Generating appendable points in encoded data

Details Generating appendable points in encoded data Generating appendable points in encoded data

1998-10-30

2001-07-31

Faber, Alan T. (Department: 2651)

Dynamic magnetic information storage or retrieval

General processing of a digital signal

Data in specific format

C360S053000, C714S748000

Reexamination Certificate

active

06268973

ABSTRACT:

TECHNICAL FIELD
The present invention relates to data storage, and in particular, but not exclusively, to methods and apparatus for encoding or formatting data for storage to, for example, a magnetic medium such as tape.
BACKGROUND ART
Taking data storage to tape as an example, a host computer system typically writes data to a storage apparatus, such as a tape drive, on a per Record basis. Further, the host computer may separate the Records themselves using Record separators such as FILE MARKs or SET MARKs.
Typically, Records comprise user data, for example, the data which makes up wordprocessor documents, computer graphics pictures or data bases. In contrast, Record separators, such as FILE MARKs, are used by a host computer to indicate the end of one wordprocessor document and the beginning of the next. In other words, Record separators typically separate groups of related Records.
Generally, the host computer determines Record length, and the order in which the Records and the Record separators are received, and, typically, the storage apparatus has no control over this.
By way of example, the diagram in FIG.
1
(
a
) illustrates a logical sequence of user data and separators that an existing type of host computer might write to a tape storage apparatus. Specifically, the host computer supplies five fixed-length Records, R
1
to R
5
, in addition to three FILE MARKs, which occur after R
1
, R
2
and R
5
.
It is known for a storage apparatus such as a tape drive to receive host computer data, arrange the data Records into fixed-sized groups independently of the Record structure, and represent the Record structure, in terms of Record and FILE MARK position, in an index forming part of each group. Such a scheme forms the basis of the DDS (Digital Date Storage) data format standard for tape drives defined in ISO/IEC Standard 10777:1991 E. EP 0 324 542 describes one example of a DDS tape drive, which implements this scheme. Once the groups of data are formed, the tape drive stores the groups to tape, typically after applying some form of error detection/correction coding.
The diagram in FIG.
1
(
b
) illustrates the organisation into DDS groups of the host computer data shown in FIG.
1
(
a
). Typically, the host computer data Records are encoded or compressed to form a continuous encoded data stream in each group. FILE MARKs are intercepted by the tape drive, and information that describes the occurrence and position of the FILE MARKs in the encoded data stream is generated by the tape drive and stored in the index of the respective group. In the present example, Records R
1
, R
2
and a part of Record R
3
are compressed into an encoded data stream and are stored in the first group, and information specifying the existence and position in the encoded data stream of the records and the first and second FILE MARKs is stored in the index of the first group. Then, the remainder of Record R
3
, and Records R
4
and R
5
, are compressed into a continuous encoded data stream and are stored in the second group, and information specifying the existence and position in the encoded data stream of the records and the third FILE MARK is stored in the index of the second group.
FIG. 2
illustrates very generally the form of the indexes for both groups shown in FIG.
1
(
b
). As shown, each index comprises two main data structures, namely a block access table (BAT) and a group information table (GIT). The number of entries in the BAT is stored in a BAT entry field in the GIT. The GIT also contains various counts, such as a FILE MARK count (FMC) which is the number of FMs written since the beginning of Recording (BOR) mark, including any contained in the current group, and Record count (RC), which is the number of Records written since the beginning of Recording (BOR) mark, including any contained in the current group. The values for the entries in this simple example are shown in parentheses. The GIT may contain other information such as the respective numbers of FILE MARKs and Records which occur in the current group only.
The BAT describes, by way of a series of entries, the ‘structure’ of a group in terms of the logical segmentation of the Record data held in the group and the position of each separator mark. The access entries in the BAT follow in the order of the contents of the group, and the BAT itself grows from the end of the group inwardly to meet the encoded data stream of the Record data.
In such a scheme, a tape drive reading the stored data, on the basis of a command from a host computer to read or write data, relies on information in the index to locate the particular Record or FILE MARK starting position in the encoded data stream.
DISCLOSURE OF THE INVENTION
In accordance with a first aspect, the present invention provides a method of arranging data received from a data source, the method including the steps of:
receiving data from the data source, the data having a data structure determined by the data source;
determining an appendable point in the data, which point is determined independently of the data structure;
encoding the data from the data source into an encoded data stream and inserting into the encoded data stream data representative of the appendable point; and
writing the data stream to a storage device or medium.
The present invention provides a means of encoding data received from a data source in such a way that any desired point, an ‘appendable point’, can be located in the resulting encoded or compressed data stream, irrespective of whether the point coincides with a Record boundary or a FILE MARK.
The applicant's co-pending patent application “Data Encoding Method and Apparatus” (applicant's reference 30970017), U.S. Pat. Ser. No. 09/182,308 filed Oct. 30, 1998, describes an invention wherein the requirement for a BAT is removed by embedding special, reserved codewords representing Record boundaries and Record separators, such as FILE MARKS, into the encoded data stream. Therein, Record boundaries and FILE MARKS can be located by the respective embedded codewords. The problem addressed herein relates to encoding data such that any required position can be located within the encoded data, in particular when the encoded data is compressed. The present invention is believed to be particularly advantageous when used in combination with the applicant's co-pending application.
In a preferred embodiment of the present invention, data from the data source is received in bursts or packets, and the beginning or end of each burst or packet is determined to be an appendable point. This is particularly advantageous for burst processing purposes, as will be described below.
In accordance with a second aspect, the present invention provides apparatus for arranging data received from a data source, the apparatus comprising:
interface means to receive data from the data source, the data having a data structure determined by the data source, and to determine an appendable point in the data, which point is determined independently of the data structure; and
encoder means to encode the data from the data source into an encoded data stream and insert into the encoded data stream data representative of the appendable point and to write the data stream to a storage device or medium.
While this present invention finds particular application in the field of tape storage, the method may equally be applied to other storage devices and storage media. One alternative storage media would be optical disk. Also, it is anticipated that data may be received from sources other than a host computer, for example a data mover system, which moves data between storage devices.
Other aspects and embodiments of the present invention will become apparent from the following detailed description and claims.


REFERENCES:
patent: 5247396 (1993-09-01), Ytaka et al.
patent: 5485321 (1996-01-01), Leonhardt et al.
patent: 5754754 (1998-05-01), Dudley et al.
patent: 0324542A (1989-07-01), None
patent: 0338781A (1989-10-01), None
patent: 0459041A (1991-12-01), None
patent: WO9110999A (1991-0

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