Q: What are the important limitations in the VMware vSphere Storage Appliance?

A: With the release of vSphere 5.0, VMware also quietly released a product called the vSphere Storage Appliance (VSA). This product intends to provide failover capabilities for VMs, but without requiring shared SAN storage. It's intended for use by smaller IT environments that either can't afford a SAN or lack the experience to use one effectively.

Although the VSA can indeed enable a high availability-like experience atop locally attached storage via its built-in replication, it does so with some significant costs. Consider the following limitations carefully:

  • The VSA doesn't support memory overcommit.
  • The VSA, once installed, prohibits adding additional storage to a VSA cluster.
  • The VSA creates a useable partition that's equivalent to the amount of disk space in the server that contains the least quantity of disk space.
  • The VSA isn't intended to be installed onto existing ESXi hosts; it prefers a "green field" installation onto essentially unconfigured hardware. ESXi hosts can't run VMs before creating the VSA cluster.
  • The VSA, in combination with VMware's RAID requirements for locally attached storage, requires a 75 percent storage overhead for redundancy. This requirement means only 25 percent of deployed storage is actually available for use.
  • The VSA can be configured in a two- or three-server configuration that, once installed, can't be altered.
  • VMware suggests you don't run vCenter Server within VSA as a VM because the loss of a datastore could prevent access to the VSA Manager. As a result, an additional and separate physical computer or VM is required to run vCenter Server and the VSA Manager.
  • The VSA reserves 33 percent of CPU and memory resources on a three-host cluster and 50 percent of CPU and memory resources on a two-host cluster for high availability admission control.
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