Decument about buffer cache mechanism

hi gurus
I want some documentations about the buffer cache internal mechanism,for example the internal structure of the buffer cache,how does the buffer cache works and the LRU mechanism.The more detailed the better.
I have searched on Oracle Documentation Linrary with buffer head or buffer descriptor as keywords.I did not get what i want.
many thanks in advance.

Hi Kevin,
I believe information which you are looking is not published by Oracle and is avaialable only to Oracle Support and other employees. You can refer to Note:62172.1 for buffer cache mechanism. It mentioned about a note with "@" in beginning meaning the remark was supposed to be unpublished.
"@ See Note 104937.1 for a description "
May be the above note has got some description. Anyways I will suggest you to go through the Note:62172.1 and get basic understanding which should be sufficient for dba's.
Cheers
Amit

Similar Messages

Buffer cache writes

Hi,
From ADDM report i saw following message.
Buffer cache writes due to small log files were consuming significant database
time.
Recommendation 1: Database Configuration
Estimated benefit is .31 active sessions, 2.99% of total activity.
Action
Increase the size of the log files to 2048 M to hold at least 20 minutes
of redo information.
From AWR which section would tell me about Buffer cache writes?
Thanks

user10698496 wrote:
From ADDM report i saw following message.
Buffer cache writes due to small log files were consuming significant database
time.
Recommendation 1: Database Configuration
Estimated benefit is .31 active sessions, 2.99% of total activity.
Action
Increase the size of the log files to 2048 M to hold at least 20 minutes
of redo information.
From AWR which section would tell me about Buffer cache writes?
As Nikolay has pointed out, the estimated benefit is only three percent, however it's not (usually) a difficult, risky, r time-consuming change to make so you might as well follow the advice.
One of the triggers that utlimately causes the database writer (dbwr) to write is the log file switch - which means if your log files are small you may find dbwr writing blocks more aggressively than needed. Oracle may be analysing history to count examples of database blocks being written more than once before being flushed from the buffer to produce this conclusion.
Jgarry has already mentioned v$instance_recovery - one of the columns in that view is "writes_logfile_size" - the number of writes due to the size of the log file; this is the figure that could be reduced by increasing the log file size. This doesn't get reported in the AWR, though. I think the best you can get is the instance activity statistic "DBWR thread checkpoint buffers written" - but there's a complicated precedence of which features cause which counter to increment so I'm not even sure how helpful that fiture may be.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
Author: <b><em>Oracle Core</em></b>

About database buffer cache issue

Hi all, for example,a DBBC of 4mb size & there is a 1gb table, How this table will be used in DBBC?

Hi,
The whole table will not be taken to cache, but only the required blocks will be placed for oracle operations and unused blocks will be flushed from cache with LRU algorithm. Googling can provide you huge data on this topic.
Oracle Database Buffer Cache Tips
Thank you!!

DB buffer cache vs. SQL query & PL/SQL function result cache

Hi all,
Started preparing for OCA cert. just myself using McGraw Hill's exam guide. Have a question about memory structures.
Actually, DB buffer cache is used to copy e.g. SELECT queries result data blocks, that can be reused by another session (server process).
There is also additional otion - SQL query & PL/SQL function result cache (from 11g), where also stored the results of such queries.
Do they do the same thing or nevertheless there is some difference, different purpose?
thanks in advance...

There is also additional otion - SQL query & PL/SQL function result cache (from 11g), where also stored the results of such queries.Result cache located in shared pool.So it is one component of shared pool.When server process execute query(and if you configured result cache) then result will store in shared pool.Then next execution time run time mechanism will detect and consider using result cache without executing this query(if data was not changed this is happen detection time)
Do they do the same thing or nevertheless there is some difference, different purpose?.Buffer cache and result cache are different things and purpose also,but result cache introduced to improve response time of query in 11g(but such mechanism also implemented in 10g subquery execution,in complex query).In buffer cache holds data blocks but not such results.
Edited by: Chinar on Nov 4, 2011 4:40 AM
(Removing lots of "But" word from sentences :-) )

Hi, I developed a web application using HTML5-Offline Application Cache mechanism. Inspite of deleting the cache as mentioned in the above steps, Firefox still maintains a copy of the page in it's cache. Also, a serious bug is, even though we delete all

