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Logical VOLUME Duplexing for DATA Availability

IP.com Disclosure Number: IPCOM000044037D
Original Publication Date: 1984-Oct-01
Included in the Prior Art Database: 2005-Feb-05
Document File: 4 page(s) / 40K

Publishing Venue

IBM

Related People

Kropff, CJ: AUTHOR [+2]

Abstract

This article describes a method of increasing the availability of critical data in a data processing system. A significant problem for many computer users is the unavailability or total loss of their data due to hardware or software failures. By duplexing logical volumes, the incidence of this can be reduced to a very low level without the expensive replication of hardware which occurs in purely hardware-based duplexing systems. The IBM Series/1 is a minicomputer system designed to meet the needs of a wide variety of applications. The Realtime Programming System is a full function operating system for the IBM Series/1. In the Realtime Programming System each disk device known to the system is divided into logical volumes (volumes, for short), and each volume has an entry in the device table of contents (TOC).

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Logical VOLUME Duplexing for DATA Availability

This article describes a method of increasing the availability of critical data in a data processing system. A significant problem for many computer users is the unavailability or total loss of their data due to hardware or software failures. By duplexing logical volumes, the incidence of this can be reduced to a very low level without the expensive replication of hardware which occurs in purely hardware-based duplexing systems. The IBM Series/1 is a minicomputer system designed to meet the needs of a wide variety of applications. The Realtime Programming System is a full function operating system for the IBM Series/1. In the Realtime Programming System each disk device known to the system is divided into logical volumes (volumes, for short), and each volume has an entry in the device table of contents (TOC). Volumes may, in turn, contain partitioned data sets, consecutive data sets and random data sets. Standard disk data access in the Realtime Programming System is done to a volume or to some substructure of a volume. The Realtime Programming System data availability scheme uses two copies of a volume with the same name on different devices to provide data redundancy. Disks which are to be accessed by the Realtime Programming System have at a fixed location on them a disk (TOC) that describes where the data and the free space are on the disk. Each entry in the disk TOC represents a single logical volume that is processed as a unit. Each volume in turn has a TOC that is in exactly the same format as the device TOC. This TOC describes the data sets within the volume which may have any one of three different organizations -- random, consecutive or partitioned. Partitioned data sets, in turn, have a TOC and contain members that may be either random or consecutive. The duplexing scheme uses two different disk devices to hold identical copies of the duplexed volume. In order to permit the system to find both devices containing a volume using only a single device name, duplexing uses paired devices. Two disk devices are said to be paired when the device name of one is at most seven non-blank characters and the device name of the other is a "$" followed by the name of the first.

This is a purely syntactic way of linking the devices and permits the system to compute, as needed, the device name of the other device in the pair. On each of the paired devices, there is a complete copy of the duplexed volume. The two copies are identical in size and in the size of the TOC, and normally their contents are also identical. However, the two copies of the duplexed volume are not necessarily located at the same physical location on the two disks, and indeed the two disks used as paired devices need not have the same device geometry. Duplexed volumes are created using the ordinary system CREATE macro. In order to create a duplexed volume, DUPVOL=YES must be specified as a parameter to CREATE. When a dup...