Block access is typically delivered over Fibre Channel, iSCSI, SAS, FICON or other protocols.
The virtualization system presents to the user a logical space for data storage and handles the process of mapping it to the actual physical location.
In this instance, "back-end" refers to a logical unit number (LUN) that is not presented to a computer, or host system for direct use.
The level of granularity, and the total addressable space both directly impact the size of the meta-data, and hence the mapping table.
These implementations utilize dynamic methods to calculate the location on access, rather than storing the information in a mapping table.
If the virtualization is implemented in the network or higher, this renders any replication services provided by the underlying storage controllers useless.
Logical disks (vdisks) are created by the virtualization software or device and are mapped (made visible) to the required host or server, thus providing a common place or way for managing all volumes in the environment.
Enhanced features are easy to provide in this environment: 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.
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 (or LUN).
[5] When all available storage capacity is pooled, system administrators no longer have to search for disks that have free space to allocate to a particular host or server.
Most implementations will provide some form of back-out procedure and with the data migration services it is at least possible, but time consuming.
It applies to the actual physical storage controllers and the hosts, their operating systems, multi-pathing software and connectivity hardware.
Switch based virtualization may not require specific host interoperability — if it uses packet cracking techniques to redirect the I/O.
If the metadata are lost, so is all the actual data as it would be virtually impossible to reconstruct the logical drives without the mapping information.
Caching however requires the visibility of the data contained within the I/O request and so is limited to in-band and symmetric virtualization software and devices.
However these implementations also directly influence the latency of an I/O request (cache miss), due to the I/O having to flow through the software or device.
Assuming the software or device is efficiently designed this impact should be minimal when compared with the latency associated with physical disk accesses.
Due to the nature of virtualization, the mapping of logical to physical requires some processing power and lookup tables.
Host-based virtualization requires additional software running on the host, as a privileged task or process.
In some cases volume management is built into the operating system, and in other instances it is offered as a separate product.
Volumes (LUN's) presented to the host system are handled by a traditional physical device driver.
However, a software layer (the volume manager) resides above the disk device driver intercepts the I/O requests, and provides the meta-data lookup and I/O mapping.
Note: Host based volume managers were in use long before the term storage virtualization had been coined.
Simple data storage devices, like single hard disk drives, do not provide any virtualization.
Generally these devices do not provide the benefits of data migration or replication across heterogeneous storage, as each vendor tends to use their own proprietary protocols.
These sit between the hosts and storage and in the case of in-band (symmetric) appliances can provide all of the benefits and services discussed in this article.
These also sit between the hosts and storage but may use different techniques to provide the metadata mapping, such as packet cracking to snoop on incoming I/O requests and perform the I/O redirection.
In-band, also known as symmetric, virtualization devices actually sit in the data path between the host and storage.
System administrators and software developers administer the virtual storage through offline operations using built-in or third-party tools.
File-based virtualization can also improve storage utilization by allowing files to be stored on devices that are not being used to their full capacity.