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Data Storage Hashing ISAM VSAM

Oracle Tips  by Burleson Consulting

High Performance Data Warehousing

Early Disk-Based Data Storage

Despite these problems, hashing is still used within commercial data warehouses, and it remains one of the fastest ways to store and retrieve disk information. Most Unix systems can take a symbolic key and convert it into a disk address in as little as 50 milliseconds. While hashing is a very old technique, it is still a very powerful method. Many C++ programmers use hashing to store and retrieve records within their object-oriented applications.

Figure 1.3  Hashing for disk data storage.

While the hashing technique is still very popular for fast storage and retrieval of individual records, it is not suitable for the type of full scans that we see in a data warehouse. As we would expect from a random key generator, records are not stored contiguously on a disk. Rather, they are randomly distributed across the disk device. While an index can help speed retrieval of hashed records, we still do not see the high I/O throughput that we see when records are stored contiguously on data blocks. With contiguous record storage (such as a relational database), we see that an 8 K file I/O will read hundreds of records into an out buffer with one I/O. We do not get this luxury with hashed file storage techniques.

It is interesting that in the early 1990s, more data was stored on magnetic tapes than in all of the other file formats combined. In fact, even now, companies with terabytes of data warehouses continue to use magnetic tapes for systems that contain large amounts of unchanging, infrequently used data. Magnetic tapes, which remain more than 10,000 times cheaper than disk storage, are still the most economical way to store large volumes of data.

Overall, data warehouse applications that access data stored in IS-AM and VSAM data structures remain popular. Commercial engines such as the Informix-SE database are basically IS-AM files that are accessed by the data warehouse.

However, the lack of robust commercial databases made sophisticated data analysis very cumbersome. The problems inherent in early disk-based systems were very serious, and an effort was undertaken to rethink the entire concept of data storage. These problems included the following issues:

This is an excerpt from "High Performance Data Warehousing", copyright 1997.
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