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Cons
• It is performed on a host-by-host basis, so it doesn’t have organization-wide impacts.
• Replication and data migration are only possible locally to that host.
• No easy way of keeping host instances in sync with other instances.

Storage Device Based
In the past, devices such as RAID controllers provided logical and physical abstraction, but
didn’t provide the ability to migrate data. Just recently, new RAID controllers have made it
possible to allow downstream attachment of additional storage devices.
It works through a primary storage controller that manages virtualization services and
allows additional storage controllers to be added. The primary controller manages metadata
services and provides pooling. It can also manage replication and migration services to
other devices.

Pros
• No additional hardware (aside from extra disk volumes) is required.
• Gives most benefits of server virtualization.
Cons
• Storage is optimized only across connected controllers.
• Replication and migration are only possible across connected controllers and may
only be possible in homogeneous environments.
• Higher latency. The primary storage controller must issue a secondary downstream
I/O request for content not in the cache.
• Higher bandwidth consumption. The primary storage controller needs the same
amount of bandwidth as the secondary storage controllers to maintain the same
level of throughput.

Network Based
The preceding types of storage virtualization really aren’t ideal for a large organization.
They can be used, of course, but the one you are most likely to use is the network-based
implementation. This is a SAN connected to a network-based device. This device sits in
the SAN and provides the management duties of processing I/O requests and managing PART V
the metadata.

Pros
• Heterogeneous storage virtualization.
• Data caching can be performed in-band.
• Single management interface.
• Replication between heterogeneous devices.