gc current and gc cr - Oracle RAC

 

The terms gc current and gc cr refer to two distinct types of block-related wait events in Oracle RAC. 

They differ in their purpose, the type of block requested, and the operations involved.

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1. gc current (Global Cache Current)

• Purpose: Represents waits associated with current blocks, which are required for DML operations (e.g., INSERT, UPDATE, DELETE).
• Definition:
  • A current block is the latest version of a data block that can be modified by a DML operation.
  • When an instance requires a current block from another node, the block must be transferred from the owning node to the requesting node over the RAC interconnect.
• Process:
  • The master instance flushes redo changes for the block to disk.
  • A Past Image (PI) of the block is created for recovery purposes.
  • The current block is sent to the requesting instance.
• Common Wait Events:
  • gc current block 2-way (two-node transfer)
  • gc current block 3-way (three-node transfer)

Examples of Operations Triggering gc current:

• Updating a row in a block that resides in another instance.
• Deleting a row where the block is held by another instance.

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2. gc cr (Global Cache Consistent Read)

• Purpose: Represents waits associated with consistent read (CR) blocks, which are required for SELECT queries or operations needing a past version of a block.
• Definition:
  • A CR block is a reconstructed version of a data block, representing its state at a specific SCN (System Change Number) for read consistency.
  • The CR block is created using undo data if the current block has uncommitted changes or if the requesting instance needs a version of the block from the past.
• Process:
  • If the current block resides on another node, it is fetched.
  • Undo data is applied to reconstruct the CR version.
  • The CR block is sent to the requesting instance.
• Common Wait Events:
  • gc cr block 2-way (two-node transfer)
  • gc cr block 3-way (three-node transfer)

Examples of Operations Triggering gc cr:

• A SELECT query fetching a block modified by another instance but not yet committed.
• A SELECT query requiring read-consistent access to a block modified in the past.

Performance Considerations

1. High gc current Waits:

   • Indicates contention due to frequent inter-node DML operations.
   • Can be reduced by:
     • Application segregation (split DML and SELECT operations by node).
     • Partitioning tables to localize DML activity.
     • Optimizing LGWR performance.

2. High gc cr Waits:

• Indicates contention due to inter-node SELECT queries or read-consistency requirements.
• Can be reduced by:
• Query optimization to limit block access.
• Partitioning tables to localize SELECT activity.
• Improving interconnect performance.

To remember it clearly:

• CURRENT Block = UPDATE (you’re editing, making the data live and up-to-date).
• CR Block = SELECT (you’re just viewing, like taking a snapshot, without affecting the data).