Electrical computers and digital processing systems: memory – Storage accessing and control – Control technique
Reexamination Certificate
1997-02-27
2001-03-13
Ellis, Kevin L. (Department: 2751)
Electrical computers and digital processing systems: memory
Storage accessing and control
Control technique
Reexamination Certificate
active
06202136
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of creating an internally consistent copy of an actively updated data set using processor memory. The invention advantageously permits a user to utilize both a computer program that requires access to the data set (e.g., a database report generator, a copy utility, or other such program) and a different computer program that might update the data set (e.g., a database management system program) while maintaining internal consistency in the data set, without the need for specialized caching hardware.
The general context of the operations associated with the invention is illustrated in FIG.
1
. Quite commonly, a computer system administrator or other user will wish to create an image copy of a data set, e.g., a data set
102
comprising one or more data pages labeled as
1
,
2
, and
3
, that are stored in a storage device
100
such as a direct access storage device (DASD). Typical copy programs accomplish this by creating an image copy of the data pages
1
,
2
,
3
in one or more buffers
105
and eventually writing out the image copy to another storage device
110
such as a magnetic tape, another DASD, and so forth.
During the copying sequence, the data set
102
represented by data pages
1
,
2
,
3
may be accessible to WRITE-type operations from another computer program such as the well-known IBM database program DB2 (not shown). The DB2 program may attempt, for example, to overwrite a data page 2 with data from a DB2 buffer pool
115
in parallel with the copying process. Under this sequence of events, if the updating takes place before the data page
2
is copied to the buffer
105
, the version of the data page
2
that is actually copied is not the same as the version that existed when the copy process was initiated. As a result, the data page
2
as copied to the buffer
105
may be internally inconsistent with associated data page
1
and/or data page
3
, possibly violating logical data consistency constraints (e.g., referential integrity constraints) within the data set.
SUMMARY OF THE INVENTION
A computer program operating in accordance with the invention monitors WRITE requests issued by a data-writing program that accesses data sets, e.g., for updating the data sets, and also monitors READ requests issued by a data-reading program that accesses data sets, e.g., for copying the data. While a series of READ requests (e.g., a data set copying operation) is in progress, the computer program monitors all WRITE requests issued for the relavent data set by the data-writing program. If the data-reading program has not yet read the data portion (e.g., a data page) that is to be updated, the computer program selectively creates a cached image of the data portion as it existed at the beginning of the series of READ requests and delivers the cached image to the data-reading program. Delivering the cached image reduces the potential for internal inconsistencies in the copied data. The computer program advantageously permits such caching to be performed without the need for specialized caching hardware.
REFERENCES:
patent: 5163148 (1992-11-01), Walls
patent: 5241670 (1993-08-01), Eastridge et al.
patent: 5375232 (1994-12-01), Legvold et al.
patent: 5487160 (1996-01-01), Bemis
patent: 5535381 (1996-07-01), Kopper
patent: 5546534 (1996-08-01), Malcolm
GNU source code for ‘cp’ file copying utility.
Ball Linda S.
Birk Carla C.
Cunningham William R.
Wikle Shawn B.
BMC Software Inc.
Ellis Kevin L.
Howrey Simon Arnold & White
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