To maintain data integrity, a new concept of past image was introduced in the 9i version of RAC. A past image (PI) of a block is kept in memory before the block is sent and serves as an indication of whether it is a dirty block. In the event of failure, GCS can reconstruct the current version of the block by reading PIs. This PI is different from a CR block, which is needed to reconstruct read-consistent images. The CR version of a block represents a consistent snapshot of the data at a point in time.
For example, Transaction-A of Instance-A has updated row-2 on block-5, and later another Transaction-B of Instance-B has updated row-6 on the same block-5. Block-5 has been transferred from Instance-A to B. At this time, the past image (PI) for block-5 is created on Instance-A.
For example, Transaction-A of Instance-A has updated row-2 on block-5, and later another Transaction-B of Instance-B has updated row-6 on the same block-5. Block-5 has been transferred from Instance-A to B. At this time, the past image (PI) for block-5 is created on Instance-A.
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