Virtualization: Tales from the trenches


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Defining terms
For the purposes of our review of these companies' activities, a virtual storage system is defined as one that virtualizes block disk storage behind it. The virtualization products support heterogeneous storage devices and can pool disk storage from any supported attached storage.

Of course, there are several different ways to implement storage virtualization: on the host, in the SAN or at the controller. Block virtualization redirects an I/O request from a virtual location to a physical location, so data can be moved anywhere within the virtualized storage pool transparently to applications and users.

Other features common to virtual storage systems include:

  • Snapshot or point-in-time copy across disparate or similar storage devices
  • Remote replication across disparate or similar storage devices
  • Data migration from one back-end disk group to another while applications continue to access the data
  • Centralized management of all virtualized storage
Virtualization products may operate in-band or out-of-band. In-band products reside within the data path of the disk storage, where they intercept I/O requests and may cache responses to the I/O requests. Out-of-band devices typically don't have cache and, although they may redirect an I/O request, they don't alter the data path to storage.


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Communications Inc., DataCore Software Corp., FalconStor Software Inc. and IBM offer in-band products at the SAN level. In-band products that work at the controller are available from HDS, Network Appliance Inc. and Sun Microsystems Inc. (and its OEM partners). EMC and Fujitsu Computer Systems Corp. offer out-of-band virtualization products that operate in the SAN. StoreAge and Symantec Corp. offer host-level, out-of-band products.

This was first published in August 2006

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