Although the server virtualization market is still dominated by one player – VMware Inc. -- competitors of VMware are attempting to capitalize on growing friction between the server virtualization market leader and the storage industry, and trying to gain an edge in the market based on how their virtual machines connect to storage.
VMware is unquestionably the dominant player in today's server virtualization market. It has been around the longest, was acquired by EMC Corp., in December, 2003, and is estimated to have achieved an 85 percent share of the server virtualization market.
VMware's most commonly used option for storage deployment uses a proprietary virtual file system to pool disk storage, and stores virtual server images and data inside files for easier management. Until recently, VMFS, VMware's cluster file system for facilitating storage virtualization, was required to support features such as VMotion, which migrates live, running virtual servers between physical hosts.
However, that's no longer strictly the case, according to Jon Bock, senior product marketing manager for VMware. Users can also run VMware with industry-standard file systems such as NFS, or use Raw Device Mapping, which maps virtual servers to storage at the block level. Raw Device Mode can also be run in Virtual Mode, which uses data-locking mechanisms from VMFS. "We don't force people to take a given route," Bock said.
Still, there can be disadvantages to forgoing VMFS. "One of the reasons we decided to use a virtual file system was that there are things that customers want, such as VMotion and VMware HA [high availability] that we have to make possible without corrupting data on disk," Bock said. A file system helps avoid "split brain" problems between servers in a failover cluster and during migration of virtual machines from one physical host to another.
Using Raw Device Mapping entails management tradeoffs, too. "It means users can't leverage disk pooling [through VMware]," Bock said. "And they have to call the storage team to provision every new LUN." Furthermore, a third-party volume manager would be required, adding to the complexity of deployment.
VMware stepping on toes of storage vendors?
But as VMware adds features to its file system, storage vendors are beginning to suspect that the company is trying to move storage system value-adds into its Hypervisor. Some storage vendors think that VMware is stepping on their toes, especially when it comes to Storage VMotion, a feature in VMware ESX Server version 3.5 that migrates the back-end storage attached to virtual machines.
It's a matter of virtualization vendors like VMware "looking for ways to add value" as the virtualization becomes commoditized, says Bruce McCorkendale, distinguished engineer in the office of the CTO for Symantec Corp.
As more companies virtualize their server environments, users who once struggled with running multiple operating systems will now struggle with running physical and virtual servers, and in some cases, with running multiple hypervisors.
This is where Symantec sees the value proposition for its vendor- and platform-agnostic storage, security and server management software. "We've been helping users do storage management and high availability in heterogeneous environments for a long time," McCorkendale said. "For [another vendor] who hasn't been doing it that long, going from zero to Symantec will take quite a while."
If it sounds like McCorkendale is drawing a line in the sand, it's because he is. VMware may insist it's not trying to compete with storage vendors. But, McCorkendale says, "They are. The question is how well they're going to be able to compete."
As for VMFS, McCorkendale doesn't just admit that Symantec competes with it, but says, "We think we can do better." For example, Symantec believes it makes sense to separate storage file systems from virtual machines for better flexibility. "Users shouldn't be limited in their selection of tools to solve problems, whether their servers are physical or virtual," he says. Using heterogeneous, centralized management tools on the storage side for all servers cuts down on management overhead and complexity, he notes, adding that this is the reason behind Symantec's partnership with XenSource. XenSource, he points out, can allow Symantec's volume manager and storage management software to be used with virtual machines, rather than supplying its own volume manager.
"We've run into this with Microsoft and many other platform vendors—when they run out of ways to extract value from their platform, they look to other things people want on it," McCorkendale said. "We've made a name in storage optimization, and we're going to be a target – the value we've found is something other people want a piece of."
FalconStor is another vendor walking the virtual tightrope. The company supports VMware, but in January said that its storage virtualization, snapshot and replication software will also support virtual servers from Virtual Iron Software Inc. According to Bernie Wu, FalconStor's vice president of business development, the use case for FalconStor's IPStor storage virtualization software is in many ways stronger with Virtual Iron, because unlike VMware, Virtual Iron doesn't have its own file system.
Wu said that users who combine storage virtualization software and VMFS will miss out on features like large-scale data migrations using storage virtualization tools and thin provisioning for storage. VMware offers guidelines to users of its software that they allocate twice the amount of disk space they actually plan to write to; the ability to expand capacity on the fly using thin provisioning can help users avoid buying twice the amount of storage they need. Other gaps in VMware's storage capabilities include support for heterogeneous array support; heterogeneous multiprotocol support; I/O caching; and heterogeneous replication support.
Bock counters that Storage VMotion doesn't change the value proposition of storage virtualization, first because it can only move one host's storage at a time, so users looking to migrate whole arrays or large volumes of storage will still need to look to storage virtualization for that support.
Problems with virtual machines and LUNs
When it comes to Storage VMotion, Bock said, it may be difficult for storage virtualization or data migration features to associate virtual machines to their particular LUNs. "Storage virtualization devices tend to move entire LUNs," he said. "But virtual machines might connect to data on multiple LUNs, or there might be multiple virtual machines associated with one LUN."
Bock also argued that storage virtualization can be used in combination with VMFS, though he acknowledges that this creates multiple layers of abstraction between the server and storage system. But he has seen no data to indicate this creates performance bottlenecks, and says that the two abstraction layers can be equally necessary for separate server and storage teams.
"The server and OS teams never touch the array," he says. "If the array is virtualized, it's not visible to the application teams how much storage is available. At the same time, for the storage team interacting with the array, the virtualization makes for better management on their end."
So why is there so much trepidation about VMware's file system? Bock says it's because VMware "is a relatively young company. There haven't been 15 years of examples of how VMware works, and many people aren't understanding how VMware is trying to work with partners."
Bock said VMware hears similar criticism in the security market because of its VMsafe feature. "We're not taking over the security market, either," he said.
At least one user of both VMware and storage virtualization software doesn't get what all the fuss is about. According to Ernest Eustace, security and network administrator for electrical and mechanical equipment manufacturers the State Group, his company began using DataCore SANmelody for pooling disk systems for disk-based backup, but the deployment quickly spread to using the software to virtualize primary HP MSA storage as well. VMware's ESX server was layered on later with no problems, he said.
"VMware sees DataCore's virtual disks," he said, and the company can use features like the storage virtualization software's thin provisioning. "DataCore understands the VMware file system—for example, even if VMware says 20 GB should be allocated, DataCore knows only 3 GB is actually used."
State Group could have used a virtualized disk array from HP, the Enterprise Virtual Array (EVA) line, but the company still has a mix of hardware in its environment and wanted software that would pool all of it.
This was first published in June 2008