== Issue
==
I have a problem with my bookmarks, cookies, history or settings
== Description
==
Hi,
I developed a web application using HTML5-Offline Application Cache mechanism. Inspite of deleting the cache as mentioned in the above steps, Firefox still maintains a copy of the page in it's cache. Also, a serious bug is, even though we delete all temp files used by Firefox, and open the previously cached page, it displays it correctly, but upon refreshing/reloading it again shows the previous version of the page maintained in the cache.
== Troubleshooting information
==
HTML5: Application Caching
.style1 {
font-family: Consolas;
font-size: small;
text-align: left;
margin-left: 80px;
function onCacheChecking(e)
printOutput("CHECKINGContents of the manifest are being checked.", 0);
function onCacheCached(e) {
printOutput("CACHEDAll the resources mentioned in the manifest have been downloaded", 0);
function onCacheNoUpdate(e)
printOutput("NOUPDATEManifest file has not been changed. No updates took place.", 0);
function onCacheUpdateReady(e)
printOutput("UPDATEREADYChanges have been made to manifest file, and were downloaded.", 0);
function onCacheError(e) {
printOutput("ERRORAn error occured while trying to process manifest file.", 0);
function onCacheObselete(e)
printOutput("OBSOLETEEither the manifest file has been deleted or renamed at the source", 0);
function onCacheDownloading(e) {
printOutput("DOWNLOADINGDownloading resources into local cache.", 0);
function onCacheProgress(e) {
printOutput("PROGRESSDownload in process.", 0);
function printOutput(statusMessages, howToTell)
* Outputs information about an event with its description
* @param statusMessages The message string to be displayed that describes the event
* @param howToTell Specifies if the output is to be written onto document(0) or alert(1) or both(2)
try {
if (howToTell == 2) {
document.getElementById("stat").innerHTML += statusMessages;
window.alert(statusMessages);
else if (howToTell == 0) {
document.getElementById("stat").innerHTML += statusMessages;
else if (howToTell == 1) {
window.alert(statusMessages);
catch (IOExceptionOutput) {
window.alert(IOExceptionOutput);
function initiateCaching()
var ONLY_DOC = 0;
var ONLY_ALERT = 1;
var BOTH_DOC_ALERT = 2;
try
if (window.applicationCache)
var appcache = window.applicationCache;
printOutput("BROWSER COMPATIBILITYSUCCESS!! AppCache works on this browser.", 0);
appcache.addEventListener('checking', onCacheChecking, false);
appcache.addEventListener('cached', onCacheCached, false);
appcache.addEventListener('noupdate', onCacheNoUpdate, false);
appcache.addEventListener('downloading', onCacheDownloading, false);
appcache.addEventListener('progress', onCacheProgress, false);
appcache.addEventListener('updateready', onCacheUpdateReady, false);
appcache.addEventListener('error', onCacheError, false);
appcache.addEventListener('obsolete', onCacheObselete, false);
else
document.getElementById("stat").innerHTML = "Failure! I cant work.";
catch (UnknownError)
window.alert('Internet Explorer does not support Application Caching yet.\nPlease run me on Safari or Firefox browsers\n\n');
stat.innerHTML = "Failure! I cant work.";
== Firefox version
==
3.6.3
== Operating system
==
Windows XP
== User Agent
==
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 ( .NET CLR 3.5.30729; .NET4.0E)
== Plugins installed
==
*-Shockwave Flash 10.0 r45
*Default Plug-in
*Adobe PDF Plug-In For Firefox and Netscape "9.3.2"
*NPRuntime Script Plug-in Library for Java(TM) Deploy
*The QuickTime Plugin allows you to view a wide variety of multimedia content in Web pages. For more information, visit the QuickTime Web site.
*Google Update
*4.0.50524.0
*Office Live Update v1.4
*NPWLPG
*Windows Presentation Foundation (WPF) plug-in for Mozilla browsers
*Next Generation Java Plug-in 1.6.0_20 for Mozilla browsers
*Npdsplay dll
*DRM Store Netscape Plugin
*DRM Netscape Network Object
Thanks & Regards,
Kandarpa Chandrasekhar Omkar
Software Engineer
Wipro Technologies
Bangalore.
[email protected]
[email protected]

