Dec 31, 2013

Buffer Aging , LRU Lists,Dirty List & DBWR

All Oracle data is obtained by users from the buffer cache.The basic purpose of the cache is to minimize physical disk I/O by holding (buffering) copies of requested data blocks in memory.

Buffer Aging and LRU Lists

    An Oracle database typically has many more data blocks on disk than memory buffers in the buffer cache. Since not all blocks can be buffered in the cache at once, new block requests (cache misses) must find room in the cache to be read in from disk. When this happens, another block in the cache is usually discarded since the cache is normally full (and fixed in size).

 The buffer cache is carefully designed to favor keeping frequently requested blocks in memory and allow less popular blocks to be replaced by new block requests.
These decisions about which blocks to replace are made using a least recently used (or LRU) algorithm. This algorithm uses a data structure called the LRU list. This list basically orders the buffers in the cache according to when they were last accessed by a user.

 When a block is accessed, it is moved to the MRU (most recently used) end of the list. Blocks in the cache that are not accessed for awhile will find more and more blocks ahead of them in the list, and they will be closer to the LRU end of the list. This is also known as buffer aging in the LRU list.

Buffers are replaced in the cache from the least recently used end of the LRU list. This helps insure that frequently accessed buffers are not discarded, as they are regularly moved to the MRU end of the list with each access.

This mechanism of keeping the most requested blocks in the buffer cache is normally very effective at minimizing disk I/O. Managing a single LRU list can sometimes be a bottleneck in a heavily loaded database

DBWR and the Dirty List

In addition to the LRU list Oracle keeps a list of buffers that contain data that has been modified by users. This list is called the dirty list. Changed data committed by users must eventually be written to disk, as this is the permanent storage of the database.

 The DBWR background process is responsible for moving blocks from the dirty list to their permanent locations in disk files.

Dirty blocks cannot be replaced in the cache until they have been written to disk, otherwise the changes would be lost. An overabundance of dirty buffers can negatively impact cache efficiency by reducing available slots for new blocks.

This can happen when DBWR is unable to keep up with the volume of update activity.

 Multiple DBWR processes can be configured in this case to increase the capacity to write out dirty blocks.

Buffer Cache Problems 

The LRU algorithm of the Oracle buffer cache is normally very good at providing efficient minimization of physical disk I/O. However, there are some situations where normal buffer cache aging may not be the best option for overall performance, for instance:

•Blocks that should not go to the MRU end of the list
•Blocks that should be excluded from aging and stay in the cache .

The first situation can occur when very large tables are accessed randomly by users with very little block use overlap between users. In this case, the MRU end of the list is flooded by blocks that will not result in subsequent cache hits yet age other blocks down to the LRU end of the list.

These other blocks may be replaced when they could have resulted in cache hits had they been kept. The second situation occurs when there are data blocks that will definitely be requested regularly and we want to insure that physical disk I/O is not necessary to obtain their data.
An example might be small lookup tables, or perhaps specific indexes.

The multiple buffer pool feature allows greater control over buffer cache usage to help address these problems and obtain even better cache performance.


PGA stands for Process Global Area which is also known as Program Global Area. It is called global area because is keeps information which is required by all modules of Oracle Code. PGA keeps information specific to the sever process upon which Oracle code acts.

PGA also keeps information about oracle shared resources so that it can free those resources .
Each session contains specific information like bind variables and runtime structures in a private SQL area.

Whenever a session executes a statement, a private SQL area is assigned to that session.
Even if multiple users are issuing the same statement using the same shared SQL area, each session will have its own dedicated private SQL area.

A private SQL area contains data such as bind information and runtime memory structures.
Each user that submits the same SQL statement has his or her own private SQL area that uses a single shared SQL area.

Thus, many private SQL areas can be associated with the same shared SQL area. A private SQL area itself is divided into run-time area and persistent area.

Persistent area contains information like bind variable and will be freed once the cursor is closed.
Run-time area is allocated with the first step of the execute request and will be freed when execution is completed.

