1Z0-084 practice question 5 of 15

Which two statements are true about the use and monitoring of Buffer Cache Hit ratios and their value in tuning Database I/O performance?

Answer options

A. The performance of workloads that primarily generate full table scans and fast full index scans are always affected by the cache hit ratio.

B. A 99% cache hit ratio can be observed for database instances which have very poor I/O performance.

Correct answer

C. The buffer cache advisory view v$db_cache_advice provides advice on cache hit ratios appropriate for the instance workload.

Correct answer

D. Both the RECYCLE and KEEP buffer caches should always have a very high cache hit ratio.

E. A 60% cache hit ratio can be observed for database instances which have very good I/O performance.

Correct Answer:

B. B. A 99% cache hit ratio can be observed for database instances which have very poor I/O performance.; C. C. The buffer cache advisory view v$db_cache_advice provides advice on cache hit ratios appropriate for the instance workload.

Explanation

Buffer cache hit ratios can be misleading performance indicators. A 99% hit ratio may exist even with poor I/O performance if the workload repeatedly accesses cached data while actual disk I/O is slow (B). The V$DBCACHEADVICE view provides workload-specific guidance on cache sizing and expected hit ratios, making it a valuable tuning tool (C).

Why Incorrect

A: Full table scans typically use direct path reads, bypassing the buffer cache entirely. D: RECYCLE buffer pool is designed for low hit ratios to prevent cache pollution. E: A 60% hit ratio doesn't necessarily indicate poor performance; it depends on workload characteristics. ---

References

1. Oracle Database Performance Tuning Guide 19c, Section 10.2.3 "Interpreting Buffer Cache Hit Ratio" - Details on limitations of hit ratio metrics

2. Oracle Database Reference 19c, "V$DBCACHEADVICE" view documentation - Describes the cache advisory functionality

3. Oracle Database Administrator's Guide 19c, Section 13.2.4 "Configuring Multiple Buffer Pools" - Explains KEEP and RECYCLE pool purposes

4. Oracle Database Concepts 19c, Chapter 14 "Memory Architecture", Section "Buffer Cache" - Details on direct path reads and cache bypass

5. "Oracle Wait Interface: A Practical Guide to Performance Diagnostics & Tuning" (2004), ISBN: 978-0072224726, Chapter 5 - Discussion on hit ratio limitations

Keep practicing

Take the free 1Z0-084 practice test

Ten exam-style questions with answers and explanations, plus the exam facts and study guides.

Start the free practice test