gc buffer busy acquire vs gc buffer busy release

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.

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.

The oradebug command can be used to verify which network is being used for RAC traffic:
SQL> oradebug setmypid
SQL> oradebug ipc

From 11g onwards, this wait event is split in to ‘gc buffer busy acquire’ and ‘gc buffer busy release’. An attendee asked me to show the differentiation between these two wait events. Fortunately, we had a problem with LGWR writes and we were able to inspect the waits with much clarity during the class.

Remember that Global cache enqueues are considered to be owned by an instance. From 11g onwards, gc buffer busy event differentiated between two cases:

  1. If existing GC open request originated from the local instance, then current session will wait for ‘gc buffer busy acquire’. Essentially, current process is waiting for another process in the local instance to acquire GC lock, on behalf of the local instance. Once GC lock is acquired, current process can access that buffer without additional GC processing (if the lock is acquired in a compatible mode).
  2. If existing GC open request originated from a remote instance, then current session will wait for ‘gc buffer busy release’ event. In this case session is waiting for another remote session (hence another instance) to release the GC lock, so that local instance can acquire buffer.

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