We have had this issue many, many times before including on the latest 3.6 rev. It appears that when the applicationCache has an update triggered by a new manifest file, the browser may still use only its local network cache to check for updates to the files in the manifest, instead of forcing an HTTP request and revalidating both the browser cache and the applicationCache versions of the cached file (seems there is more than one). I have to assume this is a browser bug, as one should not have to frig with server Cache-Control headers and such to get this to work as expected (and even then it still doesn't sometimes).
The only thing that seems to fix the problem is setting network.http.use-cache to false (default is true) in about:config . This helps my case because we only ever run offline (applicationCache driven) apps in the affected browser (our managed mobile apps platform), but it will otherwise slow down your browser experience considerably.

Will I increase my Buffer Cache ?

Oracle 9i
Shared Pool 2112 Mb
Buffer Cache 1728 Mb
Large Pool 32Mb
Java Pool 32 Mb
Total 3907.358 Mb
SGA Max Size 17011.494 Mb
PGA
Aggregate PGA Target 2450 Mb
Current PGA Allocated 3286059 KB
Maximum PGA Allocated (since Startup) 3462747 KB
Cache Hit Percentage 98.71%
The Buffer Cache Size advise is telling me that if I increase the Buffer Cache to 1930Mb i will get a 8.83 decrease in phyiscal reads (And its get better the more I increase it)
The question is .. can I safely increase it (In light of my current memory allocations) ? Is it worth it .. ?

Two things stand out:
Your sga max size is 17Gb, but you are only using about 4Gb of it - so you seem to have 13Gb that you are not making best use of.
Your pga aggregate target is 2.4Gb, but you've already hit a peak of 3.4Gb - which means your target may be too small - so it's lucky you had all that spare memory which hadn't gone into the SGA. Despite the availability of memory, some of your queries may have been rationed at run-time to try to minimise the excess demand.
Is this OLTP or DSS - where do you really need the memory ? (Have a look in v$process to see the pga usage on a process by process level).
How many processes are allowed to connect to the database ? (You ought to allow about 2Mb - 4Mb per process to the pga_aggregate_target for OLTP) and at least 1Mb per process for the buffer cache.
Where do you see time lost ? time on disk I/O, or time on CPU ? What type of disk I/O, what's the nature of the CPU usage. These figures alone do not tell us what you should do with the spare memory you seem to have.
A simple response to your original question would be that you probably need to increase the pga_aggregate_target, and you might as well increase the buffer size since you seem to have the memory for both.
On the downside, changing the pga_aggregate_target could cause some execution plans to change; and changing the buffer size does change the limit size on a 'short' table, which can cause an increase in I/O as an unlucky side effect if you're a little heavy on "long" tablescans.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk

Will Oracle look into the database buffer cache in this scenario?

hi guys,
say I have a table with a million rows, there are no indexes on it, and I did a
select * from t where t.id=522,000.
About 5 minutes later (while that particular (call it blockA) block is still in the database buffer cache) I do a
select * from t where t.id >400,000 and t.id < 600,000
Would Oracle still pick blockA up from the database buffer cache? if so, how? How would it know that that block is part of our query?
thanks

Without an Index, Oracle would have done a FullTableScan on the first query. The blocks would be very quickly aged out of the buffer cache as they have been retrieved for an FTS on a large table. It is unlikely that block 'A' would be in the buffer_cache after 5minutes.
However, assuming that block 'A' is still in the buffer_cache, how does Oracle know that records for the second query are in block 'A' ? It doesn't. Oracle will attempt another FullTableScan for the second query -- even if, as in the first query -- the resultset returned is only 1 row.
Now, if the table were indexed and rows were being retrieved via the Index, Oracle would use the ROWID to get the "DBA" (DataBlockAddress) and get the hash value of that DBA to identify the 'cache buffers chain' where the block is likely to be found. Oracle will make a read request if the block is not present in the expected location.
Hemant K Chitale
http://hemantoracledba.blogspot.com

