Low buffer hit ratio

Hi all,
could anyone please let me know how I can improve upon the buffer hit ratio in Oracle 8.0
I have increased my buffer size by 40% in the init.ora parameters but there has been no gain at all in the buffer hit ratio after the system has been bounced and up by 42 hrs so could any let me know..
Thanks
Akkama

Why don't u try pinning the objects accessed frequently - even pinning tables are possible.

Similar Messages

How should i increase over all buffer hit ratio.....

Hi all,
As shown below if my DB2 databse buffer Qulaity is low .. How should i increase over all buffer hit ratio..
Please advice on any sap standrd notes or procedures
Number                                    1
Total Size                           80,000 KB
Physical Reads                         6.65 ms
Physical Writes                        0.00 ms
Overall Buffer Quality                86.05 %
Data Hit Ratio                        85.79 %
Index Hit Ratio                       87.50 %
No Victim Buffers                259,079,295
--rahul

One of the options is to simply increase the bufferpool size using the following command
db2 alter bufferpool immediate <bufferpool name> size <new bufferpool size>
However, this will affect the hit ration for a particular bufferpool. If you have more than one bufferpool, you need to identify the bufferpool(s) with worst hit ratio. In the SAP DBA Cockpit, check using
Performance -> Bufferpool
The victim buffer information is only useful in case you use alternate page cleaning.
Note that there are other options to fight bad bufferpool hit ratio - however, with your small bufferpool size (80MB) maybe increasing the size is the appropriate step.
Malte

Buffer hit ratio

I am using the following:
SELECT ROUND(((1-(SUM(DECODE(NAME, 'physical reads', value, 0)) /
(SUM(DECODE(NAME, 'db block gets', value, 0))+
(SUM(DECODE(NAME, 'CONSISTENT GETS', value, 0))))))*100), 2) || '%' BCHR
FROM V$SYSSTAT
to calculate the buffer hit ratio. This query is returning: -1753.28%
Can someone explain why I am getting this crazy number?
Thanks,
mdp

>>
Many folks misunderstand that bit about "setting your own BHR", and falsely conclude that it's a useless metric. It's not useless.
<<
The buffer cache ratio is useful only when considered in relation to other statistics. The problem is that the majority of users seem to think that that a high ratio value is good and a low ratio value is bad based on absolute values and do not understand that the static is dependent on how SQL plans are being solved. If you measure the ratio when the dominant work on the system is being done via hash joins, full scans that touch the target blocks only once, or make use of PQO during the process you can get a fairly low value, but the system is performing well. On the other had poorly performing SQL can result in a high value for the statistic. The value of the statistics bears no direct relationship to performance of the system and it needs to be emphasized that the ratio must be used in conjunction with other available information. The ratio by itself should be considered useless.
>>
If the BHR was totally useless, why does Oracle continue to include it in OEM alert thresholds, and STATSPACK and AWR reports?
<<
Over the years Oracle has done lots of things that turned out to be wrong so just because Oracle includes the statistics in certain products does not really provide a lot of support for the validity of the statistic. Known errors in the documentation have made it through two full releases. Again it is the misapplication of the statistic that is really at issue. Unfortunately, many poorly written DBA Administration and Tuning books in the past claimed that ratio could be used to measure database performance, and in point of fact the ratio has only a passing relationship to performance depending on the application.
HTH -- Mark D Powell --

STATSPACK REPORT (BUFFER HIT RATIO)

my statspack report shows that my buffer ration is 83%...what factors i need to look to imporve the buffer hit ratio. Thanks

