11gR2 RAC
PI - Past image is a copy of a globally dirty block and is maintained in the database buffer cache.It can be createdand saved when dirty block is shipped across to another instance after setting the resource role and global(if it was not already set).
PI must be maintainde by an instance until it or a later version of block is written to disk.
GCS is responsible for informing an instance that its PI is no longer needed after another instance writes a newer version of the same block.
Please are discarded when GCS posts all the holding instances that a new and consistent version of that particular block is now on disk.
check logs and session logs into shows this type
st: PI md: NULL tch: 1 le: 0x7bfa6000 -- Past Image
PI - Past image is a copy of a globally dirty block and is maintained in the database buffer cache.It can be createdand saved when dirty block is shipped across to another instance after setting the resource role and global(if it was not already set).
PI must be maintainde by an instance until it or a later version of block is written to disk.
GCS is responsible for informing an instance that its PI is no longer needed after another instance writes a newer version of the same block.
Please are discarded when GCS posts all the holding instances that a new and consistent version of that particular block is now on disk.
check logs and session logs into shows this type
st: PI md: NULL tch: 1 le: 0x7bfa6000 -- Past Image
Note:- In simple Words It's Eleminates Write/Write contentions
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