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VSphere VVOLs are a framework for vSphere and storage arrays. VMware does not specify the capabilities that must be surfaced up from the underlying array; VVOLs simply supply the mechanism from which the storage capabilities can be surfaced up. The various storage vendors supporting vSphere VVOLs differentiate themselves by surfacing up diverse capabilities.
Storage vendors use VVOLs to expose the capabilities of their arrays to specific virtual machines (VMs). For example, Dell EqualLogic arrays allow the selection of RAID types, disk speed, disk type and disk encryption to be surfaced up through VVOLs. NexGen surfaces up five different quality of service policies. Hitachi Data Systems surfaces up, among other characteristics, availability, cost, array type, and allows the creation of custom tags such as maintenance window restrictions, availability zone and campus location. These are just a sampling of the capabilities that can be exposed from the array to vSphere VMs using VVOLs.
That said, some capabilities are common to all the arrays I have examined, while other capabilities are unique to a specific vendor's arrays. I've looked at more than a dozen storage vendors and all of them support array-based snapshots. Snapshots have always been problematic in vSphere. VMware has many caveats around the use of snapshots, such as only allowing the use of two to three snapshots per VMDK, a limit of 72 hours for using a snapshot and performance issues caused by the snapshot process. VVOLs have mitigated many of these issues because they allow the array to take snapshots rather than vSphere. Array vendors may have their own limitations for snapshots, but the vSphere limitations are no longer valid. For example, some vendors have stated their snapshots can be kept indefinitely and support more than 1,000 snapshots of a VM.
With vSphere VVOLs, there are two types of snapshots:
- Managed snapshots are performed by the array, but are initiated and managed by vSphere. A maximum of 32 snapshots are supported per VMDK.
- Unmanaged snapshots are initiated and managed by the storage array. The maximum number of snapshots is dictated by the storage array rather than by vSphere.
VVOLs are a revolutionary concept for the storage of VMs, and storage vendors are just starting to explore and exploit their relationship with vSphere via VVOLs. As the product matures, I fully expect that we will see storage arrays take advantage of VVOLs in other unique ways.
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