I deleted because i realized that i took the statspack report of 1 day period.
Below is the Statspack report of 1 hour. Can you please let me know if i still need to increase database buffer cache?
STATSPACK report for
Database DB Id Instance Inst Num Startup Time Release RAC
~~~~~~~~ ----------- ------------ -------- --------------- ----------- ---
4254163 TEST1 1 28-Jun-07 23:30 10.2.0.3.0 NO
Host Name: Linux3 Num CPUs: 2 Phys Memory (MB): 7,968
~~~~
Snapshot Snap Id Snap Time Sessions Curs/Sess Comment
~~~~~~~~ ---------- ------------------ -------- --------- -------------------
Begin Snap: 32 03-Jul-07 11:59:13 23 11.0
End Snap: 42 03-Jul-07 14:07:33 26 11.3
Elapsed: 128.33 (mins)
Cache Sizes Begin End
~~~~~~~~~~~ ---------- ----------
Buffer Cache: 100M Std Block Size: 8K
Shared Pool Size: 100M Log Buffer: 33,823K
Load Profile Per Second Per Transaction
~~~~~~~~~~~~ --------------- ---------------
Redo size: 1,259.57 8,598.13
Logical reads: 148.39 1,012.92
Block changes: 6.41 43.76
Physical reads: 41.91 286.09
Physical writes: 0.73 5.02
User calls: 15.66 106.91
Parses: 4.07 27.77
Hard parses: 0.27 1.85
Sorts: 1.70 11.61
Logons: 0.01 0.07
Executes: 9.59 65.47
Transactions: 0.15
% Blocks changed per Read: 4.32 Recursive Call %: 83.09
Rollback per transaction %: 6.03 Rows per Sort: 11.39
Instance Efficiency Percentages
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Buffer Nowait %: 100.00 Redo NoWait %: 100.00
Buffer Hit %: 71.77 In-memory Sort %: 100.00
Library Hit %: 93.15 Soft Parse %: 93.34
Execute to Parse %: 57.58 Latch Hit %: 100.00
Parse CPU to Parse Elapsd %: 97.12 % Non-Parse CPU: 86.74
Shared Pool Statistics Begin End
Memory Usage %: 91.37 92.38
% SQL with executions>1: 77.55 80.43
% Memory for SQL w/exec>1: 83.11 84.69
Top 5 Timed Events Avg %Total
~~~~~~~~~~~~~~~~~~ wait Call
Event Waits Time (s) (ms) Time
CPU time 132 48.3
db file sequential read 89,745 91 1 33.4
db file scattered read 29,289 35 1 13.0
control file parallel write 2,558 6 2 2.1
log file parallel write 2,294 3 1 1.0
Host CPU (CPUs: 2)
~~~~~~~~ Load Average
Begin End User System Idle WIO WCPU
0.11 0.11 2.26 2.65 95.09 0.90 0.24
Instance CPU
~~~~~~~~~~~~
% of total CPU for Instance: 1.06
% of busy CPU for Instance: 21.63
%DB time waiting for CPU - Resource Mgr:
Memory Statistics Begin End
~~~~~~~~~~~~~~~~~ ------------ ------------
Host Mem (MB): 7,967.6 7,967.6
SGA use (MB): 316.0 316.0
PGA use (MB): 57.8 62.6
% Host Mem used for SGA+PGA: 4.7 4.8
Time Model System Stats DB/Inst: TEST1/TEST1 Snaps: 32-42
-> Ordered by % of DB time desc, Statistic name
Statistic Time (s) % of DB time
sql execute elapsed time 212.3 92.7
DB CPU 124.2 54.2
parse time elapsed 21.6 9.4
hard parse elapsed time 19.7 8.6
PL/SQL execution elapsed time 4.3 1.9
hard parse (sharing criteria) elaps 1.4 .6
connection management call elapsed 1.4 .6
PL/SQL compilation elapsed time 1.2 .5
repeated bind elapsed time 0.1 .0
hard parse (bind mismatch) elapsed 0.1 .0
sequence load elapsed time 0.0 .0
DB time 228.9
background elapsed time 48.2
background cpu time 39.3
Wait Events DB/Inst: TEST1/TEST1 Snaps: 32-42
-> s - second, cs - centisecond, ms - millisecond, us - microsecond
-> %Timeouts: value of 0 indicates value was < .5%. Value of null is truly 0
-> Only events with Total Wait Time (s) >= .001 are shown
-> ordered by Total Wait Time desc, Waits desc (idle events last)
Avg
%Time Total Wait wait Waits
Event Waits -outs Time (s) (ms) /txn
db file sequential read 89,745 0 91 1 79.6
db file scattered read 29,289 0 35 1 26.0
control file parallel write 2,558 0 6 2 2.3
log file parallel write 2,294 0 3 1 2.0
db file parallel write 2,179 0 3 1 1.9
log file sync 1,089 0 2 2 1.0
os thread startup 7 0 1 120 0.0
latch free 3 0 0 89 0.0
SQL*Net break/reset to client 640 0 0 0 0.6
direct path read 140 0 0 1 0.1
control file sequential read 3,599 0 0 0 3.2
SQL*Net more data to client 2,121 0 0 0 1.9
db file parallel read 49 0 0 1 0.0
cursor: pin S wait on X 2 100 0 16 0.0
read by other session 4 0 0 5 0.0
direct path write 24 0 0 0 0.0
latch: shared pool 1 0 0 2 0.0
SQL*Net message from client 120,211 0 47,282 393 106.6
wait for unread message on broadc 7,631 100 7,517 985 6.8
Streams AQ: waiting for messages 1,540 100 7,512 4878 1.4
Streams AQ: qmn slave idle wait 275 0 7,508 27302 0.2
Streams AQ: qmn coordinator idle 554 51 7,508 13553 0.5
Streams AQ: waiting for time mana 25 52 6,643 ###### 0.0
SQL*Net message to client 120,215 0 0 0 106.6
class slave wait 7 0 0 1 0.0
