To help ensure excellent performance, you want to keep your cache hit ratio in the. If free memory is close to zero and either the library cache hit ratio or the row cache. Therefore can i ignore this hit cache ratio altogether. Monitoring the data buffer efficiency is a critical oracle tuning task, and dont be fooled by beginners who claim that the data buffer hit ratio is meaningless simply because it does not apply in all cases. Best settings for oracle library cache reload and library. A low cache hit ratio does not imply that increasing the size of the cache would be beneficial for performance. For optimal performance, keep the oracle library cache reload ratio at zero and the library cache hit ratio greater than 95 percent. With regarding to the sizing or the row cache miss ratio, oracle recommends that it. The queries in the following listings help you investigate this area.
Jdbc connection url with extra connection settings to the jdbc driver, such as timeout. One of our production database is having buffer cache hit raio as 15% from the last few days. Set pause on set pause press return to continue set pagesize 60 set linesize 300 select round 1 phy. Buffer cache hit ratio show buffer cache hit ratio. Typically, a dba makes a first estimate for the cache size, then runs a representative workload on the instance, and examines the relevant statistics to see whether the cache is underconfigured or overconfigured. How to check the library cache hit ratio dba to dba. Here is a query to get your databases current hit ratio. December 16, 2007 posted by richard foote in buffer cache hit ratio, oracle general, oracle myths, oracle opinion, richards musings. If the queries seem to be well tuned, the size of the buffer cache also determines how often buffers need to be fetched from disk. As a percentage, this would be a cache hit ratio of 95. Avoid repeated scanning of frequently accessed data by performing the processing in. Cache hit ratio gives very important information regarding the physical reads vs logical reads. Many dbas do their best to get a 99% or better buffer cache hit ratio, but quickly discover that the performance of their database isnt improving as the hit ratio gets better. I ve read from metalink that the buffer cache hit ratio formula stated in oracle 8i tuning guide is wrong.
Moreover, a high cache hit ratio may wrongly indicate that the buffer cache is adequately sized for the workload. So from the above query i can see that hit ratio is hit ratio not enough to make this determination. The buffer cache hit ratio can be used to verify the physical io as predicted by. Home oracle how tos how to check the overall dictionary cache hit ratio navigation home. Oracles shared pool is made up of two main memory areas the library.
Caching occurs by default at the subrequest level, which results in multiple cache entries for some sql statements. Alternatively, jdbc provides a sql statement cache within the jdbc client using. This metric represents the library cache efficiency, as measured by the percentage of times the fully parsed or compiled representation of plsql blocks and sql statements are already in memory. Library cache hit ratio oracle database sql scripts. The query cache is not shared across clustered servers except ibots. The hit ratio determines if oracle is performing more physical reads to retrieve the data. Cache hit ratio is a metric that applies to any cache. If a negative value is not impossible, i would like to understand how that could be. If the information is here the data is sent to the user or modified in function of the user request. If the server process can find out the required block from the sga, then that is called a memory hit.
Best settings for oracle data dictionary cache hit ratio. It contains articles and scripts about oracle database it contains articles and scripts about oracle database. Is the oracle buffer hit ratio a useless metric for monitoring and tuning and optimal data buffer cache hit ratio tips. High buffer cache hit ratio does not necessarily mean that your database is doing right, and low buffer cache hit ratio does not necessarily mean that your database is doing wrong. Library cache hit ratio when configuring a brand new instance, it is impossible to know the correct size to make the shared pool cache. If free memory is close to zero and either the library cache hit ratio is less than 0. The proportion of requests for a lock on an object. If the amount of free memory in the shared and java pools is relatively high, no ora04031 errors are occurring, and the library cache hit ratio is low, then poor code sharing is probably occurring. A low buffer cache hit ratio means that the server must often go to disk to retrieve the buffers required to satisfy a query. The sizes range from 10% of the current shared pool size or the amount of pinned. I know that the optimal buffer cache hit ratio in oracle7 was 90% for oltp databases and the sap documentation also mentions to keep you data buffer cache hit ratio optimized at 95% or above. Script to measure the buffer cache hit ratio blogger.
