Browse Prior Art Database

Online RECOVERY of Duplexed Logical Volumes

IP.com Disclosure Number: IPCOM000044039D
Original Publication Date: 1984-Oct-01
Included in the Prior Art Database: 2005-Feb-05
Document File: 4 page(s) / 34K

Publishing Venue

IBM

Related People

Kropff, CJ: AUTHOR [+2]

Abstract

This article describes a method which permits the two copies of a duplexed logical volume to be resynchronized while the volume is being accessed by other programs. A significant problem for many computer users is the unavailability or total loss of their data due to hardware or software failure. To solve this problem a computer system can use duplexed copies of the logical volume on different devices. When a single failure occurs, one of the copies may become down level with respect to the other. Once the problem has been corrected, it is highly desirable to restore the second copy to service without interrupting the work being done on the system, even when that work involves the use of the volume itself.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 29% of the total text.

Page 1 of 4

Online RECOVERY of Duplexed Logical Volumes

This article describes a method which permits the two copies of a duplexed logical volume to be resynchronized while the volume is being accessed by other programs. A significant problem for many computer users is the unavailability or total loss of their data due to hardware or software failure. To solve this problem a computer system can use duplexed copies of the logical volume on different devices. When a single failure occurs, one of the copies may become down level with respect to the other. Once the problem has been corrected, it is highly desirable to restore the second copy to service without interrupting the work being done on the system, even when that work involves the use of the volume itself. In the Realtime Programming System for IBM Series/1 minicomputer 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 process for recovery of duplexed volumes does some analysis to determine which copy of the data is the good copy and then uses a combination of system and application software to restore the bad copy to the same level as the good. The recovery process begins by determining the status of each copy of the duplexed volume. This status is kept in bit flags in the system table of contents (SYSTOC) entry for each copy. If either copy indicates that a previous recovery operation has failed or if neither copy indicates that the other copy is down level, the recovery process terminates immediately. Otherwise, the recovery program marks the down level copy as recovery in progress and turns off the down level indicator. In order to guarantee proper access to the data stored in the internal TOCs imbedded within the volume (the volume TOC and any partitioned data set TOCs), the recovery program next locates and copies each internal TOC from the correct copy to the incorrect copy. This is done in supervisor state, holding various gates to prevent other tasks from updating the TOCs during the copy. The TOC entry for the good copy of the volume is marked to indicate that the other copy is no longer downlevel. This permits OPEN to construct all of the control blocks that it normally does for a duplexed volume OPEN. At this point both copies of the volume are opened when an OPEN is done against the volume or a data object within it, but the system will not read from the copy marked as recovery in progress. Once the TOCs have been copied, the recovery program locates all OPENs that have been done against the volume and the objects within it, and it ensures that the control block structure includes control blocks for both copies of the volume. If necessary, the program d...