Browse Prior Art Database

Check Point Copy through Address Space Mapping

IP.com Disclosure Number: IPCOM000115828D
Original Publication Date: 1995-Jun-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 126K

Publishing Venue

IBM

Related People

Ouchi, NK: AUTHOR

Abstract

Disclosed is a mechanism for a point in time copy, a Check Point Copy, where the address space of the Check Point Copy appears as the address range of another real DASD file. This provides addressability to the Check Point Copy and the real DASD space to create the image of data elements before alteration. The concept of a Cursor was disclosed to provide a mechanism to reduce the amount of interesting Active Space as the Check Point Copy is read onto another medium. The concept of a Window was disclosed to provide a smaller address space to access the Check Point Space and a smaller amount of real DASD space for copying the data before it is changed.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 51% of the total text.

Check Point Copy through Address Space Mapping

      Disclosed is a mechanism for a point in time copy, a Check
Point Copy, where the address space of the Check Point Copy appears
as the address range of another real DASD file.  This provides
addressability to the Check Point Copy and the real DASD space to
create the image of data elements before alteration.  The concept of
a Cursor was disclosed to provide a mechanism to reduce the amount of
interesting Active Space as the Check Point Copy is read onto another
medium.  The concept of a Window was disclosed to provide a smaller
address space to access the Check Point Space and a smaller amount of
real DASD space for copying the data before it is changed.

      Disclosed is a mechanism to facilitate "point in time" copy of
a data set for purposes of data recovery in the event of loss of the
active data set.  The point in time copy and associated journal of
transaction are used to recreate the lost data set.  The mechanism
provides that the point in time copy of a data set be addressed as
another data set.  Data are not copied unless changed and are access
through a mapping of address space.  An image of data that are
changed is copied into the point in time copy data set.  Disclosed
are means to minimize the real DASD space.

      Performance of point in time copy or check point copy has
become important since many systems run 24 hours a day.  There have
been several published disclosures that teach that in a storage
system that permits a level of indirection such as virtual
addressing, it is possible to create a copy of data very rapidly by
copying the pointers of one data space into another.  This disclosure
describes a mechanism where the check point copy of a data space
appears in the address space of another data space.  This differs
from a pointer based storage system in that all of the data space of
a "real" DASD system has real storage associated with each
addressable unit.  Most pointer based systems create a virtual data
space and need not consume addressing to real DASD.  However, this
type of storage system is complex in that the real space can be
over-committed, that is there may be more virtual space than real
space, and the management of storage can be complex.  In the
disclosed mechanism, the space management is simplified by using real
space.  That is, both the Active Space and Check Point Space consume
real DASD and DASD address space: cylinders, heads, records.  To aid
in conserving DASD space while keeping storage management simple, the
concepts of CURSOR and WINDOW are disclosed.

      Since the check point copy is usually read and copied to
another device such as tape, the copy is read sequentially.  A CURSO...