Buffer Hit 0 % User Experiencing Slowness
- Thread starter Mike
- Start date
TomBascom
Curmudgeon
There are only a few ways for buffer hits to be 0%
1) The db isn't doing anything. At all.
Users might think that is "slow". If they have actually asked it to do something then they are right. More likely the client side of the equation is unable to communicate with the db -- in that case the issue isn't that "the db is slow" but rather that something in the path between the db and the clients is not working.
2) You can also get fairly small hit ratios when you are writing a lot of data and not reading very much (although I can't say that I've ever seen *0* in that case).
3) Laughably ancient, obsolete, unsupported and criminally irresponsible to still be running releases such as 9.1d also suffered from bugs in the calculation of the hit ratio. If the db has been running for a while certain inputs to the hit ratio calculation will overflow and specious values will be displayed.
If this is your situation then the hit ratio is telling you nothing useful about why the system may, or may not, be "slow". In this case you are also almost certainly looking at the values since startup and over a very long period of time. Which is also basically useless. To get a useful perspective on why your system is slow at a particular time you need to _sample_ the metrics during the slow performance. In PROMON you use the "s" command (this is only available within the "R&D" sub menu).
You will need a lot more data than any single metric if you are going to do any serious performance analysis. (Or if you are going to ask other people to do that analysis for you.)
Also, FWIW "hit ratio" is not a very useful metric.
1) The db isn't doing anything. At all.
Users might think that is "slow". If they have actually asked it to do something then they are right. More likely the client side of the equation is unable to communicate with the db -- in that case the issue isn't that "the db is slow" but rather that something in the path between the db and the clients is not working.
2) You can also get fairly small hit ratios when you are writing a lot of data and not reading very much (although I can't say that I've ever seen *0* in that case).
3) Laughably ancient, obsolete, unsupported and criminally irresponsible to still be running releases such as 9.1d also suffered from bugs in the calculation of the hit ratio. If the db has been running for a while certain inputs to the hit ratio calculation will overflow and specious values will be displayed.
If this is your situation then the hit ratio is telling you nothing useful about why the system may, or may not, be "slow". In this case you are also almost certainly looking at the values since startup and over a very long period of time. Which is also basically useless. To get a useful perspective on why your system is slow at a particular time you need to _sample_ the metrics during the slow performance. In PROMON you use the "s" command (this is only available within the "R&D" sub menu).
You will need a lot more data than any single metric if you are going to do any serious performance analysis. (Or if you are going to ask other people to do that analysis for you.)
Also, FWIW "hit ratio" is not a very useful metric.