This is a bad thing, no matter your opinion on buffer cache hit ratio. The buffer cache hit ratio calculates how often a requested block has been found. The bchr represents the percentage of lios in which the required block is already loaded in the buffer cache s. Oracle 9i 10g 11g12c dba rac and dataguard and grid administration. Another question, if have aix with oracle db and some 100 gb of ram, where numperm oscilates between 45 65. Buffer cache, library cache, dictionary cache hit ratio when a user submits a query to the oracle session, oracle will parse the query and check out for the result set blocks in the memory sga. If your buffer cache hit ratio is lower than 90%, you should consider adding more ram, if possible. If the information is not present in this area in the buffer cache the information is read from the disk and put in this memory zone in the buffer cache. Read hit ratio below 90 are usually a sign of poor indexing but distortion of hit ratio number is possible 6. Now there can be so many reasons for this low buffer cache hit ratio as according to the manual this may be because of 1 may be because of full table scans.
Large databases may require a shared pool of at least 10 mb. Even as the year 2009 draws to a close there are oracle forum posts asking for help in improving the buffer cache hit ratio bchr. Dss applications then the data buffer hit ratio is not meaningful, but that does not mean that the data buffer hit ratio. Tuning the shared pool and the large pool oracle docs. When an instance is started, the oracle caches are empty and the cache hit rate is low, so running the monitor at this time returns misleading results.
Repeated scanning of the same large table or index can. The top statement may not be executing currently, and thus not responsible for the current poor buffer cache hit ratio. If there is no single optimal data buffer hit ratio, then what rules apply to the optimization of the data buffer hit ratio and the size of the data buffers. Of course, if your database does not reread data frequently i. In the name of allah, most compassionate, most merciful. Since the days of oracle7 when oracle corporation recommended keeping the buffer cache hit ratio above a fixed threshold, research has show some important facts. Probably it could be some oracle 10g bug which made this odd display of hit ratio information in data dictionary. Is the oracle buffer hit ratio a useless metric for monitoring and tuning. Now there can be so many reasons for this low buffer cache hit ratio. If the data being queried is found in the buffer cache, the hit ratio will be higher than if the data is not cached a oracle has to retrieve it from disk. The buffer cache hit ratio oracle metric monitors the rate at which oracle finds the data blocks it needs in memory over the lifetime of an instance. Here is an interesting history of recommendations for the buffer cache hit ratio bchr, as presented in user conferences for a particular erp platform.
The top sql view shows all sql that is still in the cache. Faulty quotes 4 buffer cache hit ratio bchr charles. I thought it might be worth briefly mentioning the topic here. The cache hit ratio can also be expressed as a percentage by multiplying this result by 100. Caching subrequests improves performance and the cache hit ratio, especially. Finding cache hit ratios history oraclenext solution. The question of whether the buffer cache hit ratio bchr is a useful metric or not is one of those discussions in oracle that seems to crop up on a regular basis. Oracle apps 11ir12 oracle 10gasweblogic server 11g oracle 9i, 10g,11g oracle apex. How to check the overall dictionary cache hit ratio dba. Another great blog to share the knowledge of my friend cyrille, ocp 12c oracle dba based in montreal.
Another aspect in our situation is that we have mostly oracle db running on servers and i read that they have own caching and filesystem cache is not important for them. Articles oracle 8i oracle 9i oracle 10g oracle 11g oracle 12c oracle c oracle 18c oracle 19c miscellaneous plsql sql oracle rac oracle apps weblogic linux mysql. Do not run the monitor until the instance has run long enough to have a representative workload. If you assume that a disk today can fulfill a pio call in 10 ms, then the pio capacity. A good cache hit ratio could wrongly indicate that the cache is adequately sized for the workload. To interpret the buffer cache hit ratio, consider the following factors. It is just a way to inflate your cache hit ratio, which seems to make people happy yeah. Ask tom library cache latch contention oracle ask tom. You can check historical values and trends of your cache hit ratios using following combination of queries in 10g and above databases. But solely dependence on higher cache ratio can not be true. Any single segment that accounts for a substantial portion more than 10% of. To interpret the buffer cache hit ratio, you should consider the following.
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