Low database cache hit ratio (85%)

Hi Guys,
I understand that high db cache hit ratio doesn't indicates that the database is healthy.
The database might be doing additional "physical" reads due to un-tuned SQL.
However, can explain why a low cache hit ratio might not indicate the db is unhealthy, as in the db need additional memory allocated?
What i can think of is probably:
1. the database might query different data most of the time. As such the data is not read again from cache before it aged out. Even if i add additional memory, the data might not be read again (from memory).
2. ?
3. ?
I'm quite reluctant to list out the databases with below 90% hit ratio as part of the monthly report to the management. To them, below 90% means unhealthy.
If these ratios to be used in monthly report, there will be a long section to explain why these ratios cannot meet but there is no performance concern.
As such will need your expert advise on this.
thanks
Edited by: Chewy on Mar 13, 2012 1:23 AM

Nikolay Savvinov wrote:
In isolation, ratios are useless, but trends in ratios can point to potential problem. If your BCHR is steadily degrading over time, this is something to worry about (you'll have to examine your application for scalability issues)
I used to think that there was a case for trending through a ratio in the days when databases were small, simple and (by modern standards) not very busy. But I'm no longer sure it was even a good idea then. How much of a change do you need to see before you start worrying - and what time-granularity would you take as your baseline. When a ratio varies between 98% and 99% during daylight hours, how do you spot a very large problem that's only going make a change of 0.01% over the course of a couple of weeks ?
I really don't think there's any good SIMPLE way of producing a management sound-bite for every database in the system; each database needs a personal touch, and the number of figures you need to supply on each is not going to be easy to grasp without some graphic assistance. A suggestion I have is simply to pick three "representative" queries from the application (one "small", one "medium" and one "large") and run them once every hour, capturing the plan_hash_value, elapsed time, disk-reads, buffer gets, and CPU for each. A daily graph - 4 lines each - of each query will give management the big picture of variation in response time; a longer term graph based on the daily average with (say) best and worst excluded will give a trend warning. Obvously each database (or even application within database) needs its own three queries, and there may be periods during the day when it is not relevant to worry about a particular query.
(NB In the past I've run baseline queries from a pl/sql package called by dbms_job, or dbms_scheduler, and stored the resulting cost figures in the database - capturing all the session stats, wait event and time model information)
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
Author: <b><em>Oracle Core</em></b>

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    >
    quote:
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    >
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    >
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    >
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    "SELECT STATEMENT"     "ALL_ROWS"     "2563"     "586110"     "15238860"     ""     ""     ""     ""     ""
    "UNION-ALL"     ""     ""     ""     ""     ""     ""     ""     ""     ""
    "TABLE ACCESS(FULL) SONO99.PROFILE"     ""     "512"     "117222"     "3047772"     ""     ""     ""     ""     ""
    "TABLE ACCESS(FULL) SONO99.PROFILE"     ""     "513"     "117222"     "3047772"     ""     ""     ""     ""     ""
    "TABLE ACCESS(FULL) SONO99.PROFILE"     ""     "513"     "117222"     "3047772"     ""     ""     ""     ""     ""
    "TABLE ACCESS(FULL) SONO99.PROFILE"     ""     "513"     "117222"     "3047772"     ""     ""     ""     ""     ""
    "TABLE ACCESS(FULL) SONO99.PROFILE"     ""     "513"     "117222"     "3047772"     ""     ""     ""     ""     ""
    This query plan was taken from sql developer, exported to txt, and the PROFILE table here has only 100k tuples.
    Right now I am more concerned with testing the MODEL query. Which Oracle doesn't seem to be able any more... but that is a matter for another thread.
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    >
    My best regards,
    Nuno (99sono xp).

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    My Oracle 10.2.0.4.0 database starts off in the day with a buffer cache hit ratio of almost 100% but this drops gradually in the course of the day as the system gets busier. Is this something I need to be concerned about if no performance problem has been reported by the users?
    I would have thought this should be a normal situation, i.e. as the system gets busier, I would expect that less number of calls to the buffer cache will be satisfied because many more calls are using up the memory?
    Please note that I've done some reading up on this but would like some suggestions from more experienced people than myself on what I should normally expect and what should or should not be a concern.
    thanks

    user8869798 wrote:
    hi,
    thanks again for your response and apologies for not being able to get back yesterday.No problem :) .
    >
    - What i am doing and how - it is the backend database r our Finance application, many users with various transactions.Okay , that sounds like a "normal" database with normal workload.
    - configuring buffer cache size - I haven't done anything manually yet, it's all been as installed, this is what I'm trying to figure out whether it is something I should be looking into doing simply because of the dropping hit ratio and not because of any reported performance problem.If you don't have much of the knowledge, its the best to take use of the advisories which would tell you in a better and in a graphical way that whether you should or shouldn't be worries. Look at the view v$db_cache_advice which can suggest to you that whether you would need to tweak the buffer cache or not.
    >
    - looking at the queries - What is the easiest way of doing this in an environment where many users are running different queries? What's the easiest way to identify queries that we may need to have a closer look at?Easiest way? Well, let the users come back to you ;-) .
    >
    - Basically, I'm just trying to ascertain whether or not I need to be concerned that my hit ratio drops below 89% even though no performance problem has been reported. If it is something that I should look into, then what is the best way to go about it?
    I believe that's answered by couple of us already, nope.
    Aman....

