Pctfree vs Pctused Oracle Khaled Jouini

7/28/2019 Pctfree vs Pctused Oracle Khaled Jouini

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PCTFREE Vs PCTUSED

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02-23-2001 09:12 PM

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Oct 2000250

#1

Senior Member

ckwan

Forum


2/9

Dear all,I want to pour some questions on the

above topic questions :

1. I come across a statement saying thatPCTUSED Parameter explanation - After a

data block becomes full, as determined byPCTFREE, Oracle does not consider the

block for the insertion of new rows untilthe percentage of the block being used

falls below the parameter PCTUSED.Before this value is archived, Oracle uses

the free space of the data block only forupdates to rows already contained in the

data block.

Q : What is the reason for this - PCTUSED? Why they want to control that parameter

? Can I set it to 0 ?

Q: Is free list this pre-allocated ? Is thisonly use for PCTFREE ? Where does it

store ?

Your answer is very much appreciated.Thanks.

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02-24-2001 03:04 AM

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Feb 2001

7

re: pctused vs pctfree

hope this will provide you your answer...

The PCTFREE parameter specifies the

percentage of

space in each data block that is reservedfor growth

resulting from updates of rows in thatdata block. This

parameter has a default value of 10percent.

For example, the value of the PCTFREEparameter is

specified as 20 in a CREATE TABLEstatement. This

indicates that inserts to the block should

#2

Junior Member

IT_girl200


3/9

free space drops to 20 percent or less.The free space

thereafter can only be used for updates.

The PCTUSED parameter represents the

minimum

percentage of the used space that the

Oracle server tries

to maintain for each data block of the

table. This

parameter has a default value of 40percent.

When a data block is filled to the limit

determined by the

value of the PCTFREE parameter, Oracleconsiders the

block unavailable for the insertion of new

rows. The block

is unavailable for the insertion of newrows until the

percentage filled by the data of that block

falls below the

value of the PCTUSED parameter.

Until the percentage of the block fallsbelow the value of

the PCTUSED parameter, Oracle uses the

free space of

the data block only for updating the rows

contained in the

data block.

For example, if PCTUSED is defined as 40

percent, the

block is reused for inserts as soon asutilization drops to

less than 40 percent. Inserts continue

until utilization

reaches 80 percent and the cycle repeats.

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4/9

02-24-2001 03:31 AM

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530

Pctused and Pctfree are block storage

parameters.Pctused is used to find out how much

percentage of the block will be used tostore rows.Pctfree is used to find out how

much percentage of the block will be usedto store rows resulting from further

updates to the rows in the same

datablock.

Eg. If u keep pctused to 40% and pctfree

20.so u can insert rows till 40 %.if thelimit exceeds 40%,still also u can insert

rows in the datablock till the limit reaches80% (100%-20%) as u have kept pctfree

to 20%.Now if one goes on deleting therows,the block is not said to be free unless

and until pctused falls below 40%.As soonas pctused falls below 40% from deleting

the rows, that block can be used to insertthe rows.In this way the cycle

continous.So it is recommended that unever sum up

pctused+pctfree=100.Always have some

gap between them this helps in reducingur Oracle server for allocation anddisallocation of freelists.

If any further doubts write to

[email protected]

Advisor

rohitsn

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02-25-2001 12:48 PM

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ATLANTA, GA, USA

3,136

This is an interesting question. I wouldlike to share my experience.

In the last 3 months, I had interviewed 15Oracle DBAs and asked the same

question. Only one person answeredcorrectly.

Both the parameters are applicable and

used for each data block in the Database.I hope an example will give you the right

answer.

#4

Moderator

tamilselvan


5/9

Consider 8K block size. The total bytes 8 x

1024 = 8196 bytesEach block requires approximately 117

bytes for the header. Please note that the

header size varies depending upon theblock size.The total available bytes for data = ( 8196

117) = 8079 bytes.

