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 Oracle RAC Concept of Past Image (PI)
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

The past image concept was introduced in the RAC version of Oracle 9i to maintain data integrity. In an Oracle database, a typical data block is not written to the disk immediately, even after it is dirtied. When the same dirty data block is requested by another instance for write or read purposes, an image of the block is created at the owning instance, and only that block is shipped to the requesting instance. This backup image of the block is called the past image (PI) and is kept in memory. In the event of failure, Oracle can reconstruct the current version of the block by reading PIs. It is also possible to have more than one past image in the memory depending on how many times the data block was requested in the dirty stage.


A past image copy of the data block is different from a CR block, which is needed for reconstructing a read-consistent image. A CR version of a block represents a consistent snapshot of the data at a point in time. It is constructed by applying information from the undo/rollback segments. The PI image copy helps the recovery process and aids in maintaining data integrity.


For example, suppose user A of Instance 1 has updated row 2 on block 5. Later, user B of Instance 2 intends to update row 6 on the same block 5. The GCS transfers block 5 from Instance A to Instance B. At this point, the past image (PI) for block 5 is created on Instance A.

Lock Modes

From the examination of resource roles, resource modes, and past images, the next step is to consider the possible resource access modes as shown in Table 2.2.


There are three characters that distinguish lock or block access modes. The first letter represents the lock mode, the second character represents the lock role, and the third character (a number) indicates any past images for the lock in the local instance.





Null Local and No past images


Shared Local with no past image


Exclusive Local with no past image


Null Global – Instance owns current block image


Global Shared Lock – Instance owns current image


Global Exclusive Lock – Instance own current image


Global Null – Instance owns the past image block


Shared Global – Instance owns past image


Global Exclusive Lock – Instance owns past image.

Table 2.2:  Lock Modes


When a block is brought into the local cache of an instance, it is acquired with the local role. But if a dirty buffer for the same data block is present in a remote instance, a past image is created in the remote instance before the data block is sent to the requesting instance’s cache. Therefore, the data block resource acquires a global role.


For recovery purposes, instances that have past images will keep those past images in their buffer cache until the master instance prompts the lock to release them. When the buffers are discarded, the instance holding the past image will write a block written redo (BWR) to the redo stream. The BWR indicates that the block has already been written to disk and is not needed for recovery by the instance. Buffers are discarded when the disk write is initiated on the master instance. The master instance is where the current status and position of the data block is maintained.


This has been a review of how a GCS resource maintains its access mode and its role. There is another feature called the buffer state, which is covered in the next section.


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