volumes to physical disks. A functional volume is a logical volume still defined by track size, capacity, and address. This mapping structure is contained in a series of tables stored in the control unit. These tables are updated at each write on functional volume, and have to be maintained when previously used space is released.
Data from all functional volumes could reside on one array device or many array devices. Functional volumes can be defined and configured
Defining a logical volume by tables brings capabilities such as easy and almost instantaneous volume duplication when both source and target volumes are controlled by the same set of tables inside the same storage server. However the actual data copy has yet to take place. Either background storage server tasks or system utilities implement the movement of data. Virtual volume definition by tables brings another function: it allows the instant volume duplication by creating two independent host access views of the same data, simply sharing the same data with no replication. The IBM RVA SnapShot enables instantaneous duplication with no physical space utilization at duplication time. This advantage comes from the other architecture improvement, the LSF concept, on which the virtual volume architecture is based.
9.2 Disk Control Units
DB2 uses VSAM Media Manager for its I/O operations. Like any access method, VSAM Media Manager builds for every I/O a channel program and sends a request to the I/O supervisor. The I/O supervisor enqueues this request on a device number for the channel subsystem.
The channel program consists of standard commands, described in the ECKD disk architecture, that specify I/O demand to the control unit. The control unit executes these commands, propagates them and controls their requests to logical volumes and physical devices. It also manages data delivery to the channel subsystem.
The channel subsystem manages the transfer of channel commands and of data through links to the control unit. This linking can be complex and involves ESCON Directors, channel extenders, and even telecommunication devices for remote I/Os.
There are two views of a disk control unit. Physically it is a storage server to which disk drives are attached and the channel links from hosts are connected. The storage server contains all the facilities to perform the I/O operations. Logically the disk control unit is an aggregate of subunits known as logical control units (LCUs) or control unit images, doing the I/O operations.
9.2.1 Storage Server
The storage server contains all shared resources and processes to support LCU activities. It often consists of two or more clusters. A cluster can take over the
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