A table is created with PCTFREE 20

PCTUSED 50 .

PCTRFREE in bytes = 1615

PCTUSED in bytes = 4039

Now the data available for insert andupdate = (8079 (20 * 8079)/100 ) =

6463 Bytes.

Now user can insert new rows into thisblock as long as the old rows total bytes

+ new rows total byte is less than orequal to 6463 bytes. If the new rows total

byte cannot be put into this block, thenOracle will get the next block from the

free list, and inserts into it.

When a row is updated and the rows datais expanded, then PCTFREE come into

play. The updated rows data is placedinto PCTFREEs area, provided the updated

rows new data can be fit into PCTFREEarea. If it is not fit into that area, another

new block will be obtained from theFreelist, and the row will be migrated. But

the original row info (pointer) is kept inthe old block. For subsequent access to

this row involves 2 read I/O. That is whyrow migration should be avoided because

of excessive I/Os.

ROW DELETION:

The PCTUSED parameter value (in thisexample 50 %) is the threshold limit for

the old block to be added in the FREELIST.To understand better, let us assume that a

block is of full data. Now the user startsdeleting rows from the block. When a row

is deleted, Oracle does not put the blockinto the FREELIST because it requires

many recursive calls to update theFREELIST. The PCTUSED % (50)

determines when the block should beadded into FREELIST. When the total

bytes in the block is less than or equal to4039 bytes, then the block will be added

into FREELIST.

If a table has high inserts and highdeletion, then ou should decrease the


6/9

PCTUSED value in order to minimize the

frequent update of FREELIST.

The best confidence booster is knowledge.

Good Luck, guys.

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02-26-2001 12:03 PM

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3,136

Correction in earlier post.

The overhead for each block is 107 bytes,not 117 bytes.

Tamilselvan

#5

Moderator

tamilselvan

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02-26-2001 12:27 PM

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Saskatoon, SK,

Canada3,925

#6

Senior Advisor

sambavan


7/9

That was one damn good explanation. Buton reading this thread, I'm getting more

questions than that of answers.

1. How do one get to check the over headfor different block_sizes.

2. What happens when one sets the

PCTFREE to 80%

Sorry for such a lame question, but was

curious to know the answer, atleast to thefirst one.

Thanx,Sam

Thanx

Sam

Life is a journey, not a destination!

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02-26-2001 12:41 PM

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Oracle says the overhead for each block

varies from 84 bytes to 107 bytes.I do not know how to cross check over

head bytes for various block sizes.

Does anybody know the answer?

#7

Moderator

tamilselvan

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02-26-2001 12:44 PM

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Madrid, Spain7,448

if pctfree is high then basically we are

wasting hard drive space since only 10%of block is used for inserts, of course if

you consider that you will update the rowsso often and fill the 80% up then its up

#8

Pando & Company

pando


8/9

to you but I think its pretty rare

I say 10% but it can be less sincepctfree+pctused cant be more than 100

and if we set pctused 20 and pctfree 80

most probably we will face perfomanceissues because the block has to be put onfreelist and taken off free list all the time.

As for block overhead I think there is a

formula in Oracle DBA Handbook

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02-26-2001 12:47 PM

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Jan 2001

642

To add on to the question pool,

I understand that the pctused is used byoracle to put the block into the free list for

future inserts.

1) Will any new blocks(extends) added willdirectly put into the freelist ?

2) I know that I have a table, in which I

rarely have any updates and deletes but

only insertions, then can I set theparameters for

PCT FREE = 100and PCT USED=0?

Badrinath

#9

Advisor

badrinathn

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02-26-2001 01:11 PM

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Nov 2000

212

badrinathn:on site [url]http://www.ixora.com.au

/q+a/datablock.htm[/url] is gooddiscussion of that issue. from my

understanding, new blocks goes to socalled master free list.

pctused=0 and pctfree=100 would make

sense for insert only table.

#10

Member

LND

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9/9

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