What is th optimal Database Buffer size?

Good evening SAP/Oracle experts,

I am interested to know how many of you have chosen to size your Oracle data buffer (db_cache_size). There seems to be two trains of thought about this

- one that says size the data buffer as large as you can because buffer accesses are 1000s of times quicker than disk accesses

- another that says that "excessive buffer visits are the morbid obesity of the database. Extra logical I/O can degrade the perfromance of virtually every subsystem in an Oracle application" (quote Cary Millsap)

As I tend to prefer the second argument, I originally sized the data buffer on our ECC5 Production system (with CI/DB running on a server with 8Gb RAM) at 500Mb. A few months after go-live, I increased the buffer to 750Mb and recently I increased it again to 1Gb. I found a slight performance improvement with each increase, but database time remained at around 50% of total response time (from ST03N) throughout. Our average response time is currently around 700-750ms, with database time around 300-400ms. Our database is currently around 450Gb.

I therefore ask you, the SDN community, to tell me how large you tend to size your data buffer in comparison to the total amount of RAM on your database server, and what % of your average response time is spent down at the database.

Thanks in advance for any help you can provide.

Arwel Owen

SAP Infrastructure Manager,

Princes Ltd.