Suitable Caching Mechanism

Apoplogies if this is the wrong newsgroup. My J2EE App runs on Weblogic 8.1 and uses Oracle 9. It involves processing at times up to 50000 database records at a go. The Application applies an algorithm to each database record and then writes it's results to an output table which has a one to one mapping with the input table i.e I will be inserting about 50000 records. In order to improve performance I want to use JDBC batch inserts, however each output record is produced by a MDB (Message Driven Bean) from a pool.I guess I need a sort of cacheing mechanism which holds the records from the MDBs and some business logic to apply the JDBC batch insert to a chunk of the cache. I will probably need a timed event to perform this "flushing" of the cache periodically. Anybody have experience of doing something similar or able to provide a more elegant solution to the problem?
TIA

Hi Sailendra,
In this case you can make use of BPM object if I understood your question correctly.
In the component Cataloge make a BPM object and under that create attribute/attributes/array. Then the value you want to save/store keep it in a bpm object attribue and pass the bpm object from one process layer to another/as per your requirement. ex: bpmobject.attribute1Name = "Bibhu";
You can acccess those values in another process/activities by bpmobject.attributeName
bpmobject is the object that you created and the instance variable is of type created bpmobject.
attributeName is the attribute where you stored the value.
Regards
Bibhu

Is dictionary cache double buffered (shared pool, buffer cache)

Hi,
I'm trying to get idea about how dictionary cache is buffered .
Let us say we're talking about dc_objects .
It is dba_tables view related so all underlying sys.obj$ sys.user$ ... tables block are cached in buffer cache.
So why we are caching them in dictionary cache space in shared pool additionally ?
Looks like double buffering and wasting SGA .
Please explain .
Regards
GregG

HI,
Dictionary cache will not cache data of tables, rather it will cache the structural information of table (in your case).
If i will do "select ename from emp", during statement compilation, it needs to check whether "ename" is a real column? and for this it needs to query data dictionary information (from using physical read of system data file or from data dictionary cache if information is there). It also need to check whether i have (logged in user) rights to access this table/column and all this information comes from data dictionary.
This is a simple example, otherwise dictionary cache need to store a lot of other information also (but purely the information present in data dictionary)
Salman

How to remove blocks from buffer cache for a specific object

hi everybody,
is it possible to remove blocks which belogns to a specific object (a table for ex) from buffer cache.
as you know, there is
alter system flush buffer_cache;command but it does it's job for all buffer cache. if you ask me why i want this, for tuning reasons. I want to test some plsql codes when they run as if they are running for the first time (reading from disk).
ps: I use oracle 11g r2

Hi mustafa,
Your performance will not degrade if you run the query second time ( if i understood correctly, you worry about the performance if you execute the procedure second time). Executing/running the code/sql statements over and over again will have following two good benefits.
1) This will avoid hard parsing (Hard parsing is resource intensive operation and this generally increase the overall processing time.
2) This will avoid physical read IO (You gonna see the benefits if data blocks already cached and you dont have to spend time in reading blocks from disk. Reading from disk is much costlier and time consuming operation as compared to data in RAM)
Having that said sometime bad written queries will acquire more blocks then required and consume most part of buffer cache, and this can some times effect the other important blocks and force to flush out from buffer cache.
Oracle have built some intelligence for large full table scan operations for e.g will doing full table scan(I hope you already know what is fts) oracle will put its blocks at end of LRU chain. So these will be the buffers will would flush out first then any other.
From oracle documentation:
"When the user process is performing a full table scan, it reads the blocks of the table into buffers and puts them on the LRU end (instead of the MRU end) of the LRU list. This is because a fully scanned table usually is needed only briefly, so the blocks should be moved out quickly to leave more frequently used blocks in the cache.
You can control this default behavior of blocks involved in table scans on a table-by-table basis. To specify that blocks of the table are to be placed at the MRU end of the list during a full table scan, use the CACHE clause when creating or altering a table or cluster. You can specify this behavior for small lookup tables or large static historical tables to avoid I/O on subsequent accesses of the table."
Regards
Edited by: 909592 on Feb 6, 2012 4:37 PM

ICM Host name buffer cache parameter

Hello,
Does any body know about the ICM Host name buffer cache parameter?
I mean how long does it hold this value and cache refresh rate..
REgards,
Atul

Hello Prem,
Thanks for information.
I know that parameter and how to check.
My specific question is about hostname buffer cache rate and how long does it hold that value in buffer and refresh rate.
Regards,
Atul

Buffer cache hit ratio query

Hi,
I would appreciate some advice please.
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 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

Data Buffer Cache Quality

Hi All,
Can somebody please please tell some ways in which i can improve the data buffer quality? Presently it is 51.2%. The DB is 10.2.0.2.0
I want to know, wat all factors do i need to keep in mind if i want to increase DB_CACHE_SIZE?
Also, i want to know how can i find out Cache Hit ratio?
Further, i want to know which are the most frequently accessed objects in my DB?
Thanks and Regards,
Nick.

