Earlier in the week I looked at the impact idle VM’s can have on CPU utilization and memory overhead today I’m going to look at the amount of Disk or Datastore space usage per Idle VM.
Each one will have associated VMDK (disk) files. The files are stored within a Datastore, which in most cases is hosted SAN or NAS storage and shared between the cluster host members. If VMDKs are provisioned as "Thick Disks" then the provisioned space is locked out within the Datastore for those disks.
To illustrate this an example of a least worst case scenario would be: 100 Windows idle VMs have been identified across the Virtual Infrastructure and each VM has a "Thick" single VMDK of 20GB used to house the operating system. This would then equate to 2TB of Datastore space being locked for use by VMs that are idle. You can expand this further by, making an assumption that some if not all VMs are likely to have more disks and of differing sizes.
The simple math will show you how much Datastore space is being wasted.There is a counter to this, known as Thin Provisioning. By using Thin disks, in which the provisioned disk size is reserved but not locked you would not waste the same amount of space as you would by using Thick Disks. Using Thin Provisioning also has the added benefit of being able to over allocate disk space thus leading to a reduction in the amount of up front storage capacity required, but only incurring minimal overhead.
Idle VMs - why you should care.
Identifying Idle VMs, questioning whether they are required, finding out who owns them and removing them completely will reduce or help eliminate VM sprawl and help to improve the performance and capacity of the Virtual Infrastructure by:
· reducing unnecessary timer interrupts
· reducing allocated vCPUs
· reducing unnecessary CPU and Memory overhead
· reduce used Datastore space
· lead to more efficient use of your Virtual Infrastructure, including improved VM to Host ratios and reduction in additional hardware.
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