Chapter 4: Storage Network Management

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4.1 Introduction

With the rapidly increasing volume of mission-critical information being accumulated in today’s business environment, companies are demanding better performance, availability, manageability, and security of their data storage assets. To meet these needs, companies are implementing specialized networks that are capable of helping them realize operational efficiencies and maximize revenue-generating opportunities. These objectives can be achieved with a combination of transport technologies and distributed storage resources, which together comprise a separate high-performance SAN. While most of today’s SAN offerings are proprietary, standards are beginning to emerge to facilitate multi-vendor interoperability and management.

The need to balance the cost, capacity, and efficiency of different storage media according to the frequency of data usage became apparent in the mid-1990s. Hierarchical storage management (HSM) solutions were introduced to provide all clients and servers with expanded on-line storage through the migration of files between hard disks, magnetic tape, and optical media, according to predefined rules.

HSM solutions use two or more levels of storage (three is typical: on-line, offline, and near-line) to provide a cost-effective and efficient solution to meet the demand for increased storage space and appropriate data retrieval response time. HSM came about because of the need to move low-volume and infrequently accessed files from disk, thus freeing up valuable on-line storage space.

Although more disks can be added to file servers to keep up with storage needs, budget constraints often limit the long-term viability of this solution. In an HSM scheme, data can be categorized according to its frequency of usage and stored appropriately: on-line, near-line, or offline. Different storage media come into play for each of these categories, and migration operations are under control of a storage management system (see Figure 4.1).

Figure 4.1: Hierarchical data storage spanning magnetic disk, optical disk, and tape options.

Frequently used files are stored on-line on local disk drives installed in a server or workstation. Occasionally used files are stored near-line on secondary storage devices such as optical disks installed in a server-like device called an auto-changer or jukebox. Infrequently used files are usually migrated off line to tape cartridges that are stored in a tape jukebox or a library facility capable of holding hundreds or thousands of tapes. The library facility uses sophisticated robotics to retrieve individual bar-coded cartridges and inserts them into a tape drive so the data can be moved to local storage media. The exchange time can be several minutes.

For organizations with mixed needs for on-line and offline storage, a near-time automated tape library offers the best compromise between price and performance. These systems bridge the gap between fast, expensive on-line disk storage and slow, high-capacity offline tape libraries. The exchange time is about 30 seconds.

A management system determines when a file should be transferred or retrieved, initiates the transfer, and keeps track of its new location. As files are moved from one type of medium to another, they are put into the proper directory for user access. The management system automatically optimizes storage utilization across different media types by removing files from one to the other until they are permanently archived in the most economical way, usually a tape library. At the same time, individual files or whole directories can be excluded from migration.

Data migration can be controlled according to such criteria as file size and last date of access. Files can also be migrated when the hard disk reaches a specified capacity threshold. When a disk reaches the threshold of 80% full, for example, files are migrated automatically to tape storage, freeing space on the hard disk until it reaches another specified threshold, say, 60%. When files are no longer needed but must be stored permanently, they can be migrated off the tape drive to optical media.

HSM concepts have been incorporated into both SANs and storage virtualization solutions, both of which offer organizations even more flexibility in how storage is used and managed.

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