As server virtualisation has evolved over the past few years, it has had a strong effect on the physical location of the storage that supports it. What started as direct-attached storage in the server has travelled full circle, via the array, and now we have the so-called virtual storage appliance (VSA).
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At the advent of server virtualisation, storage was often directly attached to the physical machine that housed the hypervisor. This was like fighting today’s war with the last war’s tactics and technology. So, to answer the demands of many virtual servers in a few physical machines, shared storage became near ubiquitous, with a number of performance and scalability advantages.
Now we are seeing some signs of the movement of storage functions -- of its intelligence, if not the physical capacity -- to the virtual server hypervisor, in the form of the virtual storage appliance. What has driven this migration process, and what can we expect to see in the future as virtualisation continues to evolve?
The pull of the hypervisor
Since EMC invented the intelligent storage controller with the release of Symmetrix in 1991, all storage arrays have depended on software to provide advanced functionality and features, from data protection via RAID to optimisation features such as thin provisioning, compression and data deduplication.
In recent years, we have seen the commoditisation of storage hardware as vendors have moved to use cheaper and increasingly more reliable components, and the “value-add” or intellectual property has been focused on software.
Consequently, we have also seen the rise of storage software products. Initially, these were for use in x86-based dedicated servers to create storage virtualisation appliances that could construct an array from heterogeneous or commodity components.
Companies such as LeftHand Networks and DataCore Software offered software-based storage solutions that could be deployed on standard Intel-based servers with embedded hard drives. This turned commodity hardware into a storage appliance, using software to provide the intelligence of multiple protocols and advanced features.
The logical follow-on from storage software that ran on a physical server was to run it from the hypervisor, and that is what has emerged over the past year or two from the vendors above and new entrants to the field offering virtual storage appliances.
The driver towards increasing server virtualisation has been one of cost and simplification. Using the hypervisor to provide storage features provides a number of benefits.
- It simplifies the environment, with fewer physical boxes to manage.
- It reduces cost, making more effective use of embedded storage.
- It simplifies management, especially where solutions integrate with the hypervisor to deliver management functionality with a “single pane of glass.”
It must be noted, however, that putting multiple virtual machines and storage in one device could spell big trouble if the server failed, so you’ll need to factor in data protection.
The virtual storage appliance marketplace
There are many virtual storage appliances in the marketplace today. Examples include DataCore’s SANsymphony, the Open-E Data Storage Server, Nexenta’s NexentaStor VSA and Hewlett-Packard LeftHand’s P4000 VSA. They can be used to aggregate physical storage -- internal to one or more servers or residing on arrays or JBODs -- into a shared storage pool.
VSAs are often virtual versions of existing products intended for deployment on dedicated hardware (as is the case with LeftHand) and can be deployed on their own server. The benefit of being able to deploy them in the hypervisor or on their own hardware is that they reduce cost but also retain management consistency across differing sizes of implementation.
Such products could be deployed within a virtual machine on a server hypervisor using internal storage as part of a low-cost small branch office deployment. But, they could also be deployed on dedicated infrastructure at core data centres, making use storage replication and standardised management functions.
It is so clear that this deployment model is a viable one for many customers that VMware has entered the marketplace with the release of the vSphere Storage Appliance in vSphere 5. At this stage, however, the vSphere Storage Appliance is a first-version product and has a lot of catching up to do to draw level with more established market players.
While most virtual storage appliances focus on block-based storage, some entrants are addressing file-based access. For example, Nasuni offers a virtual version of its cloud storage NAS filer gateway. This uses Amazon S3 storage and some local cache to provide almost limitless storage to NFS and CIFS clients. The Nasuni filer potentially offers a way of delivering file storage without lots of expensive infrastructure, especially since the company offers SLA guarantees of 100% cloud access uptime, although it remains to be seen to what degree customers will trust it for primary storage.
Another way of managing storage from the hypervisor is software that enables a Windows server to act as an iSCSI array. These products implement an iSCSI target on either physical or virtual deployments of Windows, using storage on the Windows server to store virtual iSCSI LUNs. Examples of this technology include Microsoft’s iSCSI Target software (acquired from String Bean Software in 2006), Starwind’s iSCSI SAN and iSCSI Cake from Youngzsoft.
Will VSAs make dedicated arrays obsolete?
Given the small but noticeable trend of the virtual storage appliance, the question is begged, Are the days numbered for the dedicated storage array?
At the high-end enterprise level, the storage array will be here for some time to come. Enterprise arrays are built to provide high availability and high performance and are priced at a premium that reflects this. Virtualising this storage into the hypervisor just wouldn’t deliver the same level of service. Storage array vendors are also trying to find ways of making their products more attractive in virtual environments. They’re making such strides by supporting VMware’s vSphere APIs for Array Integration (VAAI) in their arrays; VAAI support helps improve common VMware storage functions such as replication and cloning.
But, for midrange and low-end requirements, virtual storage appliances may fit the bill, depending on the use case. Users will have to decide whether virtual storage can supply the performance they need. In these environments the trade-off will be performance, scalability and reliability versus cost and simplicity.
Storage vendors have worked hard to improve array performance and integration with the hypervisor, so we’re likely to see a range of solutions spanning dedicated arrays, integrated storage and multiple hybrids, for some time to come.
Chris Evans is a UK-based storage consultant. He maintains The Storage Architect blog.