  • Buffer Cache hit Ratio

    Hi All,
    My DB Version: 10.2.0
    OS: Windows Server 2003
    I run the following script to get Hit ratio's
    SELECT cur.inst_id, 'Buffer Cache Hit Ratio ' "Ratio", to_char(ROUND((1-(phy.value / (cur.value + con.value)))*100,2)) "Value"
    FROM gv$sysstat cur, gv$sysstat con, gv$sysstat phy
    WHERE cur.name = 'db block gets'
    AND con.name = 'consistent gets'
    AND phy.name = 'physical reads'
    and phy.inst_id=1
    and cur.inst_id=1
    and con.inst_id=1
    union all
    SELECT cur.inst_id,'Buffer Cache Hit Ratio ' "Ratio", to_char(ROUND((1-(phy.value / (cur.value + con.value)))*100,2)) "Buffer Cache Hit Ratio"
    FROM gv$sysstat cur, gv$sysstat con, gv$sysstat phy
    WHERE cur.name = 'db block gets'
    AND con.name = 'consistent gets'
    AND phy.name = 'physical reads'
    and phy.inst_id=2
    and cur.inst_id=2
    and con.inst_id=2
    union
    SELECT inst_id, 'Library Cache Hit Ratio ' "Ratio", to_char(Round(sum(pins) / (sum(pins)+sum(reloads)) * 100,2)) "Library Cache Hit Ratio"
    FROM gv$librarycache group by inst_id
    union
    SELECT inst_id,'Dictionary Cache Hit Ratio ' "Ratio", to_char(ROUND ((1 - (SUM (getmisses) / SUM (gets))) * 100, 2)) "Percentage"
    FROM gv$rowcache group by inst_id
    union
    Select inst_id, 'Get Hit Ratio ' "Ratio",to_char(round((sum(GETHITRATIO))*100,2)) "Get Hit"--, round((sum(PINHITRATIO))*100,2)"Pin Hit"
    FROM GV$librarycache
    where namespace in ('SQL AREA')
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    union
    Select inst_id, 'Pin Hit Ratio ' "Ratio", to_char(round((sum(PINHITRATIO))*100,2))"Pin Hit"
    FROM GV$librarycache
    where namespace in ('SQL AREA')
    group by inst_id
    union
    select a.inst_id,'Soft-Parse Ratio ' "Ratio", to_char(round(100 * ((a.value - b.value) / a.value ),2)) "Soft-Parse Ratio"
    from (select inst_id,value from gv$sysstat where name like 'parse count (total)') a,
    (select inst_id, value from gv$sysstat where name like 'parse count (hard)') b
    where a.inst_id = b.inst_id
    union
    select a.inst_id,'Execute Parse Ratio ' "Ratio", to_char(round(100 - ((a.value / b.value)* 100),2)) "Execute Parse Ratio"
    from (Select inst_id, value from gv$sysstat where name like 'parse count (total)') a,
    (select inst_id, value from gv$sysstat where name like 'execute count') b
    where a.inst_id = b.inst_id
    union
    select a.inst_id,'Parse CPU to Elapsed Ratio ' "Ratio", to_char(round((a.value / b.value)* 100,2)) "Parse CPU to Elapsed Ratio"
    from (Select inst_id, value from gv$sysstat where name like 'parse time cpu') a,
    (select inst_id, value from gv$sysstat where name like 'parse time elapsed') b
    where a.inst_id = b.inst_id
    union
    Select a.inst_id,'Chained Row Ratio ' "Ratio", to_char(round((a.val/b.val)*100,2)) "Chained Row Ratio"
    from (SELECT inst_id, SUM(value) val FROM gV$SYSSTAT WHERE name = 'table fetch continued row' group by inst_id) a,
    (SELECT inst_id, SUM(value) val FROM gV$SYSSTAT WHERE name IN ('table scan rows gotten', 'table fetch by rowid') group by inst_id) b
    where a.inst_id = b.inst_id
    union
    Select inst_id,'Latch Hit Ratio ' "Ratio", to_char(round(((sum(gets) - sum(misses))/sum(gets))*100,2)) "Latch Hit Ratio"
    from gv$latch
    group by inst_id
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    union
    select inst_id, metric_name, to_char(value)
    from gv$sysmetric
    where metric_name in ( 'Database Wait Time Ratio', 'Database CPU Time Ratio')
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    order by inst_id
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    INST_ID Ratio Value
    1 Buffer Cache Hit Ratio .83
    1 Chained Row Ratio 0
    1 Dictionary Cache Hit Ratio 77.5
    1 Execute Parse Ratio 45.32
    1 Get Hit Ratio 75.88
    1 Latch Hit Ratio 100
    1 Library Cache Hit Ratio 99.52
    1 Parse CPU to Elapsed Ratio 24.35
    1 Pin Hit Ratio 95.24
    1 Soft-Parse Ratio 89.73
    i have a doubt with buffer cache hit ratio, can anyone please help me to understand this

    Buffer Cache Hit Ratio .83Quite weird value. It seems your system is doing all physical reads, which seems unrealistic.
    I had a 10.2.0.1 database where i saw this kind of result for cache hit ratio and after patching it to 10.2.0.4, it started showing results correctly.
    Probably it could be some Oracle 10g bug which made this odd display of hit ratio information in data dictionary. Can you try patching your database to latest 10g PSU, or contact Oracle support for a one-off patch for this problem
    Salman

  • Buffer Cache Hit ratio 48

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    Thank You
    Prasad

    Maybe, maybe not.
    You might want to post your question into a more appropriate forum : General Database Discussions
    Nicolas.

  • Cache Hit Ratio -19

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