Extents, segments and blocks in detail

Database structureDatabase

Table spaces

Segment

Extent

Oracle block

O/S block

Data file

logical physical

Table spaces

Belongs only to one databaseConsists of one or more operating system files (i.e. data files)Brought online while db is runningExcept for the SYSTEM table space or one with an active rollback segment, they can be taken offline while db is running

Sizing your table spaces

Identify the max size of each record Estimate the number of rows in each table at creation/start up This could be stated as instance volume

Estimate the growth of the tables. Determine how many years data you are

to hold etc. What is the increase in records per year.

Storage clauses for table spaces

Storage parameters Initial extent size of the first extent Next extent size of second extent Pctincrease % increase for each

extent SET TO 0 !!!! Minextents min number of extents Maxextents max number of extents

How extents work

Segments need one or more extents.If pctincrease is anything other than 0 then the table can grow out of hand.50

40

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0

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0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Pctincrease > 0 pctincrease = 0

Table size in MB

Creating table spaces

Create tablespace productsDatafile ‘d:/oradata/product1.dbf’

size 100MDefault storage (initial 500K

next 500Kpctincrease 0minextents 1)

Online;Creates a table space called products which is allocated 100M of disk space in a dbf file called product1.dbf (this is what is backed up) each extent will be 500K in size and will not grow.

Parameters for creating Table spaces

Table space name given when createdDatafile specifies the data file or files

for the table spaceDefault storage parameters for all objects in

table spaceMinimum ensures extents are multiples

of thisOnline/offline start statePermanent will hole permanent objectsTemporary holds temp objects e.g those used in sorts etc.

Cont …Storage parameters influence the segment storage allocation Initial size of first extent Next size of second extent Maxextents max number allowed Minextents min number allocated when

segment is created (default = 1)

Pcincrease % by which extent grows

Adding data files to table space

ALTER TABLESPACE app_data

ADD DATAFILE

‘/data/app03.dbf’ SIZE 200m;

NOTE: you should always specify the full path of file location or oracle will store them in the default directory of the database server

Changing size

Instead of adding space by adding files you can increase the existing file size.Used to enlarge a tablespace

ALTER DATABASE DATAFILE

‘/data/app02.dbf’ RESIZE 200m;

NEW SIZE

Moving datafiles

The tablespace must be offlineTarget datafiles must exist (use OS commands)Delete original file after movingMust not be SYSTEM tablespace files or have active rollbacks or temporary segments

ALTER TABLESPACE app_data RENAME DATAFILE ‘/data/app01.dbf’ TO ‘/newdata/app01.dbf’;

Dropping table spaces

Removes all contents of tablespace from databaseIf data is in tablespace INCLUDING CONTENTS option must be usedDoes not matter if tablespace is read-only or notRecommend that take tablespace offline to check effect before dropping

DROP TABLESPACE app_data INCLUDING CONTENTS;

Extents Set of contiguous number of blocks. Each segment has one or more extentsMay NOT span data files but must exist in one file

Data blocks One block corresponds to one or more

physical file blocks allocated from physical file

Set at initialisation parameters of file DB_BLOCK_SIZE

Types of table spaces

SYSTEM Data Dictionary information SYSTEM rollback segment Required in all dbs or db operations Should not contain user information but is allowed.

Non-SYSTEM Rollback segments Temporary segments Application data Application indexes

PCTFREE & PCTUSED

Block empty

Records inserted

Inserts are stopped when pctfree is met

Records deleted and amount of free space in block increases Amount of free space in block falls below the pctused and block becomes available for inserts and is added to segments free list, t remains on free list until block reaches pctfree again

pctused pctfree

Pctfree and pctused control the free space available for inserts. Pctused does not apply to indexes

PCTfree

Setting low means less space wasted in block but may not have enough room for future updatesIf not enough space to update the entire row is migrated to another blockMigration is a serious performance problem10% is normally sufficient but if chaining (migration) is common it should be monitored.

Calculation of PCTfree

Final record length – initial record lengthFinal record length

Express above as a %As final and initial record lengths in most systems are approximate then above formula will only be approximate and should be rounded to next integer

Automatic segment space management

This is new in oracle 9i and the benefits include (according to oracle)

Ease of use Better space initialisation Better concurrency handling Better performance

It users bitmaps in the data file header to map the blocks, their state and how much amount of data is in each blocksIt replaces freelist, freelist groups and pctused.Set in create tablespace command (add option SEGMENT SPACE MANAGEMENT AUTO)

Extents

An extent is a logical storage unit made up from contiguous data blocksFirst allocated when segment is created and then added when all blocks in segment are fullOracle can manage the extent allocation through DD or locallyThe header block of each segment contains a directory of extents in that segment

Allocating extents

•Oracle allocates an extent when an object is first created or when all blocks in a segment are full

•For locally managed tablespaces space for the next extent is allocated by

•Searching each data files bitmap making up the tablespace for the contiguous space required

•Exceptions raised if none of the files have enough free space.

Allocating extentsFor dictionary managed tablespaces, space for the next extent is allocated by

1. Oracle searches the tablespaces for a free extent with an exact match (or match +1)

2. If no match occurs, Oracles searches free blocks again for a free extent larger than required value

1. If one found and it is less or equal to 5 blocks bigger than requested size then it will allocate all blocks in free space to segment

2. If size greater then 5 blocks then only exact requested size is allocated

3. If step 2 fails then oracle coalesces free space in tablespace and repeats step 2

4. If step 3 fails then if auto extendible is on oracle will try to extend file and repeat step 2

5. If oracle can not extend file or auto extendible if not set then an exception is raised.

Querying extent informationQuery the extent information using:

•DBA_EXTENTS - This will list the extents allocated in the db for all segments

•DBA_FREE_SPACE - This will list information about free extents in each tablespace. This will also give and indication of how badly a tablespace is fragmented

Select tablespace_name, count(*) cnt, max(bytes)/1024 maxbytes, min (bytes)/1024 minbytes, avg (bytes)/1024 avgbytes,sum (bytes)/1024 sumbytes

From dba_free_space

Group by tablespace_name;

Segments

A logical storage unit made up of one or more extents

Every object in DB that requires space to store data is allocated a segment

If no segment storage parameters are specified when segment created then default tablespace parameters are used, (or db server defaults)

Changes to storage parameters only effect new extents nor extents in existing segments.

Querying segment information

Use:

DBA_SEGMENTS – this includes information on parameter and type of segment

V$SORT_SEGMENT – this view contains information about every sort segment in temporary tablespaces

Note: in DBA_SEGMENTS lob segments are listed as LOBINDEX for indexes and LOBSEGMENT for data

Managing undo segments

Undo segments record old values of data that were changed by a transaction

They are used to provide read consistent images of the data and to rollback uncommitted transactions when requested by user or at instance recovery.

Create UNDO TABLESPACE <name>

DATAFILE ‘undo.dbf’ size 30M

The undo spaces can be dropped by not until all transactions using them are complete and committed.

Must be large enough to handle the workload of all concurrent transactions.

Querying UNDO

You can query undo information using

DBA_ROLLBACK_SEGS – provides information bout undo segments (on and offline)

V$ROLLNAME – lists all the undo segments online use USN column to link to V$ROLLSTAT

V$ROLLSTAT – this lists undo statistics

V$UNDOSTAT – view collects 10 minute snapshots that reflect the performance of the undo tablespaces

Sample question

What is row migration?

1. A single row spread across multiple blocks

2. Move a table from one tablespace to another

3. Storing a row in a different block when there is not enough room in the current block for the row to expand

4. Deleting a row and adding it back to the same table.