Oracle database bypasses the buffer cache for particular operations, such as sorting and parallel reads. The top sql view shows all sql that is still in the cache. He specializes in creating database architectures for very large online databases and he has worked with some of the worlds most powerful and complex systems. For optimal performance, keep the oracle library cache reload ratio at zero and the library cache hit ratio greater than 95 percent. 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. Any single segment that accounts for a substantial portion more than 10% of. Home oracle how tos how to check the overall dictionary cache hit ratio navigation home. Automatic performance diagnosis and tuning in oracle.
Is the oracle buffer hit ratio a useless metric for monitoring and tuning. Db block gets the number of blocks accessed via single block gets not through the consistent get mechanism. How to check the library cache hit ratio dba to dba. Below is an example of a modern, generic cache hit ratio graph. Figure 2 the graph in figure 2 is expressing the same behavior, but is expressed for one location in this case the controller, the vertical axis is expressed in % cache hits vs. To be precise about the oracle 10g, 10g is oracles network calculating product group counting among other things a database management system dbms and. 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. Best settings for oracle library cache reload and library cache. Do not run the monitor until the instance has run long enough to have a representative workload. Consistent gets the number of accesses made to the block buffer to retrieve data in a consistent mode db blk gets the number of blocks accessed via single block gets i.
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. Similar target values were suggested for other ratios such as the rowcache hit ratio or the latch hit ratio. Oracle database performance tuning guide, 10g release 2 10. The bchr represents the percentage of lios in which the required block is already loaded in the buffer caches. I thought it might be worth briefly mentioning the topic here. Burleson is one of the worlds top oracle database experts with more than 25 years of fulltime dba experience. The optimizer uses these data to estimate the number of cached blocks for each index or table access. It is just a way to inflate your cache hit ratio, which seems to make people happy yeah. Automatic performance diagnosis performance of various components of the database are measured using different metrics.
A good result is, if the buffer hit ratio is not less than 95%. 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. Actually it is fluctuating sometimes it drops to 56%,65% and sometimes it looks fine above 95%. But solely dependence on higher cache ratio can not be true. The buffer cache hit ratio percent metric calculates the percentage of data pages sql server found in memory without reading the data from the disk subsystem. Faulty quotes 4 buffer cache hit ratio bchr charles.
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. A correctly tuned dictionary cache can significantly improve overall database performance. The top statement may not be executing currently, and thus not responsible for the current poor buffer cache hit ratio. Set pause on set pause press return to continue set pagesize 60 set linesize 300 select round 1 phy. So, to clear all your confusions on interview locations, interview questions we have given detail on our wisdomjobs site. Best settings for oracle library cache reload and library. A sort job that oracle performs entirely in memory is said to be an optimal sort.
The buffer hit ratio bhr indicates the current ratio of buffer cache hits to total requests, essentially the probability that a data block will. 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. Oracle 8i, 9i, 10g, and 11g red hat customer portal. The total io and cpu cost of a query will be the combined. Increasing the buffer cache until the ra tio reached 9095% was often suggested.
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. For example, the efficiency of the datablock buffer cache is expressed as a. Hi folks, we have noticed in our sap servers databuffer cache hit ratio is low. This is a bad thing, no matter your opinion on buffer cache hit ratio. Statspackawr report resources charles hoopers oracle notes.
Dss applications then the data buffer hit ratio is not meaningful, but that does not mean that the data buffer hit ratio. December 16, 2007 posted by richard foote in buffer cache hit ratio, oracle general, oracle myths, oracle opinion, richards musings. 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. Such ratios include the buffer cache hit ratio, the softparse ratio, and the latch hit ratio. The views expressed on this blog are those of the author and do not necessarily reflect the views of oracle or any other organization. According to my understanding of the oracle database, a negative value for this particular metric buffer cache hit ratio is impossible, and its presence indicates an anomaly somewhere in the stack. Here is a query to get your databases current hit ratio.
Explain plan and computing cache hit ratio in oracle. Per the oracle 10g r2 performance tuning guide and the. Probably it could be some oracle 10g bug which made this odd display of hit ratio information in data dictionary. Oracle database performance tuning guide oracle docs. One of our production database is having buffer cache hit raio as 15% from the last few days. 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. For many types of operations, oracle database uses the buffer cache to store data blocks read from disk. Understanding the nature of the cache hit ratio curve. The higher this value is the more efficiently the system is running.
Oracle 10g have a performance diagnosis and tuning solution that automatically identifies the rootcauses affecting total database throughput. The oraclebuffer cache probe computes the buffer cache hit ratio so as to optimize the system global area sga database buffer cache size. A low cache hit ratio does not imply that increasing the size of the cache would be beneficial for performance. Searching for a new job can be more difficult know a day even searching itself might become a big job for you. Such ratios include the buffer cache hit ratio, the softparse ratio, and the latch hit. A good cache hit ratio could wrongly indicate that the cache is adequately sized for the workload. The library cache miss ratio tells the remote dba whether or not to add space to the shared pool, and it represents the ratio of the sum of library cache reloads to the sum of pins. The buffer cache hit ratio calculates how often a requested block has been.
Buffer cache hit ratio percent sql diagnostic manager. 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. Buffer cache hit ratio metric represents the data block buffer cache efficiency, as measured by the percentage of times the data block requested by the query is in memory. 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. Effective use of the buffer cache can greatly reduce the io load on the database. Moreover, a high cache hit ratio may wrongly indicate that the buffer cache is adequately sized for the workload. Cache hit ratio gives very important information regarding the physical reads vs logical reads. Security awareness would you like your company to implement gamification into your security awareness program.
Here is an interesting history of recommendations for the buffer cache hit ratio bchr, as presented in user conferences for a particular erp platform. Tools and methods of databases optimization in oracle database. How to check the overall dictionary cache hit ratio dba. The author has made best efforts to validate the information provided here for the correctness. I am no way experienced in oracle tuning, but your tip appears quite strange to me. To interpret the buffer cache hit ratio, you should consider the following. Of course, if your database does not reread data frequently i. Systematic oracle tuning yet another database blog. Oracle database performance tuning guide, 10g release 1 10. This value is measured by reading the buffer cache hit ratio from sysperfinfo. Likewise, if the library cache hit ratio comes in less than 95 percent, then the.