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Additional Wait Events of Concern

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.


Most of the events reported in the dynamic performance views or in a STATSPACK report that show a high total time are actually normal. However, if monitored response times increase and the STATSPACK report shows a high proportion of wait time for cluster accesses, the cause of these waits needs to be determined. STATSPACK reports provide a breakdown of the wait events, with the five highest values sorted by percentages. The following specific RAC-related events should be monitored:

* global cache open s:  A block was selected.

* global cache open x:  A block was selected for IUD.

* global cache null to s:  A block was transferred for SELECT.

* global cache null to x:  A block was transferred for IUD.

* global cache cr request:  A block was requested transferred for consistent read purposes.

* Global Cache Service Utilization for Logical Reads.

The following sections will provide more information on these events to help show why they are important to monitor.

The global cache open s and global cache open x Events

The initial access of a particular data block by an instance generates these events. The duration of the wait should be short, and the completion of the wait is most likely followed by a read from disk. This wait is a result of the blocks that are being requested and not being cached in any instance in the cluster database.  This necessitates a disk read.

When these events are associated with high totals or high per-transaction wait times, it is likely that data blocks are not cached in the local instance and that the blocks cannot be obtained from another instance, which results in a disk read. At the same time, suboptimal buffer cache hit ratios may also be observed. Unfortunately, other than preloading heavily used tables into the buffer caches, there is little that can be done about this type of wait event.

The global cache null to s and global cache null to x Events

These events are generated by inter-instance block ping across the network. Inter-instance block ping is when two instances exchange the same block back and forth. Processes waiting for global cache null to s events are waiting for a block to be transferred from the instance that last changed it. When one instance repeatedly requests cached data blocks from the other RAC instances, these events consume a greater proportion of the total wait time. The only method for reducing these events is to reduce the number of rows per block to eliminate the need for block swapping between two instances in the RAC cluster.

The global cache cr request Event

This event is generated when an instance has requested a consistent read data block and the block to be transferred had not arrived at the requesting instance. Other than examining the cluster interconnects for possible problems, there is nothing that can be done about this event other than to modify objects to reduce the possibility of contention.

Global Cache Service Times

When global cache waits constitute a large proportion of the wait time, as listed on the first page of the STATSPACK or AWRRPT report, and if response time or throughput does not conform to service level requirements, the Global Cache Service workload characteristics on the cluster statistics page of the STATSPACK or AWRRPT reports should be examined. The STATSPACK or AWRRPT reports should be taken during heavy RAC workloads.

If the STATSPACK report shows that the average GCS time per request is high, it is the result of one of the following:

* Contention for blocks.

* System loads.

* Network issues.

The operating system logs and operating system statistics are used to indicate whether a network link is congested. A network link can be congested if:

* Packets are being routed through the public network instead of the private interconnect.

* The sizes of the run queues are increasing.

If CPU usage is maxed out and processes are queuing for the CPU, the priority of the GCS processes (LMSn) can be raised over other processes to lower GCS times. The load on the server can also be alleviated by reducing the number of processes on the database server, increasing capacity by adding CPUs to the server, or adding nodes to the cluster database.

 


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

http://www.rampant-books.com/book_2004_1_10g_grid.htm


 

 
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