Second log processing

IMS DEDB Fast Recovery performs the second log processing to reconstruct the latest image of the data CI that needs recovery.

In the second log processing, to minimize log reading, IMS DEDB Fast Recovery starts reading with the log block that has the oldest log record that contains the data CI update image to be recovered.

Subsections:

Reading the log record with the update image of a data CI

IMS DEDB Fast Recovery again reads the log record that contains the update image of a data CI, which is read while reconstructing the data CI update status in the first log processing step.

Related reading: For information about the data CI update status reconstruction, see Reconstructing the data CI update status.

Opening the related areas

IMS DEDB Fast Recovery dynamically allocates and opens the areas related to the log records that contain the update image of the data CI. The block-level data-sharing VSO areas are already opened while connecting to the block-level data-sharing VSO areas in the previous step.

Related reading: For information about making connection to the block-level data-sharing VSO areas, see Connection to block-level data-sharing VSO areas.

Reading the data CI from DEDB

If the CI that might be recovered is for a block-level data-sharing area, or if the log records do not have an entire image of the data CI, IMS DEDB Fast Recovery reads the CI from the related area into the I/O buffer.

Checking the read CI

IMS DEDB Fast Recovery checks if the CI read has been damaged by write-padding caused by a write error when IMS failed because of a hardware (channel) problem. For multiple area data sets, the buffer can be reconstructed from the normal CI in another area data set even if the CI has been damaged. If the damaged portion can be repaired by the update log records, the buffer can be reconstructed even if the DEDB does not constitute multiple area data sets. In other cases, IMS DEDB Fast Recovery marks the area as recovery needed. IMS DEDB Fast Recovery does not check the pointers in the segment prefix and segment data.

Checking the CUSN

For the block-level data-sharing areas, IMS DEDB Fast Recovery compares the CUSN of the read CI and the latest CUSN in the related log record for the CI that was saved and updated while reconstructing the data CI update status in the first log processing step. IMS DEDB Fast Recovery checks if the CI has been already updated up to the latest image as follows:
  • If (the latest CUSN in log record) = (CUSN in the area CI) + 1, the area CI needs recovery.
  • If (the latest CUSN in log record) = (CUSN in the area CI), the area CI does not need recovery.
  • If (the latest CUSN in log record) < (CUSN in the area CI), the area CI is assumed to was updated by another IMS subsystem in the data sharing environment. Therefore, the area CI does not need recovery.
Related reading: For information about the data CI update status reconstruction, see Reconstructing the data CI update status.

Reconstructing the latest image of the data CI

IMS DEDB Fast Recovery moves the read data CI from I/O buffer to the buffer in data space, and then overlaps the update image of the log records to reconstruct the latest image.