What are the differences in Oracle RAC wait events?
Oracle RAC is somewhat of a unique case of an Oracle environment, but everything learned about wait events in the single instance database also applies to clustered databases. However, the special use of a global buffer cache in RAC makes it imperative to monitor inter-instance communication via the cluster-specific wait events such as gc cr request and gc buffer busy. Understanding these wait events will help in the diagnosis of problems and pinpointing solutions in a RAC database.
Focus on the buffer cache and its operations
The main difference to keep in mind when monitoring a RAC database versus a single-instance database is the buffer cache and its operation. In a RAC environment, the buffer cache is global across all instances in the cluster and hence the processing differs. When a process in a RAC database needs to modify or read data, Oracle will first check to see if it already exists in the local buffer cache. If the data is not in the local buffer cache the global buffer cache will be reviewed to see if another instance already has it in their buffer cache. In this case the remote instance will send the data to the local instance via the high-speed interconnect, thus avoiding a disk read.
Monitoring an Oracle RAC database often means monitoring this situation and the amount of requests going back and forth over the RAC interconnect. The most common wait events related to this are gc cr request and gc buffer busy (note that in Oracle RAC 9i and earlier these wait events were known as “global cache cr request”and “global cache buffer busy” wait events).
gc cr request
The gc cr request wait event specifies the time it takes to retrieve the data from the remote cache. In Oracle 9i and prior, gc cr request was known as global cache cr request. High wait times for this wait event often are because of:
RAC Traffic Using Slow Connection – typically RAC traffic should use a high-speed interconnect to transfer data between instances, however, sometimes Oracle may not pick the correct connection and instead route traffic over the slower public network. This will significantly increase the amount of wait time for the gc cr request event. The oradebug command can be used to verify which network is being used for RAC traffic:
SQL> oradebug setmypid SQL> oradebug ipc
This will dump a trace file to the location specified by the user_dump_dest Oracle parameter containing information about the network and protocols being used for the RAC interconnect.
Inefficient Queries – poorly tuned queries will increase the amount of data blocks requested by an Oracle session. The more blocks requested typically means the more often a block will need to be read from a remote instance via the interconnect.
gc buffer busy acquire and gc buffer busy release
The gc buffer busy acquire and gc buffer busy release wait events specify the time the remote instance locally spends accessing the requested data block. In Oracle 11g you will see gc buffer busy acquire wait event when the global cache open request originated from the local instance and gc buffer busy release when the open request originated from a remote instance. In Oracle 10g these two wait events were represented in a single gc buffer busy wait, and in Oracle 9i and prior the “gc” was spelled out as “global cache” in the global cache buffer busy wait event. These wait events are all very similar to the buffer busy wait events in a single-instance database and are often the result of:
Hot Blocks – multiple sessions may be requesting a block that is either not in buffer cache or is in an incompatible mode. Deleting some of the hot rows and re-inserting them back into the table may alleviate the problem. Most of the time the rows will be placed into a different block and reduce contention on the block. The DBA may also need to adjust the pctfree and/or pctused parameters for the table to ensure the rows are placed into a different block.
Inefficient Queries – as with the gc cr request wait event, the more blocks requested from the buffer cache the more likelihood of a session having to wait for other sessions. Tuning queries to access fewer blocks will often result in less contention for the same block.