Disk - Free Space Status

Monitoring Benefit

The Disk Free Space test reports how much space remains on each logical disk of a monitored server, and changes state as that falls through the thresholds you set.

It earns its place because running out of disk is both the most predictable server failure and one of the most disruptive. On an Exchange server a full database volume dismounts the database and stops mail; on a domain controller a full volume can stop replication and SYSVOL; on the ENow web server it stops monitoring itself. All of those are avoidable with a few days' warning, which is exactly what this test provides.

Disk is presented as two related tests. This one measures capacity. Disk Performance Status measures speed — read and write times and queue lengths. A disk can be comfortably empty and still be the bottleneck, so read them together.

How to use it

The Disk Space Status page lists each drive with its role, a free space figure, a White Space column where applicable, and a status indicator.

Treat the trend as the real signal, not the current value. A volume sitting at 15% free and stable is a different situation from one that was at 40% last week. The threshold tells you where you are; the direction tells you how long you have.

Two things commonly cause a surprise here:

• A percentage threshold on a very large volume leaves less warning than it appears. 10% of a 4 TB volume is 400 GB, which sounds ample until something writes 400 GB overnight. Where a volume is large, set the threshold by absolute size rather than percentage if the console allows it.
• White space is not free space. On an Exchange database volume, white space is recoverable space inside the database file — it is available to Exchange for new data, but the operating system still counts it as used. A database volume can look tight while having plenty of usable room.

How do we verify the monitoring test results?

From EMS 8.0 onward these results are held in the ENow SQL database rather than in a file on the web server. Open the monitoring page for this server in the ENow console to review what ENow recorded.

How do we verify the results received on the ENow Client?

The ENow Client runs a WMI query periodically to collect disk space information. This information is not logged, but the results are stored on the monitored server:

\Program Files (x86)\ENow\Mailscape Agent\Cache\DiskAgentMessage.xml

To capture the same data manually, run the following in PowerShell on that server:

Get-WmiObject -Class Win32_LogicalDisk

Or, for a more readable view in gigabytes:

Get-CimInstance Win32_LogicalDisk -Filter "DriveType=3" |
    Select-Object DeviceID,
        @{n='FreeGB';e={[math]::Round($_.FreeSpace/1GB,1)}},
        @{n='SizeGB';e={[math]::Round($_.Size/1GB,1)}},
        @{n='Free%';e={[math]::Round(($_.FreeSpace/$_.Size)*100,1)}}

The status of the test depends on the free space reported here against the threshold set in the console. Below is sample output; values will differ in your environment.

PS C:\Users\Administrator> Get-WmiObject -Class Win32_LogicalDisk

DeviceID     : C:
DriveType    : 3
ProviderName :
FreeSpace    : 35143770112
Size         : 68124930048
VolumeName   :

Common warning or error results, and potential solutions