VM Whitespace & Negative Whitespace Values

Sam Kirchoff

VM Whitespace is the opposite but related value to VM Occupancy

VM Whitespace = 100% - VM Occupancy %.

If a VM is 33% Occupied, then the VM contains 67% Whitespace; 100% - 33% = 67%.

Understanding Negative Whitespace values

Less common but total plausible, it is entirely possible to get a valid negative Whitespace value! And it’s important that you know why. It’s likely not an error.

Guest iSCSI connections will often produce negative whitespace values. The Guest iSCSI technique is fading in popularity, but it is wildly misunderstood as a VMware RDM. However, they are not the same.

What’s the difference?

Whitespace.png

An RDM is a disk that is created and known to VMware. It’s passed to the Guest OS by VMware. The bottom line is that VMware is aware of it.

A Guest iSCSI Disk is when the Guest OS uses it’s Microsoft/Linux guest iSCSI initiator to mount external storage directly. They key here is that VMware has NO knowledge or management of that capacity. The exception to this is that VMTools will see this capacity because it’s installed in the guest OS and talking directly to the disk management services.

The net result is that it’s entirely possible for a VM to report its capacity as much higher than the Virtual Disk Capacity numbers suggest is physically possible…and those higher numbers will likely be correct. 

Example:
This Virtual Machine has mounted a 500TB Guest iSCSI volume. This creates a 1-to-1 relationship between the Guest VM OS and the external storage provider. However, the VM is only using 325GBs of the 500GBs allocated.

In this situation, the 325GBs is real capacity managed by the VM and needs to be considered or capacity would be undersized.

Virtual Disk Capacity

100GBs

 vCenter VM size limitation

Virtual Disk Used

75GBs

 vCenter VM measured size

Guest VM Disk Capacity

500GBs

 OS measured capacity limitation

Guest VM Disk Used

325GBs

 OS measured capacity used

 

In this scenario 325GBs/75GBs = 4.33 leading to a 433% VMware VM Occupancy.

Whitespace is now naturally negative! Or 100% - 433% = -333% Whitepsace.

In Live Optics if you encounter any VM Occupancy greater than 100%, you will see the Guest iSCSI technique value set to True.

If an environment uses this technique aggressively then the average VM characteristics can show a negative number. When examining the Virtual Machines of any environment with a negative whitespace value be sure to look at the VM Occupancy column. There is likely many VMs with a VM Occupancy greater than 100%.

Simply put, this means that the environment has a lot more data managed by the VMs than VMware is reporting.  

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    Nir Shiloni

    Great article!

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