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Administrator
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It is basically used in scenarios like the following;
instance A holds a block..
Instance B in the same RAC wants to modify that block.
Instance B receives that block and Instance A keeps a Past Image of the block.
So now suppose -> Instance B crashed while modifying the block. (before writing to disk)
In this case, ORacle is able to build that data block using Past Image.
However, once that block in question is written to disk, the past image won’t be neededed for a crash recovery.
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Locked
