vSphere Storage Appliance: The fruit of a conflict of interest

The new vSphere Storage Appliance has serious limits. Is the EMC relationship keeping VMware from realizing its full potential?

As I mentioned in my wrap-up of VMworld 2011, one of the shiny new VMware products to hit the shelves this year is the vSphere Storage Appliance, or VSA. The VSA delivers SAN-like features to small businesses that may not have the budget, staffing, or technical sophistication to purchase a real SAN.

But the initial release of the VSA is marred by a number of serious deployment and scalability limitations. The real story behind VMware's introduction of the VSA has less to do with the VSA itself and more with VMware's relationship with its partners and its parent company, EMC.

What is the VSA?

The vSphere Storage Appliance is a cluster of virtual machine appliances that offer local storage as a unified, highly redundant datastore. A VSA cluster can currently span either two or three vSphere hosts, and it uses network-based synchronous mirroring (RAID1) combined with local RAID10 on the hosts to ensure data availability and to deliver reasonable performance. The vSphere cluster hosts are then able to connect to the VSA cluster's datastores via NFS -- effectively allowing the hosts to see and share each other's storage in real time.

In a two-host deployment, the first appliance operates a datastore roughly half the size of its local storage while the other half is used to store a mirror image of the second host's datastore, thereby distributing the storage load across both hosts. When a third host is added to the picture, there are three total datastores with each host backing up one of the other two in a round-robin fashion.

Although the network-based mirroring introduces a degree of latency, a VMware engineer told me informally that a three-host cluster using a total of 24 15k SAS drives can typically push around 3,000 IOPS in real life -- generally enough for most small-business customers. There's another trade-off: The combination of RAID10 on the local hosts and mirroring in between the hosts is fantastically inefficient from a capacity perspective, effectively resulting in about a quarter of the physical storage being available to store virtual machines. For that reduced capacity, small-business customers get a SAN-like solution much less costly and complex than an actual SAN.

There are other significant limitations. First, the VSA can be deployed only in a greenfield environment (if you have existing virtual machines on local storage, you have to move them somewhere else prior to implementing the VSA). Worse, the VSA cannot be scaled once it's installed. If you install with two nodes, you cannot scale to three nodes afterward, nor can you add more physical disks to your hosts and expand the footprint of the VSA. Speaking from personal experience, those two limitations alone would prevent me from wanting to implement VSA in any environment that might experience unexpected growth -- which, these days, is pretty much anywhere.

In other words, the VSA is a potentially great product with a couple of show-stopping problems. If VMware fixes them, the VSA could become the de facto answer for any small business deploying vSphere. Its simplicity, the fact that it's managed entirely from within vCenter, and its low cost compared to all but the cheapest entry-level SAN/NAS could make it a very compelling option.

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