Concept of database buffer cache

Similar Questions

• Write the list of database buffer Cache

  What is the list of write in the database buffer cache? What is he function?
  
  Thanks in advance.

  >
  What is the list of write in the database buffer cache? What is he function?
  >
  The list of writing to dirty buffers that have not been written to disk yet.

  See "Database Buffer Cache" in the Concepts of database 11g doc
  http://docs.Oracle.com/CD/B28359_01/server.111/b28318/memory.htm
  >
  Organization of the Database Buffer Cache

  The buffers in the cache are organized into two lists: list of Scripture and the list (LRU) least recently used. List of writing contains stamps Sales, but which contain data has changed, has not yet been written to disk. The LRU list holds free buffers, pinned buffers and dirty buffers that have not yet moved to the list of Scripture. Free buffers contain no useful data and can be used. Pinned buffers are being accessed.

• CKPTQ in the database buffer cache and LRU

  Hi experts
  
  
  This feature can settle in cache buffers data base Oracle 10.2 or higher.
  Forums of sources: OTN and 11.2 Concepts guide
  
  According to my readings. To improve the functionality and make it more good American cache database is divided into several zones which are called workareasNow more
  
  Zoom this each activities will store multiple lists to store tampons inside the database buffer cache.
  
  Each wrokarea can have one or more then one lists to keep the wrokordering in there. The list of what each activity will have therefore to list LRU and list CKPTQ. LRU list
  
  is a list of buffers pinned, free and sales and CKPTQ is a list of stamp Sales. We can say THAT CKPTQ is a group of stamps Sales ordering of RBA weak and ready to be flushed from the cache on the disk.
  
  CKPTQ list is maintained by ordering of low RBE.
  As novice let me clearly low RBA and RBA senior first
  
  RBA is stored in the header of the block and we will give the information on which this block is spent and how many times it is changed.
  
  Low RBE: low RBE is the address to redo the first change that was applied to the block since his own last.
  RBA high: the high RBA is the address to redo the last change has been applied to the block.
  
  Now back to CKPTQ
  It can be like this (pathetic CKPTQ diagram)
  
  lowRBA = high RBA
  (Head of the CKPTQ)                         (CKPTQ line)
  
  CKPTQ is a list of stamp Sales. According to the concept of the RBA. The most recent modified buffer is at the tail of CKPTQ.
  
  Now the oracle process starts and try to get the DB cache buffer if she gets a buffer it will put an end SRM to the list.and buffer buffer LRU will become the most
  
  recently used.
  
  Now, if the process cannot find a necessary buffer.then first, he will try to find free tampons to LRU. If he finds his most he will place a datablock to the data file in the
  
  place where free buffer was sitting. (Good enough).
  
  Now, if the process cant fnd a buffer without LRU then first step would be he will find some Sales swabs at the end of the LRU to LRU list and place them on a
  
  CKPTQ (do not forget in the low order of RBA he organize it queue of CKPT). and now the oracle process will buffer required and place it on the end of the MRU of LRU list. (Because space was acclaimed by the displacement of Sales to CKPTQ buffers).
  
  I do not know of CKPTQ buffers (to be more precise tampon Sales) will move to datafiles.all buffers are line up n lower CKPTQ RBA way first. But
  
  emptied to datafile how and in what way and to what event?
  
  That's what I understand after these last three days, flipping through the blogs, forums and concepts guide. Now miss me you please erase me on and off it
  
  I can't bind the following features at this rate... It's
  
  (1) how the work of additional checkpoint with this CKPTQ?
  
  (2) now, what is this 3 second delay?
  
  (Every 3 seconds DBWR process will wake and find if nothing to write about the data files for this DBWR will check only CKPTQ).
  
  Apartment 3) form 3 second funda, when CKPTQ buffers will be moved? (IS IT when the process is unable to find any space in CKPTQ to keep buffers LRU. Its a
  
  moment where CKPTQ buffer will be moved on the disk)
  
  (4) can you please report when the control file will be updated with checkpoint so it can reduce recovery time?
  
  In many ques but I'm trying to build the entire process in mind that its operation may be I can be wrong in any phase in any stage, please correct me upward and
  
  Take me @ the end of the flow.
  
  
  Thank you
  Philippe

  Hi Aman,

  Yes, I a soft copy of ppt or white paper "Harald van Breederode" of 2009.

  -Pavan Kumar N

• What else are stored in the database buffer cache?

  What else are stored in the database buffer cache, except the data reading of data files blocks?

  The nitty gritty on this point, you ask someone smarter than me waaay.

