins/check_nrpe -H sqldevserver -c check_drivesize -a "drive=D:"
OK All 1 drive(s) are ok|'D: used'=10.94061GB;79.89838;89.88568;0;99.87298 'D: used %'=10%;79;89;0;100

And here comes the good news: The NRPE command check_drivesize (internally configured within the NSClient agent, no need to define this command somewhere) also allows mounted volumes. From the NSClient++ documentation:

To check the size of a mounted volume (c:\volume_test)[...]

According to the documentation, only the mount-path is needed. Let's try that:

nagios@monitoring:~# /usr/lib/nagios/plugins/check_nrpe -H sqldevserver -c check_drivesize -a "drive=D:\SQL_Data"
OK All 1 drive(s) are ok|'D:\SQL_Data used'=302.15924GB;319.8976;359.8848;0;399.872 'D:\SQL_Data used %'=75%;79;89;0;100

Indeed, the returned values are different than from the D: drive!

And this is how you can monitor mounted drives/partitions without a drive-letter on Microsoft Windows.

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