Not true at all. Ratios are not useful as indicators of bad performance in there own right. Apart from anything else there are Re: Buffer hit ratio.
If all you want is a high cache hit ratio check out Connor MacDonald's site. He has a very helpful script which generates enough useless database activity so you set the buffer hit ratio to[b]to as high a value as you want.
However, it is a much better idea to isolate individual statements which are running too slowly and tune them.
The system is not performing badly but we were actually concerned why the buffer hit ration is 28% and also for one particular schema there is enormous amount of physical reads. Also I have found out that there are few missing indexes which will be built. APC's advice is really good but for that I must have some time and Now I cannot as I am in clients place.
Does it mean that low buffer hit ratio have some impact on the file system.
1 - Lots of re-read data blocks
2 - You have too small a cache to hold your working set of frequently accessed data blocks
The Data Buffer Hit Ratio Oracle metric is ONLY a measure of the effectiveness of the Oracle data block buffer. The higher the buffer hit ratio, the more frequently Oracle found a data block in memory and avoid a disk I/O.
With a super-tiny data buffer, increasing the db_cache_size will result in a large decrease in disk I/O. As you approach full caching, the marginal benefit declines greatly, Oracle 10g AMM predicts the optimal point for you, or you see the v$db_cache_advice utility output in any STATSPACK or AWR report.
Here are my notes.
I would run a STATSPACK report and look at the amount of "physical reads":
The subject says "performance issue - buffer cache hit ratio below 50% "
I had similar issue few month’s back, load on the Linux box was very high with lot of Disk IO. Noticed that db_cache_size was set to 40MB by previous DBA, upping the cache to 256M made the app work like a dream. To me it made sense to increase db_cache since db was 100GB and lot of select/updates are done.
Yes I agree it's not a general rule, but you need a decent SGA size for any application especially if your box has the memory.