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66 DS6000 Series: Concepts and Architecture
4.1 Virtualization definition
In our fast changing world, where you have to react quickly to changing business conditions,
your infrastructure must allow for on-demand changes. Virtualization is key to an on-demand
infrastructure. However, when talking about virtualization many vendors are talking about
different things.
Our definition of
virtualization is the abstraction process going from the physical disk drives
to a logical volume that the hosts and servers
see as if it were a physical disk.
4.2 The abstraction layers for disk virtualization
In this chapter, when we talk about virtualization, we are talking about the process of
preparing a bunch of physical disk drives (DDMs) to be something that can be used from an
operating system, which means we are talking about the creation of LUNs.
The DS6000 is populated with switched FC-AL disk drives that are mounted in storage
enclosures. You order disk drives in disk drive sets. A disk drive set is a group of 4 drives of
the same capacity and RPM. The disk drives can be accessed by a pair of device adapters.
Each device adapter has four paths to the disk drives. The four paths provide two FC-AL
device interfaces, each with two paths such that either path can be used to communicate with
any disk drive on that device interface (in other words, the paths are redundant). One device
interface from each device adapter is connected to a set of FC-AL devices such that either
device adapter has access to any disk drive through two independent switched fabrics (in
other words, the device adapters and switches are redundant). In normal operation, however,
disk drives are typically accessed by one device adapter and one server. Each path on each
device adapter can be active concurrently, but the set of eight paths on the two device
adapters can all be concurrently accessing independent disk drives. This avoids any
contention between the two device adapters for access to the same disk, such that all eight
ports on the two device adapters can be concurrently communicating with independent disk
drives.
Figure 4-1 on page 67 shows the physical layer on which virtualization is based.