SQL*Net more data from client 146 0 0 0 0.1
Background Wait Events DB/Inst: TEST1/TEST1 Snaps: 32-42
-> %Timeouts: value of 0 indicates value was < .5%. Value of null is truly 0
-> Only events with Total Wait Time (s) >= .001 are shown
-> ordered by Total Wait Time desc, Waits desc (idle events last)
Avg
%Time Total Wait wait Waits
Event Waits -outs Time (s) (ms) /txn
control file parallel write 2,557 0 6 2 2.3
log file parallel write 2,290 0 3 1 2.0
db file parallel write 2,179 0 3 1 1.9
os thread startup 7 0 1 120 0.0
db file sequential read 1,456 0 1 0 1.3
db file scattered read 25 0 0 8 0.0
control file sequential read 156 0 0 0 0.1
latch: shared pool 1 0 0 2 0.0
rdbms ipc message 25,017 92 59,496 2378 22.2
pmon timer 2,576 100 7,513 2917 2.3
Streams AQ: qmn slave idle wait 275 0 7,508 27302 0.2
Streams AQ: qmn coordinator idle 554 51 7,508 13553 0.5
smon timer 26 96 7,148 ###### 0.0
Streams AQ: waiting for time mana 25 52 6,643 ###### 0.0
Wait Event Histogram DB/Inst: TEST1/TEST1 Snaps: 32-42
-> Total Waits - units: K is 1000, M is 1000000, G is 1000000000
-> % of Waits - column heading: <=1s is truly <1024ms, >1s is truly >=1024ms
-> % of Waits - value: .0 indicates value was <.05%, null is truly 0
-> Ordered by Event (idle events last)
Total ----------------- % of Waits ------------------
Event Waits <1ms <2ms <4ms <8ms <16ms <32ms <=1s >1s
LGWR wait for redo copy 7 100.0
SQL*Net break/reset to cli 640 99.2 .6 .2
SQL*Net more data to clien 2121 100.0
control file parallel writ 2558 84.2 12.0 .7 1.4 1.5 .2
control file sequential re 3599 99.9 .1
cursor: pin S wait on X 2 100.0
db file parallel read 49 93.9 2.0 4.1
db file parallel write 2179 68.2 19.9 6.8 4.0 .9 .1 .1
db file scattered read 29K 90.7 6.0 .5 .5 .6 .8 .9
db file sequential read 89K 89.4 2.8 1.3 3.6 1.5 .7 .6
direct path read 140 87.1 2.9 .7 1.4 7.1 .7
direct path write 24 100.0
latch free 3 100.0
latch: messages 1 100.0
latch: shared pool 1 100.0
log file parallel write 2294 77.4 17.3 2.0 1.3 1.1 .8 .2
log file sync 1089 62.4 28.8 3.3 1.7 2.5 1.1 .2
os thread startup 7 100.0
read by other session 4 50.0 25.0 25.0
SQL*Net message from clien 120K 95.2 1.6 .9 .3 .1 .2 .1 1.7
SQL*Net message to client 120K 100.0
SQL*Net more data from cli 146 100.0
Streams AQ: qmn coordinato 554 49.1 .2 .2 50.5
Streams AQ: qmn slave idle 275 100.0
Streams AQ: waiting for me 1540 .2 99.8
Streams AQ: waiting for ti 25 36.0 16.0 48.0
class slave wait 7 85.7 14.3
pmon timer 2577 .5 .1 .1 99.3
rdbms ipc message 25K 2.3 1.3 1.4 .4 .4 .3 32.1 61.8
smon timer 26 100.0
wait for unread message on 7631 .0 .0 100.0 .0

Which is a better measure of buffer hit ratio?

which out of these gives a better measure of buffer hit ratio?
consistent gets from cache
db block gets from cache
physical reads cache
or
consistent gets
db block gets
physical reads
from v@sysstat?

Hi,
Well you can always edit your reply.There is a button out there to do that.
About the question,I am not clear with the question.What do you mean by "better measure of hit ratio"?
From the PT guide,
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/memory.htm#i56283
The formula includes all three of them in the calculation,
SELECT NAME, VALUE
FROM V$SYSSTAT
WHERE NAME IN ('db block gets from cache', 'consistent gets from cache', 'physical reads cache');
Using the values in the output of the query, calculate the hit ratio for the buffer cache with the following formula:
1 - (('physical reads cache') / ('consistent gets from cache' + 'db block gets from cache')All three of them are part of the hit ratio.When you wrote the three statistics second time,I am not sure how they were different from the first one.All you did was that you removed "cache" word.
There is no "better measure" statistics in the calculation.All three are there.
If your question had some other meaning than I would appreciate if you rephrase and elaborate it please.
Aman....

Differents values for Buffer Hit Ratio

Hi Everyone!
I have only a buffer_pool(8k) and when I execute this two querys(below). The "Buffer Hit Ratio" results are very different.
Anyone know why does this happen?? I searched on the Internet and find out just V$SYSSTAT gather dates of all buffer pools
while V$BUFFER_POOL_STATISTICS maintains statistics for each buffer pool. But i can't understand why it is so different.
Any suggestion?
Thanks!
SELECT 1- ((p.value - l.value - d.value) / s.value) "Buffer Cache Hit Ratio"
FROM v$sysstat s, v$sysstat l, v$sysstat d, v$sysstat p
WHERE s.name = 'session logical reads'