• UNIX buffer cache vs oracle database buffer cache

  1. What is the difference between the operating system buffer cache and the oracle buffer cache?
  2. in what cases, oracle uses oracle buffer cache? in this case, oracle does not use buffer cache instead of use the OS buffer cache.
  
  I'm confused about this concept.
  
  Appreciate any help.
  
  S.

  Database instance does not use the OS buffer cache in 2 cases:

  1 FILESYSTEMIO_OPTIONS is set to ARM or SETALL (see http://download.oracle.com/docs/cd/E11882_01/server.112/e16638/os.htm#PFGRF94412).

  2 database instance uses ASM with raw devices.

  Edited by: P. Forstmann on July 19. 2011 18:43
  It is not ASM with ASMLib but ASM with gross as devices explained ASMLib and Linux block devices

  Edited by: P. Forstmann on July 19. 2011 19:36

• Database buffer cache hit ratio is less than 90%

  Hi all
  
  For the following query I get the bottom 90%
  
  SELECT ROUND (((1-(phy.) VALUE /(cur.) VALUE con. ((VALUE))) * 100), 2) "HIT RATIO" +.
  OF SYS.v_$ sysstat news,.
  SYS.v_$ sysstat con,.
  SYS.v_$ sysstat phy
  WHERE cur.NAME = 'db block gets. "
  AND con.NAME = "becomes" compatible
  AND phy.NAME = 'physical reads. "
  
  RATE
  ----------
  81,79 +.
  
  Please advice me what could be the reasons and how do I make more than 90%.
  I have access to metalink. I raise this issue with Oracle metalink?
  
  Thank you and best regards,
  Arun Kumar

  user8853422 wrote:
  I'm afraid I can't do that... because I'm still new to DBA... What is my understanding that if buffer cache hit ratio is high, it means that server process do not need to recover data from data files, instead it will look in the buffer cache. Time is less and therefore better performance.

  Your understanding is "basically" correct.

  Please correct me if I'm wrong.

  However, your understanding is limited to a set of very specific circumstances. In reality there are times where it may be reasonable to pick up the disc.

  Basically, the concept of a specific BCHR is excellent if you have exactly the same kind of question asked time and time again (limited OLTP) ensure that guests have repeatedly looking at things in memory, or if you have enough memory to fit the entire database AND all potential read consistent blocks in the SGA.

  Anything less and you're looking at compromises.

  And that 'compromise' requirement that throws the usefulness of the BCHR whack. You need to look at the complete picture.

  Therefore, we can honestly say: If you follow your buffer Cache Hit Ratio over a period of time and your workload (transaction rate, type of transaction and competition) remains about the same, then you could grant using BCHR or use it as an indicator of the health of your system.

  But watch the BCHR and say, "this number is good" and "this number is bad" is essentially meaningless.

• LMD are in the DB buffer Cache?

  DB version: 11.2.0.4

  OS: RHEL 6.5

  Oracle documentation defines "database buffer cache" in the following way

  "The buffer of database cache, also called the buffer cache, is the area of memory that stores copies of data in blocks to read data files"

  http://docs.Oracle.com/CD/E25054_01/server.1111/e25789/memory.htm#i10221

  Let's say that the following UPDATE statement updates 100,000 records. The server process go get all the blocks that has the matching records and place it in the DB buffer cache and updates the blocks. This change metadata are recorded in the restore log buffer by progression. At the next checkpoint changed blocks are written to the data files. Right?

  Employee UPDATE

  SET salary = salary * 1.05

  The EMPLOYEE

  WHERE deptnum = 8;

  basically: Yes, once again. Oracle has rewritten Sales buffers (i.e. buffers with different content than the corresponding blocks on the disk) on the drive under certain conditions. If the operation is cancelled, the blocks must be read again in the cache and change must be cancelled (and written).

• Updated data are larger than the buffer cache

  Hi Experts,

  I have a small request. I have a table called CONTENT to have 12 GB of data. Here, I pulled a single update statement that updates to 8 GB of CONTENTS table of data using 1 GB of database buffer cache?

  How 1 GB of the Database Buffer Cache will be used to update the 8 GB of data? Level architectural any additional changes will happen (than usual) when executing "Updated data is larger than the buffer cache"?

  Could someone of you please response. Thank you

  Database: 10.2.0.5

  OS: Power 5-64 bit AIX system

  Hello

  the basic mechanism is the following:

  needed to update data blocks are read from the data files and cached in memory (buffer cache), the update is made to the buffer cache and the front of the (UNDO) image is stored in the segments of cancellation, operation (update here) is re-encoded to redo buffer until it goes again files If the buffer is samll or we need more space in the buffer cache, or we have a control point or... Oracle writes back the block modified data files to free the memory buffer for the more blocks.

