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Data Protection Through Detection of Overwritten Control Blocks

IP.com Disclosure Number: IPCOM000062069D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Meacock, G: AUTHOR

Abstract

In a programming system such as CICS/VS user data may be lost if a system control block, the stored bit map, is overwritten. Improved data protection is achieved by segmenting the stored bit map where each segment contains a prefix, master information bytes, backup information bytes and a suffix, the master and backup bytes provide duplication of the bit map. Tests of prefix and suffix confirm overwrite has not occurred when compare equal results. Tests of master and backup bytes confirm random bits have not occurred when compared equal results. If tests indicate damage to a segment has occurred, this segment is inhibited whilst undamaged segments may be used. CICS/VS uses a bit map to record the status of control intervals on a VSAM data set.

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Data Protection Through Detection of Overwritten Control Blocks

In a programming system such as CICS/VS user data may be lost if a system control block, the stored bit map, is overwritten.

Improved data protection is achieved by segmenting the stored bit map where each segment contains a prefix, master information bytes, backup information bytes and a suffix, the master and backup bytes provide duplication of the bit map. Tests of prefix and suffix confirm overwrite has not occurred when compare equal results. Tests of master and backup bytes confirm random bits have not occurred when compared equal results. If tests indicate damage to a segment has occurred, this segment is inhibited whilst undamaged segments may be used. CICS/VS uses a bit map to record the status of control intervals on a VSAM data set. The nth bit in the map records the status of the nth control interval; it is set to '1'B when the control interval is allocated and to '0'B when the control interval is deallocated. Control intervals are initially deallocated. Once a control interval has been allocated, CICS/VS logic requires the control interval to be deallocated before it can be allocated again.

If the bit map is corrupted, then it becomes possible for control intervals (CIs) to be reallocated without having been deallocated.

The result is the loss, potentially very damaging, of user data. The bit map can be corrupted in one of two ways: 1.by the random movement of bytes over the bit map, and 2.by the random setting of bits within the bit map. The main problem for CICS/VS is to detect corruption of the bit map before any corruption of user data can occur. A subsidiary problem for CICS/VS is to continue using the data set without having to reconstruct the bit map. It should be noted that use of the dataset must be quiesced before the bit map can be repaired. It should also be noted that quiescing use of the data set could create a significant bottleneck within CICS/VS. The solution adopted for CICS/VS is as follows: 1. The bit map is divided into one or more segments, the first segment maintaining the status of CIs 0 to m-1, the second maintaining the status of CIs m to 2m-1, etc. 2. Each segment is mapped by...