11.5 Storage-Based Virtualization

As storage vendors themselves will modestly admit, storage arrays are the perfect place to house virtualization technology. Enterprise-class storage arrays already provide low-level virtualization via RAID and mirroring, and many of them support higher-level functions such as storage pooling across multiple arrays. As with interconnection-based virtualization, array-based virtualization may be completely transparent to servers and may require no special agent software. Unlike interconnection-based virtualization, storage arrays reside at the end points of the SAN, not the middle, and so are less likely to generate SAN-wide bottlenecks to performance.

Storage-based virtualization from leading vendors provides direct disk-to-disk data replication that simplifies (at a cost) the implementation of high availability for applications such as business continuance and disaster recovery. EMC SRDF, HDS TrueCopy, HP DRM, and XIOtech REDI-SANlinks, for example, all provide means to replicate storage data without server intervention, thus fulfilling one major component of storage virtualization.

Some storage-based virtualization solutions provide additional functions, such as point-in-time snapshots of storage data to facilitate windowless tape backup, dynamic storage pool creation within an enclosure, and storage pooling between multiple enclosures, as shown in Figure 11-8.

Figure 11-8. Storage-based virtualization for pooling and data replication

If storage-based virtualization sounds too good to be true, it is. The major limitation of array-based virtualization is not technical but marketing in nature. Storage-based virtualization solutions are proprietary to each vendor, and so they can be implemented only in homogeneous storage environments.

Although that may be acceptable for customers who prefer to implement uniform storage products throughout their networks, most customers resist solutions that foster single-vendor lock-in. In addition, the spontaneous accumulation of storage products from multiple vendors on a departmental basis or through company acquisitions often results in a heterogeneous mix of storage investments, all of which could benefit from a vendor-neutral virtualization solution. This has driven attention from array-based virtualization to the upstream interconnection-based solutions as a means to accommodate multivendor environments.