Each instance will have its own SGA .The SGA contains data buffer areas, redo log buffers and the shared pool .
Each area is important to the database overall performance.

The Oracle kernel process uses an LRU (Least Recently Used) algorithm to write data back to the disks.
Data is never altered on the disks directly, but is altered in memory first.

The shared SQL pool is used to store the Dictionary Cache as well as information about SQL statements that are being run against the database.

Dec 20, 2013

Row chaining & Migration



       The datafiles are the physical storage space on the server. Storage in the datafiles is allocated by blocks to each extent that is used by an object. The block size is operating system-dependent and is determined when the database is created and cannot be changed. The database block size is set in the database parameter file DB_BLOCK_SIZE. Typically, a database block is either 2K or 4K.

•The PCTFREE parameter specifies the percentage of space in each segment's data block reserved for future record expansion into the block.

If records within a block are rarely updated after they are inserted, you should set PCTFREE lower to allow for full space usage. If records within a block are subject to many updates, you should set PCTFREE higher to allow for more growth.

Once the PCTFREE threshold is reached, no additional rows will be inserted until the PCTUSED threshold is reached due to deletion of data. If PCTFREE is set too low, row chaining and row migration will result because updates to the record will not fit into the block.
After that point, no new rows can be inserted into that block until the free space percentage falls below the PCTUSED threshold. The default for PCTFREE is 10%.

The PCTFREE and PCTUSED parameters tell Oracle when to link and unlink a block from the freelist chain. This is only relevant if you are not using the new Automatic Segment Space Management .

•The PCTUSED parameter defines the minimum percentage of data block used space that is necessary before the block is eligible for row insertion. A segment block is added to the free space list once its used space falls below this threshold. The default for PCTUSED is 40%.

If data is static, you will be able to set the PCTUSED lower and more fully use the space. If large amounts of data are inserted and deleted, you should set the PCTUSED higher to prevent block fragmentation.

You might set these parameters as follows: CREATE table emp (......) PCTFREE 5 PCTUSED 80.

The combined sum of PCTFREE and PCTUSED must be less than 100. Correct choice of these parameters can be used to improve the efficiency of table and index segments.

For example, tables that are insert-only (auditing tables, history tables, etc.) should have a PCTFREE setting of 1.

This simple change to a table's storage definition can reduce its total disk space requirements by 10%.
This means less object extension and less physical disk I/O when reading and writing: 10% more data can be read or written with a single data block read; 10% more data can be held in the database's SGA.
This is a very simple way of increasing your block buffer cache without having to buy any more memory.

INITRANS is the initial number of concurrent transactions allocated in each block header when the block is allocated. The default is 1, which is the minimum. The maximum is 255. The size of each transaction entry is operating system-dependent.

MAXTRANS is the maximum number of concurrent transactions for the blocks within the tablespace. The default is 255, which is the maximum. The minimum setting is 1.

Oracle Managed Datafiles (OMFs)

Oracle introduced Oracle Managed Files (OMF) concept in Oracle 9i. OMFs simplify the administration task of an Oracle Database. Oracle manages these files for the administrators.
Prior to Oracle9i, when you dropped a tablespace you would also have to manually remove the physical datafile associated with that tablespace from the operating system.

If a wrong datafile is removed manually from the Operating System, then the option remaining was to do the recovery of the file from the recent backup, which means either a bigger downtime or the tablespace containing the removed datafile was unavailable for the users.
This manual task of removing the datafile is taken over by Oracle. Hence, with Oracle-Managed Datafiles physical file management is left to the database itself.

Oracle-Managed Datafiles (OMFs) give Oracle the ability to manage database files for you.The database internally uses standard file system interfaces to create and delete files as needed.

OMFs can be used when creating database datafiles, tempfiles, online redo logfiles, and database control files.
Before using OMFs, the database must be configured for OMF use. After the database is configured

If the database creates an Oracle Managed Control File, and the database uses SPFILE, then the control_file parameter is automatically added to the SPFILE with the location of the control file.

If PFILE is used instead of SPFILE, then the control_file parameter needs to be added manually else the instance startup will fail.