Performance Tuning Guide - EMC Community Network

Measuring Performance
Troubleshooting high latency
Consider the case where a business transaction searches a large repository and TaskSpace is
configured to display 100 items per page. There can be so much data matching the search criteria that
it cannot all be cached in memory either by Oracle or by the storage array, in which case the query can
take 8 seconds to complete. Several factors can contribute to this situation.
• Many results per page amplifies the amount of processed data. The random nature of results
matching the search keyword indicates a low probability that Oracle or the storage array cashes
the results, especially if amount of data exceeds the available cache memory at either tier. In
the case being considered, the storage array is sized for 50,000 disk I/O per second, but is only
servicing about 800 physical disk I/Os in 8 seconds, so the storage array is not the problem.
• DQL select statements against custom object types generally execute as NESTED LOOPS with
TABLE ACCESS, the queries are not issued against an index.
• The Oracle execution plan is issuing 800 disk I/Os serially. Each I/O request runs against a 7200
RPM drive with 0.01-seconds service time per request. As a result, 800 physical disk I/Os issued
serially takes 800 * 0.01 = 8 seconds.
To improve the performance, do any or all of the following:
• Do less. Make the application retrieve less than 100 rows per page. Retrieving 50 rows per page
instead of 100 reduces the amount of data processed by 50% and reduces the service time by
the corresponding amount.
• Do something different. Create "covering indexes" and influence the execution plan to replace
TABLE ACCESS with INDEX ACCESS. Indexes are much smaller, packed tighter, more likely to
be cached, and can fit more easily into memory than a full table row. Getting data from an Oracle
buffer cache reduces service times by several orders of magnitude.
• Do it faster. Request more storage array cache and/or lower latency storage. If the Oracle data
storage used solid-state drives (SSD) with a latency of 0.001 seconds, the 8-second request takes
0.8 seconds.
Figure 27, page 71 illustrates the affect of latency on throughput.
70 EMC Documentum xCP 1.0 Performance Tuning Guide