Nick-- wud b DBA wrote:
Hi Aman,
Thanks. Can u please give the appropriate query for that?
And moreover when i'm giving:
SQL>desc V$SEGMENT-STATISTICS; It is giving the following error:
SP2-0565: Illegal identifier.
Regards,
Nick.LOL dude I put it by mistake. Its dash(-) sign but we need underscore(_) sign.
About the query, it may vary what you really mean by "most used obect". If you mean to find the object that is undergoing lots of reads,writes than this may help,
SELECT Rownum AS Rank,
Seg_Lio.*
FROM (SELECT St.Owner,
St.Obj#,
St.Object_Type,
St.Object_Name,
St.VALUE,
'LIO' AS Unit
FROM V$segment_Statistics St
WHERE St.Statistic_Name = 'logical reads'
ORDER BY St.VALUE DESC) Seg_Lio
WHERE Rownum <= 10
UNION ALL
SELECT Rownum AS Rank,
Seq_Pio_r.*
FROM (SELECT St.Owner,
St.Obj#,
St.Object_Type,
St.Object_Name,
St.VALUE,
'PIO Reads' AS Unit
FROM V$segment_Statistics St
WHERE St.Statistic_Name = 'physical reads'
ORDER BY St.VALUE DESC) Seq_Pio_r
WHERE Rownum <= 10
UNION ALL
SELECT Rownum AS Rank,
Seq_Pio_w.*
FROM (SELECT St.Owner,
St.Obj#,
St.Object_Type,
St.Object_Name,
St.VALUE,
'PIO Writes' AS Unit
FROM V$segment_Statistics St
WHERE St.Statistic_Name = 'physical writes'
ORDER BY St.VALUE DESC) Seq_Pio_w
WHERE Rownum <= 10; But if you are looking for the objects which are most highly in the waits than this query may help
select * from
   select
      DECODE
      (GROUPING(a.object_name), 1, 'All Objects', a.object_name)
   AS "Object",
sum(case when
   a.statistic_name = 'ITL waits'
then
   a.value else null end) "ITL Waits",
sum(case when
   a.statistic_name = 'buffer busy waits'
then
   a.value else null end) "Buffer Busy Waits",
sum(case when
   a.statistic_name = 'row lock waits'
then
   a.value else null end) "Row Lock Waits",
sum(case when
   a.statistic_name = 'physical reads'
then
   a.value else null end) "Physical Reads",
sum(case when
   a.statistic_name = 'logical reads'
then
   a.value else null end) "Logical Reads"
from
   v$segment_statistics a
where
   a.owner like upper('&owner')
group by
   rollup(a.object_name)) b
where (b."ITL Waits">0 or b."Buffer Busy Waits">0)This query's reference:http://www.dba-oracle.com/t_object_wait_v_segment_statistics.htm
So it would depend upon that on what ground you want to get the objects.
About the cache increase, are you seeing any wait events related to buffer cache or DBWR in the statspack report?
HTH
Aman....

Oracle RAW / Long / LOBs - Buffer cache?

Hello guys,
i think i have read sometime ago something about that LOBs are not cached in the buffer cache? Is this right?
I also think that i can remember that raw or long are stored in the sga buffer cache?
I can not find any official documentation about that topic... maybe you can help me...
Thanks and Regards
Stefan

Hi,
Depends on whether you are on about temporary lobs or lobs that are persisted as a column. If it's as a column then you have the cache/cache read/nocache option which is part of the lob storage clause. If it's nocache it's not put into the buffer cache and if it's cache it is (and I'm sure you can figure out from this what "cache read" means)
If it's a temporary lob then it exists in the temporary tablespace which if a true temp tablespace (which it should be) will be accessed using direct IO (so not buffered).
An the official documentation is easily found, Oracle wrote a whole manual dedicated to LOBS...
http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14249/toc.htm
HTH
Chris

Decument about buffer cache mechanism

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