pajama

Question

Your read cache rate (onstat -p) rises durning server use to ninety-nine percent. You suspect that you have overallocated buffers to the extent that you are wasting memory. How can you determine how much of your buffer cache memory is used?

Answer

Count the number of buffers in use at regular intervals and compare with the total allocated. You can use the following two methods to count the number of buffer pages in use at a given point in time:

• Execute the onstat -B command. The number of data lines in the output is the number of buffers in use. The onstat -B output includes all the configured buffer pools, grouped by page size. You will have to count the lines in the individual groups if you have more than one buffer pool.
  
  
  
• Open the sysmaster database and execute the following query:
  
  SELECT COUNT(*) AS pages_used
  FROM sysbufhdr
  WHERE bufsize = <buffer_page_size>
  AND bflags != 4;
  
  The buffer_page_size is the page size (in bytes) for the buffer pool you are examining. The buffer page size is listed in the onstat -b/B output. If you only have one buffer pool, you can leave out the buffer page size filter and the query becomes:
  
  SELECT COUNT(*) AS pages_used
  FROM sysbufhdr
  WHERE bflags != 4;