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This is part one of a series of articles on System-Managed Storage.
Introduction
System-managed storage allows the operating system to take over many storage management tasks that were previously performed manually. This is important because the cost of a storage device is more than just the initial cost of the hardware. Other costs include the electrical requirements, floor space, and the people needed to install, monitor, and operate the devices. Removable media, such as optical and tape storage, cost less per megabyte than online storage but require additional time and resources to locate, retrieve, and mount.
After taking all the hidden costs into account, it becomes clear that controlling and managing the growth of information systems is critical if a business is to grow and expand profitably. There are many tasks associated with storage management, such as space management, backup processing, and data set allocation.
Why should I use system managed storage?
Consider the following analogy, which was provided by storage management consultant Paul Daughenbaugh.
Suppose you were flying to New York's JFK airport. At this airport, one of the busiest in the world, traffic is controlled by air traffic controllers. If it was not, then arriving and departing aircraft could come and go whenever they wished, using whichever runway they preferred. The result would be chaos and an increased probability of collisions. Would you want to travel through such an airport?
A storage environment without system-managed storage is analogous to an airport without air traffic controllers. Allocations and deletions occur with little or no control, on whichever volume the person performing the allocation happens to choose. Some volumes may be highly utilized in terms of both space and performance while others are sitting idle. In a storage environment, a collision can be said to occur when a data set allocation fails because there is no space on the volume on which the allocation was attempted.
System-managed storage addresses this problem by placing device selection under the control of the system. The system does this using a policy established by the storage administrator who defines a volume pooling structure made up of
storage groups. The storage administrator also writes straightforward
automatic class selection (ACS) routines that define which data sets can be allocated in which storage groups. Using these ACS routines, the storage administrator can allow the system to control as much or as little allocation of storage groups as desired.
When a new allocation occurs, the system uses the ACS routines to determine a set of storage groups in which the data set is eligible to be allocated. The system then considers criteria such as space and performance to select the specific volume or volumes on which to perform the allocation. This can help:
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 Reduce the number of out of space abends
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Reduce device fragmentation
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Balance allocations across a pool of devices
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Improve storage utilization
Why should I use the IBM solution?
There are products other than MVS/DFP and DFSMS which provide the equivalent of IBM's system-managed storage concept. However, many of these products simply function as a front end to MVS common allocation. They provide a hook between the user's allocation and MVS to perform device selection. Once they select a device or set of devices, the allocation proceeds as it would in a non-managed environment. If it succeeds, that is well and good. But if it fails, there is no mechanism to retry the allocation.
The IBM solution is superior in that it is an integral part of the allocation process. It receives control back
after the allocation is attempted or performed. If the allocation succeeds, it can then proceed and issue appropriate messages. If the allocation fails, it can retry until either the allocation is successful or all possible volumes have been tried.
In summary
DFSMS provides the key to system-managed storage in an MVS environment. In a system-managed storage environment, an installation establishes policies for how hardware resources should be used, balancing available resources with user requirements for data availability, performance, and space. DFSMS implements these policies and handles many storage management tasks, freeing users from manual storage administration and making more efficient use of storage resources.
Follow these links for the rest of the story:
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Reduce the number of out of space abends
