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17 Dell EMC SC Series: Microsoft Hyper-V Best Practices | CML1009
2.3.4 Overprovisioning with dynamic virtual hard disks
When using dynamic virtual hard disks and thin provisioning, there is an inherent risk of either the host
volume or a storage pool on the SC Series array running out of space. See Figure 11 for an example. If the
dynamic disk used by VM2 on the host volume expanded far enough, it would fill up the host volume and
negatively impact 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 host volume would be full. To resolve this, an administrator
would need to move the virtual hard disk for VM1 or VM2 elsewhere to free up space on the 100 GB host
volume or expand the host volume. In either case, it may be difficult to identify the root cause of the problem,
and resolution may require a service interruption.
To mitigate risks, consider the following best practice recommendations:
• Create a Hyper-V volume on the physical host that is large enough so that current and future
expanding dynamic virtual hard disks will not fill the host volume to capacity. Creating larger Hyper-V
host volumes does not negatively impact space efficiency on SC Series because of the benefits of
thin provisioning.
• Hyper-V based checkpoints (snapshots) create differencing virtual hard disks on the same physical
volume. Allow adequate overhead on the host volume for the extra space consumed by the
differencing virtual hard disks.
• At the host level, set up monitoring on overprovisioned volumes so that if a percent-full threshold is
exceeded (such as 90 percent), an alert is generated with enough lead time to allow for remediation.
• At the SC Series level, configure thresholds and alerting so that warnings are generated before a
storage tier or storage pool reaches capacity.
- If tier 1 fills to capacity, new writes are forced into a lower performing tier (if there is capacity in a
lower tier), resulting in degraded performance.
- If all storage tiers in a storage pool fill to near capacity, the SC Series array will enter
conservation mode. Assistance from Dell EMC Support may be required to recover from
conservation mode.
2.4 Present SC Series storage to Hyper-V
There are several ways to present SC Series SAN or DAS storage volumes as LUNs to Hyper-V hosts,
nodes, and VMs.
• Present SC Series volumes as LUN 0 to physical Hyper-V hosts or nodes that will boot-from-SAN.
- Requires a FC or iSCSI adapter that supports boot-from-SAN
- Boot-from-DAS (hosts with SAS front-end adapters) is not supported
See the Dell EMC SC Series and Microsoft MPIO best practices guide for details on how to configure
boot-from-SAN.
• Present SC Series volumes as data volumes to physical Hyper-V hosts and clusters:
- Support for Fibre Channel, iSCSI, and SAS-FE
- Support for mixed transports
- Leverage server cluster objects on SC Series storage to map data volumes to multiple nodes at
the same time to support clustering and cluster shared volumes