buffer cache hit ratio. LIO vs PIO
I came across a post on AskTom that seemed to contradict everything I've been told or read about tuning the buffer cache hit ratio and reducing Physical IO by aiming to get all queries satisfied by reading the blocks in the buffers.
The article quotes a paper by Cary Millsap of Hotsos, and is titled "Why a 99%+ Database Buffer Cache Hit Ratio is NOT OK".
Whats your view on LIP vs PIO and exactly what should we look for when tuning memory?
Re: buffer cache hit ratio. LIO vs PIO
Quote:
Originally posted by JMac
I came across a post on AskTom that seemed to contradict everything I've been told or read about tuning the buffer cache hit ratio and reducing Physical IO by aiming to get all queries satisfied by reading the blocks in the buffers.
Whats your view on LIP vs PIO and exactly what should we look for when tuning memory?
Wass wrong in that link???????
wont reducing ur LIO reduce ur PIO (most often than not it will)..
U shud look for ur CONSISTENT/DB BLOCK gets and reduce as much as possible..
Re: buffer cache hit ratio. LIO vs PIO
Further thoughts - I don't want to pick on you particularly JMac, but your post is revealing:
Quote:
Originally posted by JMac
. . . tuning the buffer cache hit ratio . . . .
. . . . what should we look for when tuning memory?
I think this emphasises how easy it is to get away from the real objective. I don't want to do either of the above, I want to "maximise throughput" or "minimise response time" or something similar which relates to business efficiency. No user will congratulate you on keeping the hit ratio above 95%!
Frankly, I didn't realise 100% what I was doing when I looked at "I/O Work" in my previous. Almost by accident I stumbled upon a rule which says: if you are going to look at hit ratios or parse/execute ratios etc, you'd be advised to write down some kind of short-hand which relates them to your real objective.
(Climbs down off soap-box.)
Buffer cache hit ratio.....
When I was being taught Oracle 9i(OCP) our Instructor made the point that if buffer cache hit ratio should be at the acceptable level of 95%, and if it fell below 95% to add more ram(tuning). All but 1 article that I have read on the subject, and outside books(2) that I have said the same thing, "when buffer cache hit ratio falls below 95% throw more ram(tuning) at it until it rises back up to 95%.
I truely understand that some of you strongly suggest that we(newbies) RTM, but when we read the manual, and it says the same thing, I feel that I have been cornered into asking you folks for assistance.
As mentioned earlier in this post (All but 1 article that I have read on the subject) it only touched on design, it did not get real in depth about it.
I throw in with JMac on this one, what should a DBA tasked with buffer cache hit ratio look for in this matter?
Rick
Re: Buffer cache hit ratio.....
Quote:
Originally posted by Alchemy
When I was being taught Oracle 9i(OCP) our Instructor made the point that if buffer cache hit ratio should be at the acceptable level of 95%, and if it fell below 95% to add more ram(tuning). All but 1 article that I have read on the subject, and outside books(2) that I have said the same thing, "when buffer cache hit ratio falls below 95% throw more ram(tuning) at it until it rises back up to 95%.
I truely understand that some of you strongly suggest that we(newbies) RTM, but when we read the manual, and it says the same thing, I feel that I have been cornered into asking you folks for assistance.
As mentioned earlier in this post (All but 1 article that I have read on the subject) it only touched on design, it did not get real in depth about it.
I throw in with JMac on this one, what should a DBA tasked with buffer cache hit ratio look for in this matter?
Rick
You can use the cache advisor to indicate the effectiveness of allocating or deallocating RAM to buffer cache.
Long story short, the sources you quote were wrong. If you are tasked with improving buffer cache hit ratio, just run some badly designed sql over and over again, thus increasing LIO's and herdly touching PIO's. That tells you all you need to know about how meaningful the ratio is.