Users interested in deploying VMware’s vSphere Storage Appliance should read the fine print, according to virtualization experts, because there are some serious scalability and high-availability limitations.
The vSphere Storage Appliance (VSA) is a software add-on for vCenter Server 5 Essentials that pools ESXi hosts’ internal disks as shared storage and replicates data between them for high availability. It’s not a new concept -- HP LeftHand, for example, has offered its own VSA for years, as have startups such as StorMagic.
VMware’s VSA is meant to provide small and medium-sized businesses (SMBs) with advanced server virtualization features that have heretofore required expensive networked storage.
The VMware VSA is also a manifestation of VMware CEO Paul Maritz’s desire to “make the infrastructure uninteresting” by absorbing its functions into software.
No room to grow
The beauty of the VSA is that “it gives you the ability to have a [storage area network] where you don’t have a SAN, in a small office where you need SAN capabilities for redundancy but you don’t have the ability to buy [one],” said Tim Antonowicz, senior architect for Mosaic Technology, an IT infrastructure consultancy in Salem, N.H.
But the VSA has some major scalability limitations. For example, the product doesn’t scale beyond two or three nodes, whichever a user starts with, according to VMware’s VSA documentation, and it supports a maximum of eight disks per VSA host.
Furthermore, admins must determine the storage capacity of the VSA cluster prior to installation. VSA 1.0 does not support adding storage to data stores and their replicas after you configure the cluster. Plus, this capacity will be limited by a 75% storage overhead requirement for RAID data protection. Thus, a VSA host consisting of eight 2 TBs would have a raw capacity of 16 TB, but the 75% redundancy overhead would result in a maximum usable capacity of 4 TB.
VMware documentation cites high availability as the reason behind VSA’s capacity limitations: “The VSA cluster requires RAID10 virtual disks created from the physical disks, and the vSphere Storage Appliance uses RAID1 to maintain the VSA datastores’ replicas,” resulting in effective capacity of just 25% of the total physical hard disk capacity.
The VSA’s high availability capabilities themselves also have room for improvement. A single node can be put in “appliance maintenance mode,” but in the case of a three-node cluster, more than one node cannot be put into maintenance mode at the same time. Putting the appliance in “cluster maintenance mode” renders all virtual machines (VMs) unavailable, and putting it in “reconfigure network mode” makes the entire cluster unavailable. With traditional storage systems, doing hardware or network maintenance usually does not require taking an entire array or cluster offline.
“You are getting HA here, but you’re sort of not, because making these large-scale changes requires an outage,” according to Greg Shields, senior partner at Concentrated Technology.
Similarly, memory overcommit is not supported on VMs that use VSA data stores, because “vSphere HA admission control makes… reservations to ensure that resources are available when virtual machines need to be restarted from a failed ESXi host onto a running ESXi host,” according to VMware documentation.
VSA sales slow
With these shortcomings, it’s no surprise to hear that VSA sales are lackluster.
“I'm sure VMware could provide a VSA that didn't have these limitations, and it would be really interesting for the really small SMBs out there,” wrote Christian Mohn, network manager for Seatrans, a Norwegian shipping company, in an email. “But currently I don't see the VSA [having] much of an impact in the SMB market locally here in Norway.”
VMware doesn’t comment on sales of individual products, but U.S. resellers say the VSA isn’t flying off the shelves, possibly in part due to year-end budgetary cycles, said Mosaic’s Antonowicz. The ability to expand the VSA on the fly would help, Antonowicz said. “Unfortunately that’s not the way it works now. It would be great if it did, but you’ve got to do [capacity planning] all up front.”Bill Hill, infrastructure IT lead for a Portland-based logistics company, added that he would be concerned about going to VMware for support on a storage system.
“I kind of wish [VMware] hadn’t gone that route. There are plenty of other people out there doing storage. Just let them do it,” he said.
Finally, for a product whose niche is small offices, the costs don’t always add up. According to VMware’s website, the list price for the VSA software add-on starts at $7,254, at a time when relatively cheap storage hardware offerings are plentiful in the market.
“I looked at it for a small client and the price was more than I expected,” said Shannon Snowden, consulting partner with New Age Technologies. “We went with a relatively low-cost [network-attached storage] unit instead.”
“It's too expensive to use in our remote/branch offices,” wrote Mohn. “In most cases a simple NAS box offers what we need in terms of storage protection… and for our main data centers… only being able to use 25% of the available storage makes it not worth the effort.”
Beth Pariseau is a senior news writer for SearchServerVirtualization.com. Write to her at email@example.com.