  While the other runs the update of transactions can read before you change the image of CANCEL if validation, at the end of the transaction done change is confirmed and validation is recorded in the redo. If the cancellation is made at the end of the transaction before the image is "restored" and rollback is saved in do it again.

  Concerning

• Buckets of the interaction between the physical i/o and buffer cache hach

  I read the book Expert Oracle practices on issues of lock contention. While I was reading this chapter. I'm little bit confused the bahavior of the buffer cache when physical i/o occurs. According to Tom Kyte, when data blocks are read on disk (if missed cache) the following steps occurred. Ask Tom & amp; quot; How to work the Database Buffer Cache? & amp; quot;

  (a) access to the buffer cache and search for block

  (b) if the block isn't here, perform physical i/o and put it in the cache

  (c) return the block of memory cache

  However, I wonder what stage b has occurred which means put the data block in the buffer cache. For this, the data block is added to the associated buffer cache hash buckets?

  As far as I know, in order to cache data hit intended address of applicable block in buffer cache hash bucket. The hash function is generated during this data block address. (if acquired the lock of the child) And then find the address of the data block in the cache buffer chain to locate the block of buffer in the buffer cache.

  My second question is what stage an occur (go to the buffer cache and search block) how to block oracle look? I mean where did he look like? My third question relates to my second question, Oracle if discovered that the block in the buffer cache, does know where to find them? I guess that Oracle does not know where to locate it in the buffer cache. Therefore, it uses cache buffer hash buckets. I'm wrong?

  Last question is, I'm just trying to understand how buffer cache buffers are linked, buffer cache buffer hash, how it works?

  Thanks in advance.

  > What happens if planned lines resident for other blocks of data in the table? How can you join the other data block addresses? And, how do you know that lines which are located in what block of data?

  See this demo:

  Microsoft Windows XP [Version 5.1.2600]

  Copyright (C) 1985-2001 Microsoft Corp.

  C:\Documents and Settings\Administrateur > sqlplus scott/tiger

  SQL * more: Production release 11.2.0.1.0 Wed Dec 18 09:01:50 2013

  Copyright (c) 1982, 2010, Oracle.  All rights reserved.

  Connected to:

  Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

  With partitioning, OLAP, Data Mining and Real Application Testing options

  SQL > drop table test is serving;

  Deleted table.

  SQL > create table test in select * from object;

  Table created.

  SQL > set line 200.

  SQL > column nom_segment for a20;

  SQL > select nom_segment, segment_type, header_file, header_block dba_segments where nom_segment like 'TEST '.

  NOM_SEGMENT SEGMENT_TYPE HEADER_FILE, HEADER_BLOCK

  -------------------- ------------------ ----------- ------------

  4 1218 TEST TABLE

  Average for test table header block are 1218 that resides in file number 4.

  SQL > SELECT

  2 dbms_rowid.rowid_relative_fno (rowid) REL_FNO,

  3 dbms_rowid.rowid_block_number (rowid) BLOCKNO

  4 test where object_name = 'EMP ';

  REL_FNO BLOCKNO

  ---------- ----------

  4 2443

  SQL > variable s/n varchar2 (30)

  SQL > exec: s/n: = dbms_utility.make_data_block_address (4, 2443);

  PL/SQL procedure successfully completed.

  SQL > print s/n

  S/N

  --------------------------------

  16779659

  SQL > SELECT

  2 dbms_rowid.rowid_relative_fno (rowid) REL_FNO,

  3 dbms_rowid.rowid_block_number (rowid) BLOCKNO

  4 test where object_name = 'I_AUDIT ';

  REL_FNO BLOCKNO

  ---------- ----------

  4 1223

  SQL > exec: s/n: = dbms_utility.make_data_block_address (4, 1223);

  PL/SQL procedure successfully completed.

  SQL > print s/n

  S/N

  --------------------------------

  16778439

  SQL >

  Then, I got two dBA for different lines that are in the block No. 2443 and 1223.

  Concerning

  Girish Sharma

• Why the blocks of temporary tables are placed in the buffer cache?

  I read the following statement, which seems quite plausible to me: "Oracle7.3 and generates from close db file sequential reading of the events when a dedicated server process reads data from temporary segment of the disc." Older versions of Oracle would read temporary segment data in the database buffer cache using db file scattered reads. Releases latest exploit heuristics that data of temporary segment is not likely to be shareable or revisited, then reads it directly to a server process programs global (PGA). »
  
  To verify this statement (and also for the pleasure of seeing one of these rare close db file sequential read events), I ran a little experiment on my Oracle 10.2 Linux (see below). Not only it seems that different this v above, the blocks of temporary tables are placed in the buffer cache, but also$ BH. OBJD for these blocks does not refer to an object in the database's existing (at least not one that is listed in DBA_OBJECTS). Either incidentally, I traced the session and have not seen any file db close sequential read events.
  
