Wait-class “Cluster” indicates cluster-related waits related to Cache Fusion:
SQL> select wait_class from v$system_wait_class;
WAIT_CLASS
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Configuration
Administration
Concurrency
Commit
Idle
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User I/O
System I/O
Scheduler
RAC WaitEvent Naming Conventions
- Wait Events that start with “GCS%” and “gc%” are Cache Fusion-related waits.
In other word, they have to do with waits experienced in shipping current or consistent-read versions of blocks across instances in a RAC cluster
- Events that start with “ges%’ are related to Global Enqueue Services gc current/cr multiblock request is only a placeholder , until the block is obtained Block provided Immediate from global Cache
- “gc current block 2-way” - Indicates that a current block was sent from either the resource master requiring 1 message and 1 transfer
- “gc current block 3-way” – Indicates that a current block was forwarded to a 3rd node from which it was sent, requiring 2 messages and one block transfer.
- “gc cr block 2-way” - Indicates that a cr block was sent from either the resource master requiring 1 message and 1 transfer
- “gc cr block 3-way” -Indicates that a current block was forwarded to a 3rd node from which it was sent, requiring 2 messages and one block transfer
Grant for Disk I/O RAC Waits
- “gc current grant 2-way” – Indicates that no current block was received because it was not cached in any instance. Instead a global grant was given, enabling the requesting instance to read the block from disk.
- “gc cr grant 2-way” - – Indicates that no current block was received because it was not cached in any instance. Instead a global grant was given, enabling the requesting instance to read the block from disk and build a consistent read image for the block.
Block Contention RAC Waits
- gc current block busy” – Indicates a local instance made a request for a current version of a block and did not immediately receive the block. This indicates that the block shipping was delayed on the remote instance. Could be on account of block contention.
A buffer may also be busy locally when a session has already initiated a cache fusion operation and is waiting for its completion when another session on the same node is trying to read or modify the same data. High service times for blocks exchanged in the global cache may exacerbate the contention, which can be caused by frequent concurrent read and write accesses to the same data.
- “gc cr block busy” – Indicates a local instance made a request for a CR version of a block and did not immediately receive the block
- gc current grant busy” – Indicates a local instance made a request for a current version of a block and did not immediately receive the grant.
This can occur because of multiple current block images / blocks being formatted on account of HWM extension.
- “gc buffer busy acquire” – Indicates that the local instance cannot grant access to data in the local buffer cache because a global operation on the buffer is pending and not yet completed
- “gc buffer busy release” – Indicates that a remote instance is waiting to complete access on a block to prep for shipping to a remote instance.
- “gc buffer busy” events mean that there is block contention that is resulting from multiple local requests for the same block, and Oracle must queue these requests.
- "gc current/cr failure/retry" signifies a hardware or a network problem.
High Load wait Events
- “gc current block congested”
- “gc cr block congested”
The load wait events indicate that a delay in processing has occurred in the GCS, which is usually caused by high load, CPU saturation,paging and would have to be solved by additional CPUs, load-balancing, off loading processing to different times or a new cluster node.
AWR Reports
Check the values for:
Average global cache cr block recieve time (ms)
Average global cache current block recieve time (ms)
SQL> select wait_class from v$system_wait_class;
WAIT_CLASS
----------------------------------------------------------------
Other
Configuration
Administration
Concurrency
Commit
Idle
Network
User I/O
System I/O
Scheduler
RAC WaitEvent Naming Conventions
- Wait Events that start with “GCS%” and “gc%” are Cache Fusion-related waits.
In other word, they have to do with waits experienced in shipping current or consistent-read versions of blocks across instances in a RAC cluster
- Events that start with “ges%’ are related to Global Enqueue Services gc current/cr multiblock request is only a placeholder , until the block is obtained Block provided Immediate from global Cache
- “gc current block 2-way” - Indicates that a current block was sent from either the resource master requiring 1 message and 1 transfer
- “gc current block 3-way” – Indicates that a current block was forwarded to a 3rd node from which it was sent, requiring 2 messages and one block transfer.
- “gc cr block 2-way” - Indicates that a cr block was sent from either the resource master requiring 1 message and 1 transfer
- “gc cr block 3-way” -Indicates that a current block was forwarded to a 3rd node from which it was sent, requiring 2 messages and one block transfer
Grant for Disk I/O RAC Waits
- “gc current grant 2-way” – Indicates that no current block was received because it was not cached in any instance. Instead a global grant was given, enabling the requesting instance to read the block from disk.
- “gc cr grant 2-way” - – Indicates that no current block was received because it was not cached in any instance. Instead a global grant was given, enabling the requesting instance to read the block from disk and build a consistent read image for the block.
Block Contention RAC Waits
- gc current block busy” – Indicates a local instance made a request for a current version of a block and did not immediately receive the block. This indicates that the block shipping was delayed on the remote instance. Could be on account of block contention.
A buffer may also be busy locally when a session has already initiated a cache fusion operation and is waiting for its completion when another session on the same node is trying to read or modify the same data. High service times for blocks exchanged in the global cache may exacerbate the contention, which can be caused by frequent concurrent read and write accesses to the same data.
- “gc cr block busy” – Indicates a local instance made a request for a CR version of a block and did not immediately receive the block
- gc current grant busy” – Indicates a local instance made a request for a current version of a block and did not immediately receive the grant.
This can occur because of multiple current block images / blocks being formatted on account of HWM extension.
- “gc buffer busy acquire” – Indicates that the local instance cannot grant access to data in the local buffer cache because a global operation on the buffer is pending and not yet completed
- “gc buffer busy release” – Indicates that a remote instance is waiting to complete access on a block to prep for shipping to a remote instance.
- “gc buffer busy” events mean that there is block contention that is resulting from multiple local requests for the same block, and Oracle must queue these requests.
- "gc current/cr failure/retry" signifies a hardware or a network problem.
High Load wait Events
- “gc current block congested”
- “gc cr block congested”
The load wait events indicate that a delay in processing has occurred in the GCS, which is usually caused by high load, CPU saturation,paging and would have to be solved by additional CPUs, load-balancing, off loading processing to different times or a new cluster node.
AWR Reports
Check the values for:
Average global cache cr block recieve time (ms)
Average global cache current block recieve time (ms)
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