Dynamic magnetic information storage or retrieval – General processing of a digital signal – Data in specific format
Reexamination Certificate
1998-10-30
2001-09-25
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Data in specific format
Reexamination Certificate
active
06295177
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. Record length, and the order in which the Records and the Record separators are received, are determined by the host computer.
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.
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 strm of the Records and the third FILE MARK is stored in the index of the second group.
It will be appreciated that the length of the index in a group will vary in proportional relation to the number of separation marks and the number of Records present in the group. Accordingly, the corresponding space remaining for the Record data in the encoded data stream for the respective group will vary in inverse-proportional relation.
In such a scheme, a tape drive reading the stored data relies on information in the index to reconstruct the original host computer data for return to a host computer.
FIG. 2
illustrates very generally the formn 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 biginning 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 GIT generally provides a high level indication of which data (e.g. which Records and FILE MARKs) are in the respective group, without including any information which assists with encoding of decoding of data in the group. The values for the GIT entries in this simple example are shown in parentheses in FIG.
2
. 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 contents of a group and, in particular, the logical segmentation of the Record data held in the group (that is, it holds entries describing the length of each Record and the position of each separator mark in the group). 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 arriving at the present invention, the present applicants have recognised that the space and performance overhead of a having a BAT becomes restrictive in the event Records received from the host computer are small or there are a relatively high proportion of FILE MARKS compared with the number of Records. In terms of space overhead, more FILE MARKs mean that the BAT grows and the room for Records in a group shrinks. In effect, the storage capacity of a tape will diminish considerably if there are a large number of FILE MARKS. It should be noted that the reason for the Records being small, or there being a large number of FILE MARKs, remains unknown to, and not under the control of, the tape drive.
In terms of encoding or compression performance overhead, each time a FILE MARK is encountered, the processing of the incoming data needs to redirect to update the BAT, which is a significant overhead for an encoding pipeline when increasing data processing speed is an important consideration.
DISCLOSURE OF THE INVENTION
In accordance with a first aspect, the present invention provides a method of arranging data received in a data transfer from a data source, the method including the steps of:
receiving from the data source, one or more blocks of data and one or more data separation signals, the block(s) of data and data separation signal(s) being received in a sequence determined by the data source;
encoding the block(s) of data into an encoded data stream and including in the encoded data stream, in the same sequence, data representing the or each data separation signal; and
writing the encoded data stream to a storage device or medium.
Advantageously, the invention obviates the need for an index such as a BAT to describe the existence and position of separation signals in the encoded data stream. Accordingly, the space and processing overheads of a BAT are removed. Further, the method provides that the encoded data stream can be split back into the serial stream, or sequence, of blocks and separation signals, and returned to the data source, without reference to any separately-recorded or transmitted table. On the down-side, however, it is a more complex task to locate any specific FILE MARK or Record data in an encoded data stream, since there is no convenient index. Therefore, it is necessary to decode an encoded data stream to find any particular data, as will be described herein.
The present invention also provides tape storage apparatus, arranged to store host computer data in accordance with the above method.
While this present method of formatting data 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 exa
Bickers Richard Arthur
Southwell Simon David
Faber Alan T.
Hewlett--Packard Company
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