Storage virtualization is a tricky category to define -- it has many manifestations, and even volume management software that's been around for decades can be called storage virtualization, since it abstracts some physical element of storage from the administrator.
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More recently, however, storage virtualization has come to refer to primary, mostly block-based implementations, which share the goal of pooling heterogeneous resources, whether they are classes of disk or models of disk arrays, behind one highly automated management interface.
Some storage virtualization products can be loosely categorized by where they insert the abstraction into the network -- built into the array, on software installed between server and host, attached to the network or storage switch, or as a gateway appliance. No single approach has yet won out, although industry experts agree that the Hitachi USP array-based virtualization and IBM's SAN Volume Controller (SVC) network-based virtualization are by far the most widely deployed devices in production.
There is some doubt about whether the holy grail of storage virtualization products, the idea of a single pane of glass for management of all storage devices in the environment, is achievable. Bottlenecks in large environments, which often are most in need of storage management help from virtualization, can be caused by a single device trying to manage many. Some users in performance-sensitive environments have not seen the same flexibility when it comes to setting custom stripe depths, also known as stroking disks, to balance performance across disk spindles behind a virtualization device.
Also, a schism remains between file and block storage virtualization . Vendors say products simply don't exist today that could keep a single block within a larger file on primary storage, while sending the rest of the file, block by block, to a separate file system on a separate physical tier, then bringing the blocks back together when different parts of the file are accessed again.
For now, users adding storage virtualization to their environment should focuson the features offered by each product before worrying about where the technology resides in the network, according to Taneja Group analyst Jeff Boles. "There are some distinctions to the underlying technology in terms of performance efficiency, scalability and low latency," he said, indicating that network-based products tend to be favored by those most concerned about latency and performance on the network.
However, he added, "It really boils down to features, and there's a big variance out there in terms of products. Some network-based solutions have a hard time offering some of the same feature depth as some of the software-based solutions." Generally, users should be looking to pull as many of their storage management tasks into the virtualization layer as possible to get the most benefit out of a product.
Below is a list of storage virtualization products, grouped by their place in the network for reference. With storage virtualization still a hard-to-define market, this is not intended to be a comprehensive listing.
Array-based storage virtualization products
Hitachi Data Systems: USP-V and USP-VM -- This is the latest of three generations of array-based virtualization from HDS. The USP high-end disk array can pool storage arrays from other manufacturers behind it and includes some other abstraction and automation management features, such as thin provisioning for both internal and external storage.
Dell/EqualLogic: PS Series arrays -- EqualLogic, acquired by Dell last November, makes this list because its abstraction software pools classes of different disk types together like the USP does with entire arrays. EqualLogic's building blocks of up to 16 drives automatically load-balance data across all drives in the same class when they are added to the system. EqualLogic's array building blocks can also be subdivided into storage pools by application and data can be migrated automatically between classes of disk based on policy.
3PARdata: InServ Storage Server; Pillar Data Systems: Axiom; Compellent Technologies: StorageCenter SAN -- Each product offers similar pooling and abstraction between classes of disk. Compellent has automated block-level tiered storage that can migrate individual blocks of data between classes of disk according to frequency of access, reassembling all the blocks into a file when it's retrieved.
Software-based storage virtualization products
DataCore Software: SANmelody, SANsymphony, SANmaestro -- SANmelody converts standard Intel/AMD servers, blades or virtual machines into storage servers that virtualize disks and serve them over existing networks to application servers. SANsymphony converts heterogeneous disk arrays into homogenous pools of storage. DataCore also has SANmaestro, for performance reporting and monitoring, Traveller CPR for continuous data protection and UpTempo I/O virtualization, which can prefetch data from disk to cache for performance-intensive applications.
FalconStor Software: IPStor -- IPStor contains backup and data protection components, as well as storage virtualization software similar to DataCore's SANsymphony, called NSS. NSS is used to manage and provision iSCSI and Fibre Channel SANs. Recent updates include support for solid-state drives and InfiniBand connectivity.
LeftHand Networks: SAN/iQ -- SAN/iQ creates iSCSI SANs out of x86 servers from Dell, HP and IBM. SAN/iQ is usually integrated with server hardware by LeftHand's channel partners and delivered as a package to the end user. This is similar to other server-to-iSCSI SAN options from companies like Open-E GmbH or StorMagic, but LeftHand's server nodes can be clustered for much larger implementations and include disaster recovery and server virtualization support.
Network-based storage virtualization products
EMC: Invista -- Invista is a switch-based product built on what's known as a split-path architecture, which uses a storage services module or adapter in a switch or appliance combined with software. Invista stumbled out of the gate when it was announced more than two years ago, having only achieved general availability in December. Invista integrates into intelligent switches from Brocade and Cisco to dynamically migrate and allocate volumes. Updates made in December include clustered controllers to remove a single point of failure and support for twice as many virtual volumes and five times as many mobility sessions as version 1.0.
IBM: SAN Volume Controller -- SAN Volume Controller like Invista, is integrated with the switch, but uses a separate Linux-based server appliance attached to the switch where data is routed for management as it passes through the network. The latest version added new support for global replication from the device.
Incipient: iNSP-- iNSP, like Invista, is a split-path approach to network-based virtualization that runs on a blade inside the Cisco MDS director switch. Incipient is a startup still trying to find major OEM deals.
LSI: StoreAge -- LSI claims StoreAge can offer more granular storage provisioning support in its split-path virtualization architecture than iNSP, including small, customizable stripe depths and more flexible performance management.
Sanrad: V-Switch -- V-Switch resides between the server network and the storage network. It's mainly used to offer servers lower cost iSCSI connections to Fibre Channel SANs, though it can also be used for connectivity between heterogeneous storage devices for disaster recovery replication.
Storage virtualization gateways
NetApp: V-Series -- The V-Series gateway is a NetApp filer without internal disk. It's meant to be placed in front of other vendors' disk arrays or commodity disks to give them the "personality" of a NetApp box. This includes CIFS, NFS, iSCSI and Fibre Channel connectivity, and the data management features NetApp ships with its OnTap operating system, such as replication, snapshots and cloning.
Reldata: Unified Storage Gateways -- Like NetApp's V-Series, Unified Storage Gateways can be placed in front of existing disk arrays or shelves of commodity disk and can offer NAS or SAN connectivity, as well as data management features, though it doesn't currently offer Fibre Channel connectivity to the host. Reldata recently began adding more automation features and wizards to guide users through the storage provisioning process.