AND d.name = 'physical reads direct'
AND l.name = 'physical reads direct (lob)'
AND p.name = 'physical reads';
Buffer Cache Hit Ratio
,970655577
SELECT NAME, PHYSICAL_READS, DB_BLOCK_GETS, CONSISTENT_GETS,
1 - (PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS)) "Hit Ratio"
FROM V$BUFFER_POOL_STATISTICS;
Hit Ratio
,584760575
ORACLE VERSION: Oracle Server 9.2.0.8.0
OS: IBM/AIX

See metalink note STATISTIC "cache hit ratio" - Reference Note 33883.1 for the correct formula to use in your calculation.
The ratio is of limited value. In and of itself it tells you nothing about the performance of your database.
There have been numerous threads on the ratio in this forum. You can use search to find them if interested.
HTH -- Mark D Powell --

Oracle Buffer Hit Ratio

I m using this query to find Oracle Buffer Hit ratio .Is this query right.Is there some other way of getting more accurate ratio
select trunc((1-(sum(decode(name,'physical reads',value,0))/(sum(decode(name,'db block gets',value,0)) + (sum(decode(name,'consistent gets',value,0)))))) * 100) from v$sysstat

Asif,
Too bad we couldn't meet when I happened to come to Bdesh coz if we could , we would had talked about the same topic for hours. If you are using this hit ratio as the tuning technique than you are not alone. There are so many dbas who are using hit ratio and loose sleep seeing not to a particular %. It was a method that was given years ago by Oracle reason being that at that time, databases , their loads, they were not too much. But now things have changed so should be the troubleshooting techniques. Search over this forum for the same topic and you would see some "cool" threads where some top-notch experts are talking about the same issue.
Aman....

Buffer hit ratio (to be negative)

Hi All,
My DB Version: 10.2.0
OS: Windows Server 2003
When i am checking snapshots at peak time and comparing it with one when there is no load on the server the buffer hit ratio (to be negative), the buffer cache is too small and the data in is being aged out before it can be used so it must be retrieved again.
So i just want to know that whether should i increase the value of db_cache_size.
What exactly happens when i do this.

Hi;
DB_CACHE_SIZE specifies the size of the DEFAULT buffer pool for buffers with the primary block size (the block size defined by the DB_BLOCK_SIZE initialization parameter).
The value must be at least 4M * number of cpus * granule size (smaller values are automatically rounded up to this value). A user-specified value larger than this is rounded up to the nearest granule size. A value of zero is illegal because it is needed for the DEFAULT memory pool of the primary block size, which is the block size for the SYSTEM tablespace.
Source:
http://docs.oracle.com/cd/B19306_01/server.102/b14237/initparams043.htm
Regard
Helios

Buffer Hit Ratio % -- Whats the right query ?

Whats the right query to track Buffer Hit % ;
Using this :
prompt BUFFER HIT RATIO %
prompt ===============
select 100 * ((a.value+b.value)-c.value) / (a.value+b.value) "Buffer Hit Ratio"
from v$sysstat a, v$sysstat b, v$sysstat c
where
a.statistic# = 38
and
b.statistic# = 39
and
c.statistic# = 40;
Buffer Hit Ratio
99.9678438
However, using this :
Select
Round((Sum(Decode(name, 'consistent gets',value,0)) +
Sum(Decode(name, 'db block gets',value,0)) -
Sum(Decode(name, 'physical reads',value,0))) /
(Sum(Decode(name, 'consistent gets',value,0)) +
Sum(Decode(name, 'db block gets',value,0)) ) * 100, 4)
from V$sysstat;
Comes up as : 67.7069 %
So which is the right one ?
Thanks.

user4874781 wrote:
Well, I recently joined this organisation and that was the script that was used since long to check Buffer Hit Ratio%.
But when I ran a TOAD report, using the other query, the value came up different.
So am confused .. Whats the difference and which is the right one ?
Try running the following query:
select
        statistic#, name
from
        v$sysstat
where
        statistic# in (38,39,40)
or      name in (
                'consistent gets',
                'physical reads',
                'db block gets'
;The you will understand the point the previous answer was making. It's a bad idea to rely on things like the statistic# being consistent across different versions of Oracle - names tend to be safer.
But neither query is correct. If you want any sort of vaguely meaningful "buffer cache hit ratio", you should be quering v$buffer_pool_statistics. See also: Re: Testing of buffer cache reveals these results: and http://jonathanlewis.wordpress.com/2007/09/02/hit-ratios/
