Storage virtualization refers to the process of abstracting logical storage from physical storage. The term is today used to describe this abstraction at any layer in the storage software and hardware stack.
Address Space Remapping
Virtualization of storage helps achieve location independence by abstracting the physical location of the data. The Virtualization system presents to the user a logical space for data storage and itself handles the process of mapping it to the actual physical location.
The actual form of the mapping will depend on the chosen implementation. Some implementations may limit the granularity of the mapping which itself may limit the capabilities of the device. Typical granularities range from a single physical disk down to some small subset (multiples of megabytes or gigabytes) of the physical disk.
In a block-based storage environment, a single block of information is addressed using a logical unit identifier (LUN) and an offset within that LUN - known as a Logical Block Address (LBA). The address space mapping is between a logical disk, usually referred to as a virtual disk (vdisk) and a logical unit presented by one or more storage controllers. (Note The LUN itself is likely to be a logical disk and may even be a virtual disk).
The virtualization software or device is responsible for maintaining a consistent view of all the mapping information for the virtualized storage. This mapping information is usually called meta-data and is stored as a mapping table.
The address space may be limited by the capacity needed to maintain the mapping table. This is directly influenced by the granularity of the mapping information.
Non-Disruptive Data Migration
One of the major benefits of abstracting the host or server from the actual storage is the ability to migrate data while maintaining concurrent I/O access.
The host only knows about the logical disk (vdisk) and so any changes to the meta-data mapping is transparent to the host. This means the actual data can be moved or replicated to another physical location without affecting the operation of any client. When the data has been copied or moved, the meta-data can simply be updated to point to the new location, therefore freeing up the physical storage at the old location.
The process of moving the physical location is known as data migration Most implementations allow for this to be done in a non-disruptive manner, that is concurrently while the host continues to perform I/O to the logical disk (vdisk).
The mapping granularity dictates how quickly the meta-data can be updated, how much extra capacity is required during the migration, and how quickly the previous location is marked as free. The smaller the granularity the faster the update, less space required and quicker the old storage can be freed up.
There are many day to day tasks a storage administrator has to perform that can be simply and concurrently performed using data migration techniques.
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