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17 Dell EMC PowerVault ME4 Series and Microsoft Hyper-V | 3921-BP-WS
3.3.3 Virtual hard disks and thin provisioning with ME4 Series arrays
It does not matter which type of virtual hard disk is used to in order maximize the space utilization on ME4
Series storage when leveraging thin provisioning at the array level. Regardless of the virtual hard disk type,
only the actual data written by a guest VM will consume space on the storage array due to the advantages of
thin provisioning.
The example shown in Figure 11 illustrates an ME4 Series 100 GB volume presented to a Hyper-V host that
contains two 60 GB virtual hard disks (overprovisioned in this case to demonstrate behavior, but not as a
general best practice). One disk is fixed, and the other is dynamic. Each virtual hard disk contains 15 GB of
actual data. From the perspective of the host server, a total of 75 GB of space is consumed and can be
described as follows:
Example: 60 GB fixed disk + 15 GB of used space on the dynamic disk = 75 GB total
Note: The host server reports the entire size of a fixed virtual hard disk as consumed.
Thin provisioning with ME4 Series array
Comparatively, this is how the ME4 Series array reports storage utilization on this same volume:
Example: 15 GB of used space on the fixed disk + 15 GB of used space on the dynamic disk = 30 GB
Note: Both types of virtual hard disks (dynamic and fixed) will use the same amount of space utilization on
Dell EMC ME4 arrays when using thin provisioning. Other factors such as the I/O performance of the
workload would be primary considerations when determining the type of virtual hard disk in your environment.
3.3.4 Overprovisioning with dynamic virtual hard disks
With dynamic virtual hard disks and thin provisioning, there is an inherent risk of either the host volume or a
storage group or pool on the ME4 Series array running out of space. Figure 11 shows an example of this. If
the dynamic disk used by VM2 on the host volume expands far enough, it would fill up the underlying host
volume and negatively impact both VM1 and VM2. From the perspective of VM2, it would still see 20 GB of
free space, but would not be able to use it because the underlying physical host volume is full. To resolve this,
an administrator would move the virtual hard disk for VM1 or VM2 elsewhere to free up space, or expand the
100 GB host volume. In either case, identifying the problem may not be obvious, and resolving the problem
might incur a service outage.