Virtual storage appliance vs. traditional shared storage

A virtual storage appliance can be a viable way to allocate storage for their virtual server infrastructure; it offers cost and complexity benefits compared with traditional shared storage.

A virtual storage appliance (VSA) is a storage controller that runs in a virtual machine (VM) connecting to storage

that’s typically directly attached to the host supporting the VSA. VSAs can replace the need for a traditional shared-storage system, enabling a company to more fully leverage the benefits of server virtualization.

VSAs range from solutions that simply provide shared storage and add basic storage features to those that improve storage performance or incorporate cloud storage access. This tip will focus on the basic implementations, providing shared storage as an alternative to more expensive and complex traditional SAN or NAS systems.

Why shared storage is so important

Server virtualization is a technology designed to improve asset utilization (server hardware) while potentially reducing management overhead. Virtualizing the compute part of the equation without including the storage component isn’t completely logical. Sharing a common pool of storage capacity is the first step toward effective storage virtualization. But there’s another reason for shared storage.

Virtualizing servers actually increases vulnerability, as more software applications running on more virtual server instances are packed into a single physical host. An unclustered hypervisor is a major single point of failure, but most platforms include features that can address these potential vulnerabilities. These features require a shared-storage infrastructure.

Virtual shared storage

There are two basic ways to achieve shared storage: via a hardware-based SAN or NAS system, or via a software-based virtual storage appliance. A common use case for VSAs is in small and medium-sized businesses (SMBs) where shared storage hasn’t been implemented. In these scenarios, virtual storage appliances consolidate direct-attached capacity on each physical host and create a virtual storage pool that looks like networked storage. This enables an SMB to unlock some of the most valuable features of server virtualization, like high availability (HA), improved data protection and simplified storage management. Let’s examine some virtual storage appliances on the market.

VMware’s vSphere Storage Appliance. The VMware vSphere Storage Appliance is a software-based shared-storage file system that’s run as a VM on each ESX/i host, which are deployed in two- or three-node clusters. The VMware VSA creates a shared-storage pool from the direct-attached storage (DAS) that's connected to each host and provides RAID protection with that local capacity within each node and mirroring between nodes in the cluster, removing single points of failure on two levels. By pooling physical storage, it supports VMware HA, which automatically restarts VMs on another host in the cluster if one host goes down.

Hewlett-Packard LeftHand Virtual SAN Appliance. HP LeftHand’s P4000 VSA software is similar in functionality to VMware’s VSA, but it also supports Hyper-V. It allows for the creation of a shared-storage pool using the DAS within each host server, but it can also pool from external storage available on an iSCSI or Fibre Channel SAN. The software can turn any available storage into a LeftHand storage node while adding snapshots and thin provisioning. These nodes can also be located in remote locations to provide a DR capability by using the LeftHand P4000 distributed storage architecture to replicate data blocks between nodes.

DataCore SANsymphony. DataCore Software’s SANsymphony software is installed on each physical host (VMware, Hyper-V or Xen), creating an HA storage virtualization layer that consolidates the DAS within each into a shared storage pool. Like HP LeftHand’s P4000 VSA, this solution can include external, networked storage to increase the storage pool capacity. As a software solution, it can also be redeployed on external servers to save costs when migrating to a traditional shared storage infrastructure. SANsymphony includes a number of storage services, including snapshots, thin provisioning, auto-tiering and remote replication.

FalconStor NSS Virtual Appliance. FalconStor Software’s Network Storage Server Virtual Appliance (NSS VA) is a software version of the company’s NSS hardware product. It runs as a VM (VMware only), creating a virtual storage pool with DAS connected to that host and sharing it with other hosts. This storage pool can be expanded with any iSCSI storage that’s available on the network, and users can cluster two NSS VAs on separate physical hosts to create an HA solution. Like their hardware-based NSS, the virtual appliance includes snapshots, thin provisioning, read/write caching, remote replication and volume tiering.

VSAs vs. traditional, physical shared storage

A traditional shared-storage infrastructure typically consists of either SAN-based block storage systems using iSCSI or FC, or NAS-based file storage appliances. Implementation of these networked storage systems includes buying the storage system itself and making decisions about future storage requirements and how to scale capacity.

For most small and midsized companies, iSCSI is a more appealing block solution than Fibre Channel because of its lower cost and lower complexity, but it may require a network upgrade to support dedicated storage traffic to each host. The same is true for NAS-based storage.

The storage management features included in most arrays or NAS systems in this end of the market typically can’t compete with the VSAs described above. They also won’t usually scale beyond a single box, meaning expansion may involve buying and running multiple systems.

By comparison, a VSA involves only a software installation and some available server disk capacity and may not require additional networking upgrades. Storage provisioning is a point-and-click process, and snapshot and thin provisioning features many VSAs include can greatly improve effective capacity. When more storage is needed, a company could add hard drives at substantially less cost than that of a typical storage system with the same capacity. Also, disaster recovery can be incorporated into most VSAs easily with remote replication.

Shared storage is essentially a must-have for companies venturing beyond a single-host server virtualization environment. It’s required for high availability, improved data protection, disaster recovery, simplified management and other valuable features. Virtual storage appliances enable smaller companies without a lot of expertise or financial resources to take advantage of these fundamental aspects of storage virtualization. And, if a traditional SAN or NAS system is chosen in the future, VSAs can make data migration easier as well.

For VARs, the message is pretty simple: Get familiar with one or more VSA solutions. Their value to smaller customers is clear, which should drive interest in that end of the market for your services. But they can also be appealing to larger IT departments that are short on resources or time to find, procure and implement a traditional SAN or NAS.  For example, there are other VSA solutions that focus on increasing storage performance and some that can integrate with a cloud storage provider. As this category of products matures we should see even more functionality, and you’ll need to stay on top of that.

Eric Slack is a senior analyst with Storage Switzerland.

This was first published in December 2011

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