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Cons
• Metadata is hard to update in switch-based devices.
• Out-of-band requires specific host-based software.
• In-band may add latency to I/O.

Implementation
Implementing network virtualization involves either a standalone appliance (appliance based)
or a switch (switch based). Both architectures give the same services—pooling, data
migration, and so forth.

Appliance-Based Devices Appliance-based devices—such as the one shown in Figure 12-4 —
are devices that provide SAN connectivity. They sit between the hosts and the SAN.
I/O requests are sent to the device, which handles the metadata mapping before the
request is redirected to the storage devices.
Switch-Based Devices These devices reside in the switch hardware used in the SAN. These
also sit between the hosts and the SAN, but might use different mechanisms to provide
the metadata mapping.

In-Band In-band (or symmetric) virtualization devices sit in the data path between the host
and the storage. Hosts send I/O requests directly to the device, and they don’t directly
communicate with the storage themselves. Data caching, statistics, replication, and data
migration are all possible in an in-band device.

Out-of-Band Out-of-band (or asymmetric) virtualization devices just perform metadata
mapping services. When an I/O request is made from the host, it is first processed at the
host using special software. Then a metadata lookup is requested from the out-of-band
device, and the location of the data is sent back to the host.

NOTE This method prevents caching of data because the data never passes through the device.

Appliance-based device

Host

SAN
FIGURE 12-4 Appliance-based devices sit between the SAN and hosts.