  So, I have the following questions:
  (1) is my experimental set-up and correct my conclusions (i.e. are blocks of temporary tables really placed in the buffer cache)?
  (2) if so, what is the reason for placing blocks of temporary tables in the buffer cache? As these blocks contain private session data, the blocks in the buffer cache can be reused by another session. So why do all cache buffer management fees to the blocks in the buffer cache (and possibly remove) rather than their caching in a private in-memory session?
  (3) what V$ BH. OBJD consult for blocks belonging to temporary tables?
  
  Thanks for any help and information
  Kind regards
  Martin
  
  Experience I ran (on 10.2 /Linux)
  =============================

  SQL*Plus: Release 10.2.0.1.0 - Production on Sun Oct 24 22:25:07 2010
  
  Copyright (c) 1982, 2005, Oracle.  All rights reserved.
  
  
  Connected to:
  Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
  With the Partitioning, OLAP and Data Mining options
  
  SQL> create global temporary table temp_tab_4 on commit preserve rows as select * from dba_objects;
  
  Table created.
  
  SQL> alter system flush buffer_cache;
  
  System altered.
  
  SQL> select count(*), status from v$bh group by status order by 1 desc;
  
    COUNT(*) STATUS
  ---------- -------
        4208 free
        3 xcur
  
  SQL> select count(*) from temp_tab_4;
  
    COUNT(*)
  ----------
       11417
  
  SQL> -- NOW THE BUFFER CACHE CONTAINS USED BLOCKS, THERE WAS NO OTHER ACTIVITY ON THE DATABASE
  select count(*), status from v$bh group by status order by 1 desc;
  SQL> 
    COUNT(*) STATUS
  ---------- -------
        4060 free
         151 xcur
  
  SQL> -- THE BLOCKS WITH THE "STRANGE" OBJD HAVE BLOCK# THAT CORRESPOND TO THE TEMPORARY SEGMENT DISPLAYED
  -- IN V$TEMPSEG_USAGE
  select count(*), status, objd from v$bh where status != 'free' group by status, objd order by 1 desc;
  SQL> SQL> 
    COUNT(*) STATUS      OBJD
  ---------- ------- ----------
         145 xcur       4220937
        2 xcur        257
        2 xcur        237
        1 xcur        239
        1 xcur    4294967295
  
  SQL> -- THE OBJECT REFERENCED BY THE NEWLY USED BLOCKS IS NOT LISTED IN DBA_OBJECTS
  select * from dba_objects where object_id = 4220937 or data_object_id = 4220937;
  
  SQL> 
  no rows selected
  
  SQL> SQL> -- THE BLOCKS WITH THE "STRANGE" OBJD ARE MARKED AS TEMP IN V$BH
  select distinct temp from v$bh where objd = 4220937;
  SQL> 
  T
  -
  Y
  
  SQL> 

  Edited by: user4530562 the 25.10.2010 01:12
  
  Edited by: user4530562 the 25.10.2010 04:57

  The reason to put the blocks to the global temporary table in the buffer cache is the same thing why you put ordinary table blocks in the cache buffers-> you want some of them to be in memory.

  If you ask why don't keep us somehow temporary tables in the PGA - well what happens if this temporary table will be 50 GB? 32-bit platforms cannot even handle this and you do not want a process of becoming uncontrollable so great.

  Moreover, TWG will allow you to restore, back to a backup (or savepoint implied when an error occurs during a call DML), and this requires protection by the cancellation. Place lines / revenge in PGA would have complicated the implementation even further... now GTT is almost of the regular tables which just happened to reside in temporary files.

  If you really want to put data in the PGA only, then you can create collections of PL/SQL and even access through the use of SQL (coll CAST AS xyz_type) where xyz_type is a TABLE of an object any.

  --
  Tanel Poder
  New online seminars!
  http://tech.e2sn.com/Oracle-training-seminars

• clarification of term required for the DB buffer Cache

  Hello!!
  
  Here, I have a very basic conceptual question, DB buffer Cache contains the data read from the data in the file as well reduce the disk i/o of fom and oracle. If suppose that the table is constantly questioned and remains in the DB buffer cache all the time, how does oracle ensures the user gets the latest information?