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
"The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge." Stephen Hawking.

One reason why buffer hit ratio just might be bogus

Hi.
Still trawling through statspack trying to make some sense of it ...
But looking at this, (on undo segments) I have perhaps another reason to sneer at the old buffer cache hit ratio: (in the sense of using it to tune, or of using it to justify action / lack of action).
"The header is also a data block that is frequently modified , so it generally remains in the buffer cache. Therefore, gets of the rollback segment header block increase the buffer cache hit ratio; this artificial increase in the hit ratio can mislead you into thinking that you have allocated enough data blocks to the buffer cache"
:taken from the OP Exam 033 text, p.295

Hi Dan,
The problem is about the usefulness of ratio's in-general, it's not specifically about the BCHR. . . .
This whole thing arose from the Oracle7 performance tuning classes where Oracle Corporation suggested that the BCHR be kept over 90%. . . .
Is the BCHR "bogus"? It is what it is, a ratio. . . .
I agree wit Ben, it does have limited use as an "indicator", that the buffer cache MIGHT be too small to cache the working set of frequently-referenced data blocks. I have my notes here:
http://www.dba-oracle.com/t_buffer_cache_hit_ratio_value.htm
Oh, and as to the buffer cache advisory, it's not perfect (as is any predictive model) and I rarely see one that does not suggest that adding RAM will reduce physical I/O.
Anyway, in just a few years all this will be a moot issue, espeially since solid-state disk has hit $100 per gig. Without spinning platters, there is no need for a large buffer cache at all . . . .
Hope this helps. . .
Donald K. Burleson
Oracle Press author
Author of "Oracle Tuning: The Definitive Reference":
http://www.dba-oracle.com/bp/s_oracle_tuning_book.htm

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>

No swaps anyware still hit Ratio around 4% for Initial Record Buffer

Dear Gurus
I am using ABAP NW7.0 System.
in the system all buffers have hit ratio 99+%
but the initial record buffer hit ratio it below 10%
any way there is no swaps at all still hit Ratio is 4%
The current parameters are:
rsdb/ntab/irbdsize   --->6000
rsdb/ntab/entrycount---> 20000
HITRATIO -
>        %         4
HITS         -
>              64
REQUESTS    -
>             1.729
DB access quality %   -
>       4
DB access      -
>          1.656
DB access saved    -
>         64
Reorgs        -
>               0
Allocated     -
>    KB     6.625
Available     -
>    KB     6.000
Used         -
>     KB     1.099*
Free         -
>     KB     4.901
Available     -
>           5.000
Used         -
>            1.656
Free           -
>         * 3.344*
Objects swapped    -
>          0
Frames swapped    -
>           0
Total          -
>              0
Pl suggest me how can i have the hitratio more.
Thanks in advance

Hello,
Unfortunately we can not tell exactly why the value is low,
however it is not necessarily an incorrect value.
The quality of a buffer and how often it is accessed is measured by the
'%Hit Ratio'. This value will indicate if the information stored in the
buffers, such as table entries, programs and screens, is being hit
directly from the buffer itself or, on the other hand, if the system
needs to bring that data from the database since it was not found in the
buffer.
To find out the buffers with poor quality, first check the system
startup time. When the system is started, all buffers (except the
program buffer which has a pre-load) are initially empty. Therefore,
all objects that are accessed for the first time have to be read from
the database and then loaded into the buffers.
If objects are not yet in the buffer, the hit ratio for the buffer will
be low. The hit ratio increases from the time objects are loaded into
the buffers. The rate of the increase depends on the workload in the
system and is different for each buffer.So it is to be noted that how often you restart the system which resuls in the loading of object again and causes hit rate to be low.
Poor buffer quality is not always due to a real problem. For example,
transports into a system can reduce buffer quality. Keep in mind though
that a value lower does not always shows that you have a problem.
A more pressing concern would be if we saw swaps on the system. As you
