Get ready for virtualization


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Storage vendors offer network-based block virtualization in two configurations: combined and split path. Combined-path architectures handle the data and management functions in the same logical design and appear in the following implementations:

  • Appliances. Cloverleaf Communications Inc., DataCore Software Corp., FalconStor Software and IBM Corp. provide their virtualization software on off-the-shelf, Intel-based server hardware. Configured to reside in the data path between the server and storage, they're deployed as either clustered pairs or an N+1 configuration. All traffic for virtualized storage is routed through the appliance.

  • Fibre Channel (FC) director blade. FC directors from Cisco Systems Inc. and Maxxan Systems Inc. support blades that run virtualization software from IBM and FalconStor, respectively. This eliminates the need for separate appliances and centralizes switch and storage hardware.

  • Array-based. Hewlett-Packard (HP) Co., Hitachi Data Systems (HDS) Inc. and Sun Microsystems Inc. extend their respective StorageWorks XP12000, TagmaStore and StorEdge 9900 arrays' existing virtualization functionality to virtualize other vendors' arrays.
Split-path architectures separate the data and control functions so that a different appliance handles each function. These show up in the following ways:

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  • Host-based. StoreAge Networking Technologies' Storage Virtualization Manager (SVM) uses host-based agents in conjunction with a management appliance that splits the data and control paths at the host level. Management of features like replication and snapshots occur over IP, while write I/Os between the server and storage occur on FC unimpeded by the appliance.

  • Storage Services Platform (SSP). Brocade Communications Systems Inc., Cisco and Troika Networks Inc. offer fabric platforms that allow the data path portion of virtualization software like EMC Corp.'s Invista to be loaded onto them. These platforms deliver the high reliability and performance typically associated with FC switches by removing cache from the switch. They use Application-Specific Integrated Circuits (ASICs) to process FC traffic and execute virtualization software delivered by storage vendors.
Network-based virtualization options
Click here for a comprehensive list of Network-based virtualization options (PDF).
Both approaches require access to the FC director as a blade or through FC connections, as well as the ability to access and manage the LUNs on the storage arrays they virtualize. Also, whether host-based or delivered as part of the SSP, the virtualization functions must be able to manipulate the LUNs presented by the storage arrays by either combining them to create a larger logical volume or carving out just a portion of a LUN and presenting it as a small volume to the host.

The devil, of course, is in the details: There are differences in how vendors configure their hardware, manage cache and handle I/O. For example, vendors with split-path approaches leave cache out of the data path. Split-path providers EMC and StoreAge find it simpler and safer to let cache reside closer to the application server or storage array and to remove the hassles of managing the data in the network cache. Virtualization applications that integrate with switch vendors' SSPs turn control of the I/O processing over to the ASICs on the FC ports on these directors. Vendors contend these switches can process I/Os faster and more effectively than the hardware provided by appliances, arrays or blades.

Despite the differences between combined- and split-path architectures, storage administrators will choose a virtualization product based less on its architecture and more on how comfortable they are with moving into a virtual environment and the vendor lock-in that will likely ensue. Ease of implementation, software code maturity and how the virtualization software is licensed will ultimately drive the wide-scale adoption of this technology.

This was first published in December 2005

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