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Storage virtualization has been around ever since IBM released the first disk drive in 1956, although some might assert that core and magnetic drum memory were also primitive forms of virtualization. In the context of disk drives, the physical location on which a piece of data was placed was obscured to the application writing or reading the data—disk drives keep indices that track data-to-physical block mapping (abstraction). Disk drives also have the ability to change the mapping to accommodate minor failures (“revectoring” or “bad block replacement” mechanisms). Today this is not even thought of as virtualization—it is totally accepted, invisible, and “uninteresting” from an IT perspective.
Figure 1 shows a representative evolution of more interesting forms of storage virtualization that HP has delivered. Initially, various forms of RAID implementations were shipped. The first was VMS Volume Shadowing—a host-based (implemented with server-resident software) RAID 1 implementation that was delivered several years before the term “RAID” was invented. After more comprehensive RAID algorithms were developed, HP incorporated them into modular array families like the RA/EMA and SmartArray product lines. Even more interesting, the VA family incorporated a self-adjusting RAID mechanism called AutoRAID—an industry-leading approach that automatically migrated data onto the most appropriate level of RAID within a subsystem, as well as automatic and dynamic RAID selection. Also, the VA7400 incorporated online dynamic capacity expansion—a harbinger of virtualization capabilities that started entering the mainstream in 2007.
All RAID schemes virtualize by creating a single pool consisting of a number of disk drives. Application data is algorithmically deposited across the disks by array controllers. In some cases (RAID 3, RAID 4, RAID 5, RAID 6), one or more disks’ worth of parity data is also generated by the array and stored on its constituent disks to provide resilience to disk failures. Although RAID was initially invented to allow inexpensive commodity disk drives to reach the availability characteristics of higher-cost proprietary drives, the enduring benefits of RAID include:
Higher and more uniform device (disk) utilization
Better performance, achieved through automatic load balancing among the disks aggregated into the RAID
Improved data availability, because built-in redundancy enables greater uptime
Easier manageability: one RAID is easier to manage than the corresponding number of independent disk drives
In 2001, HP extended RAID implementation to encompass the entire contents of a storage subsystem, as opposed to creating multiple physically discrete RAIDs within a single subsystem. The result was the HP StorageWorks Enterprise Virtual Array family (EVA). The EVA amplified the benefits of RAID to encompass a complete system that could enhance the utilization of many disks simultaneously, and warrant even greater availability through the proprietary application of RAID algorithms. The system-wide approach also made EVA easier to manage than peer products.
Remarkably, the EVA extended storage-based virtualization (virtualization embedded within the storage system) in interesting and useful ways. It transcends conventional RAID by delivering “advanced storage-based whole-system virtualization” with capabilities like:
Dynamic expansion of the virtual disk pool: disks inserted into a configured array are automatically added to the existing virtualized pool (by the EVA controller), and the workload is automatically redistributed to all members—without administrative involvement
Dynamic expansion of the virtual disks (“LUNs”) presented to servers, although the ability to dynamically present the new capacity is dependent on host operating system ability to recognize the changed LUNs
Snapshot and other point-in-time replication technologies: these “in-the-box” replication technologies create virtual images—copies of the LUNs—and can be used for rapid data protection and recovery, and for time-shifting backup operations
Remote replication—with synchronous and asynchronous options—invokes mirroring and other technologies to provide resilience against site outages, as well as being useful for remote backup and other purposes
While general attention has been focused on disk storage virtualization, it is useful to point out that tape libraries, Redundant Arrays of Independent Tapes (RAIT), and other technologies apply virtualization to tape-based storage.
As the millennium changed, the industry began delivering network-based virtualization capable of pooling heterogeneous, multi-vendor storage systems using virtualization technologies embedded in the fabric (SAN or LAN) to which the storage resources are attached. HP offered the HP OpenView Continuous Access Storage Appliance (CASA) to aggregate multiple disk subsystems into a single pool for data migration, capacity expansion, management simplification, and other capabilities. Network-based virtualization seemed to be ahead of its time back then, and faded from the limelight until recently—in part for reasons discussed later in this paper.
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