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Storage virtualization: It’s ready, are you?

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Device-based virtualization

In the traditional scale-up architecture where the controllers are separate from the disk shelves, virtualization at the storage device level is typically built into the controller operating system. As a standard feature it essentially provides a workable solution for provisioning the tens or hundreds of terabytes that modern storage arrays can contain. Most systems include the ability to create tiers of storage within a single virtualized system or among discrete systems, using different storage types (performance drives, capacity drives or SSDs) and different RAID levels. Some also include a policy engine and the ability to move file or sub-file data blocks among the tiers based on activity, application and so on. Most systems allow data to be copied to a second chassis for high availability or moved to a second system at a remote site for DR. While the majority of storage systems include virtualization, most don’t support storage from other vendors. For a heterogeneous virtualization solution, one that can consolidate different vendors’ storage systems, most options are network based.

Network-based virtualization

A number of years ago, the conventional storage wisdom was that storage services, like virtualization, and to an extent storage control, would eventually reside in “smart switches” on the storage-area network (SAN). While at least one storage virtualization product is

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moving in that direction, the network implementation of storage virtualization technology has commonly been in the form of appliances. These appliances are essentially storage controllers that connect to disk arrays or storage systems from certified vendors, or they’re software that’s installed on user-supplied servers or virtual machines (VMs). Storage virtualization appliances connect to heterogeneous storage arrays directly, or via Fibre Channel (FC) or iSCSI SANs, but most provide the option of using their own disk capacity as well. Most solutions include some storage services, like file sharing, snapshots, data deduplication, thin provisioning, replication, continuous data protection (CDP) and so on.

In-band and out-of-band virtualization

Early on in the lifecycle of storage virtualization technology two primary architectures emerged: in-band and out-of-band virtualization. In-band implementations placed a controller between users and physical storage or the SAN, and passed all storage requests and data through that controller. Out-of-band products placed a metadata controller on the network that remapped storage requests to physical locations, but didn’t handle the actual data. That added complexity to the process but reduced the CPU load compared to in-band virtualization. Out-of-band storage virtualization also removed the potential disruption associated with decommissioning an in-band device, as users are disconnected from their data while storage is remapped. Most network-based virtualization solutions today use the in-band architecture, probably because CPU power is relatively plentiful compared to when storage virtualization first appeared. Another reason for the popularity of in-band solutions is that they’re easier to implement, which means faster time to market and fewer problems.

This was first published in August 2011

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