  The database buffer cache is a part of the zone system Global (SGA), which is responsible for caching of blocks accessed frequently for a segment. The subsequent transactions involving the same blocks can then access them from memory, instead of from the hard disk. The works of cache buffer of database on the basis of least recently used (LRU algorithm), according to which the most frequently accessed blocks are kept in memory while the less frequent are gradually.

  See this link

  http://www.Stanford.edu/dept/ITSS/docs/Oracle/10G/server.101/b10743/memory.htm :)

• Oracle 11g R2 buffer Cache

  Hello

  We have a production Oracle 11 g r2 standard database with the following general settings:

  Shared pool: 608 MB

  Buffer cache: 608 MB

  ...

  SGA 1417 MB total

  We have noticed that over time, shared pool takes memory from the buffer cache, even if automatic shared memory management is disabled.

  The only way we can redistribute the memory is returned to the original settings by restarting the database service. We tried to redistribution of memory by using the following commands unsuccessfully.

  alter system set shared_pool_size = 608M;

  change the value system DB_cache_size = 608M.

  The application provider insists that automatic memory management should not be enabled.

  Could someone provide guidance as to 1. Why the buffer cache loses the memory of the shared pool 2. How can we redistribute the memory without having to restart the database service.

  Thank you

  Heck

  Hemant asked that means "standard"?

  In the meantime, you can pass by http://www.ora600.be/_memory_imm_mode_without_autosga+-+no+really+! + n % 27T + resizing + my + CMS +! + I + medium + TI +!

  HTH

  Anand

• Shared pool larger than the buffer cache

  Hi all
  
  My database is 10.2.0.4 running linux platform
  
  No .of 2 cpu, RAM-2 GB
  
  SGA_TARGET wa set to 1 GB.
  
  Initially the memory have been configured as a shared pool around 300 MB and the buffer cache about 600 MB.
  
  When I questioned the v$ sga_resize_ops views I found some interesting results.
  
  Many operations and growth reduction were happened and the current size of the shared pool is about 600 MB and buffer cache is 300 MB. (this happened during last 1)
  
  I guess that the buffer cache must always be larger than the size compared to a shared pool. My assumption is right?
  
  Is it because of sql code using the do not bind variables resulting in growth shared pool? No relaods and radiation are almost ignored I think it should not be the case.
  
  Also no lock events listd in the top5
  
  I've also seen the 15% of the shared pool is marked as being of kGH:NO ACCESS, which means that the part is used for the cache buffers.
  
  Should I set the lower limit for the shared pool and the buffer cache or can I just ignore it.
  
  Thank you
  rajdhanvi

  You change your question now... your question was that he has sharedpool large size > buffer cache is acceptable... . Check your own second post... for your new question now is why pool continues to increase and partly used as buffer cache... the proof is given by tanel poder y what happens when EAMA is used... For the Kingston general hospital: NO ACCESS means that no one else could touch...

  Concerning
  Karan

• DB buffer Cache

  Hi all
  
  As I walked through the oracle architecture, a doubt was is me appeared and thought it would be the right place to ask.
  
  What happens when a user tries to read a complete picture (table scan) say about 10 GB in size for a database with only 2 or 3 gb DB size cache buffer.
  
  Your contributions will help greatly.
  
  Thank you.

  Full scan of the table, to add to what has been already said, will not replace your entire buffer cache. Tables above a certain threshold are considered "large" and will use only a fraction of the buffer cache. They use and reuse the same portion of the cache, cycling through the same cache blocks. So in your example, if you have a cache of pads/3 GB and 10 GB table to perform a full scan, assuming that it's a classic way to read (use the cache and do not playback via the direct path), you will never have 3 GB of the table in the cache of the buffers at any time.

• How to increase! Buffer Cache Hit Ratio

  Hi all
  
  my database performance is low how to increase the buffer Cache Hit Ratio
  
  Database version: 8.1.7.0.0
  ~ Data upward since: 09:56:23, September 23, 2010
  ! Buffer Cache Hit Ratio: 81.6157
  ~ The library Cache Miss Ratio: 0.03
  ~ Dictionary Cache Miss ratio: 6.6979
  
  [Use of the shared pool] Exec time 0 seconds
  ~ Total unused Mb: 251.88
  ~ Total MB used: 98.12
  ~ MB total: 350
  ~ % Pool used: 28.04

  Buffer Cache Hit Ratio is an indicator of sense of the performance of the system.
  Are there users complain? If there are no complaints, there is also no problem.

  The best way to increase the buffer Cache Hit Ratio is executing statspack identify offensive SQL and to grant this SQL offensive.

  ----------
  Sybrand Bakker
  Senior Oracle DBA