Chapter 13 ■ tuning global CaChe
In this scenario, the following statistics are updated by Oracle. Considering this was a read-only block request, only the cr (consistent read) statistics are updated.
gc cr block send time: When the block is requested by another instance, the value represents the total time taken to send the blocks. The value is influenced by conditions such as locks and time taken to downgrade the lock, and so forth.
gc cr block read wait time: Represents the time taken waiting to read the consistent read block from buffer or into the buffer from disk.
10 × gc current block receive time ÷ gc current blocks received
Depending on the type of data block requests, the following wait events could also be updated:
As illustrated in Figure 13-5, there are two possibilities when the block is not found on the instance that is the master of the object (resource). Figure 13-5 is a four-node RAC cluster. All instances maintain masters for various objects represented by R1 thru R8. In our discussions, we refer to data blocks that are mastered on instance 4 (SSKY4).
2BAST(Blocking asynchronous system trap), a notification mechanism to release a block used by the global cache service.