Patent
1991-11-18
1994-01-18
Richardson, Robert L.
G06F 300, G06F 1300
Patent
active
052806006
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the storage of user data and associated information received from a host computer onto tape and to the retrieval of such stored data. The invention is particularly concerned with the storage and retrieval of data which has undergone data compression (DC).
BACKGROUND OF THE INVENTION
It is known to provide a tape drive having data compression hardware (a DC drive) so that, as data arrives from a host, it is compressed before being written to tape thus increasing the tape storage capacity. A DC drive is also able to read compressed data from a tape and to decompress the data before sending it to a host.
There is more than one type of data compression. For example, removing separating marks such as record marks and file marks from user data and storing information regarding the separation marks in an index compresses the data. However, references to data compression (DC) in this specification will mean removing redundancy from the data (eg. replacing the user data by codewords form which the original data can be recovered). There are various algorithms which are used for data compression and it is envisaged that each of these algorithms will be given a universally recognized number. In practice, the application of a data compression algorithm can sometimes lead to expansion of the data (eg. if the data has previously been compressed).
In a DC drive, data compression and decompression may be carried out by a single DC chip. It is possible for a DC drive to contain more than one type of DC chip so that the drive may be able to perform data compression according to more than one data compression algorithm. An alternative approach is to perform data compression and data decompression in a host using the appropriate software.
SUMMARY OF THE PRESENT INVENTION
The present invention provides a data storage method for storing compressed user data and associated information on magnetic tape. The user data from a host computer is organized into a plurality of records. The user data is then compressed according to a compression algorithm. The compressed data and information related to the data compression (DC) algorithm are then stored on magnetic tape.
An advantage of a data storage method according to the present invention is that, if a tape which is written to in accordance with the present invention is subsequently loaded into a non-DC drive, such a drive may (if suitably configured) be able to inform a host which compression algorithm was used to compress the data to enable the host to perform software decompression.
The invention has the following further advantages:
In a DC drive utilizing one type of DC algorithm it has the above advantage and also enables the DC drive to selectively decompress only those records which have been compressed according to the algorithm which that drive uses;
In a DC drive utilizing more than one DC algorithm, the above two advantages apply and, in addition, it enables the use of more than one DC algorithm to decompress data records on a tape.
Preferably, a data storage method in accordance with the present invention comprises writing DC algorithm information in the datastream so that it is associated with one or more records.
In a preferred embodiment to be described the method comprises writing DC algorithm information to a header in the datastream associated with one or more records the DC algorithm information is, for instance, a DC algorithm number.
One way of compressing user data is dynamically to generate a dictionary of code words from the user data and to recreate the dictionary from the compressed data during decompression. An algorithm which adopts this approach is the Lempel Ziv Welch (LZW) algorithm. The DC algorithm information may contain an indication of whether a new data dictionary is being started at a particular point in the datastream.
Furthermore, the DC algorithm information may indicate that the data has not undergone data compression.
A data storage method according to the present invention may c
REFERENCES:
patent: 4891784 (1990-01-01), Kato et al.
"Maximizing Tape Capacity by Super-Blocking", David J. Van Maren et al., Hewlett-Packard Journal, vol. 40, No. 3, Jun. 1989, pp. 32-34.
Bramhall Peter
Van Maren David J.
Hewlett--Packard Company
Richardson Robert L.
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