can see, there are no swaps.
Swapping occurs when the buffer is full, and the SAP System has to
load additional objects into the buffer. Objects in the buffer that
were used the least recently are removed. In this context, the term
"swap" means the objects removed from the buffer are lost and cannot
be replaced until a new database access is performed (replacing what
was lost).
There are two possible reasons for swapping
1 There is no space left in the buffer data area
The buffer is too small. You should increase the buffer size.
2 There are no directory entries left.
Therefore, to conclude, although the hitratio appears low it does
not mean that there are any performance issues. The fact that there is
sufficient free space and there are no swaps confirm this.
You can try to increase size of intial record buffer(in steps) as from current setting it seems to be small.
I hope this helps.
Regards,
Archana

Buffer Hit % discussion

Hi Guys,
I read few sites about buffer hit % vs performance of the database.
I understand that a high % of hit ratio doesn't mean the performance is good. It's might means that the queries running in the database is doing alot of unwanted huge I/O by the user of unselective indexes.
However can i Conclude that a low % of hit ratio = bad performance where we have to either look into the sql <reduce logical i/o which in term reduce physical i/o> and adding memory to the buffer cahe if we confirm all the sqls are good but still low hit %.
Kindly share ur thoughts.
thanks!

dbaing wrote:
However can i Conclude that a low % of hit ratio = bad performance where we have to either look into the sql <reduce logical i/o which in term reduce physical i/o> and adding memory to the buffer cahe if we confirm all the sqls are good but still low hit %.
Kindly share ur thoughts.If you have a model of how you expect your system to behave then there may be cases, or times, when you can decide that some ratio is signficantly out of the range you expect.
As far as the BCHR is concerned, you could imagine approximating an OLTP system with the (pessimistic) assumption that a single row access requires thee index block visits (root, branch, leaf) and one table block visit to pick up the row. If that's really the case you could also decide that for your model you have enough memory to buffer the indexes, but no memory for keep the tables buffered because of the extreme randomness of the table visits. If that model makes sense for you then you might expect a hit ratio of around 75% - and therefore start to worry if the ratio is significantly lower. Of course the OLTP system might run regular reports - which do large tablescans and hash joins and distort the figures; it might accumulate a large number of indexes over time which could invalidate your "all indexes buffered" model; there are probably a number of small tables which are constantly buffered that (ought to) push the hit ratio up.
Without a reasonable model of what your system is supposed to do at what times of day, and what variation to expect over the week it's quite hard to make any comment about what constitutes a low or high BCHR - and it's hard to say how long and how far the figure should deviate from your expectation before you consider it to be showing threatening behaviour.
Regards
Jonathan Lewis

Low Hit Ratio for Export/Import Buffer

Hi,
Do low hit ratio (less than 70%) for Export/Import buffer causes performance issue?
I didnt get any Note related to this.
Any help will be appreciated.
Thanks,
Saurabh

Thanks everyone,
Basicaly all the buffers(Program/Nametab/CUA/Screen/Genric Key/Single record) has hit ratio around 99% , but for export/import buffer its 60%. which is indeed less.
This is what i intended to ask, if this low hit ratio for export/import buffer has much impact ?
Any note if anyone remembers?
Thanks,
Saurabh

Buffer Cache hit ratio low (40.42%)

Can someone help me with a good guide or advice in order to increase my Buffer Cache hit ratio? If I am not wrong databases should have +85% hit ratio for a good performance.
thanks

Here is a script that will bump up your BCHR
http://www.oracledba.co.uk/tips/choose.htm
I think you understood from that you shouldn't worry about your bchr but better focus on your real problems i.e. parts of your app that run slowly and people are expecting quicker resposnse.
Gints Plivna
http://www.gplivna.eu

Low buffer hit ratio

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