Pctfree vs pctused

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PCTFREE Vs PCTUSED
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02-23-2001 09:12 PM
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#1
Senior Member
ckwan
Forum
PCTFREE Vs PCTUSED http://www.dbasupport.com/forums/showthread.php?7235-PCTFREE-...
1 of 9 12/02/2013 01:40

Dear all,
I want to pour some questions on the
above topic questions :
1. I come across a statement saying that
PCTUSED Parameter explanation - After a
data block becomes full, as determined by
PCTFREE, Oracle does not consider the
block for the insertion of new rows until
the 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 for
updates 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 this
only 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 reserved
for growth
resulting from updates of rows in that
data block. This
parameter has a default value of 10
percent.
For example, the value of the PCTFREE
parameter is
specified as 20 in a CREATE TABLE
statement. This
indicates that inserts to the block should
stop as soon as
#2
Junior Member
IT_girl200
2 of 9 12/02/2013 01:40

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 40
percent.
When a data block is filled to the limit
determined by the
value of the PCTFREE parameter, Oracle
considers the
block unavailable for the insertion of new
rows. The block
is unavailable for the insertion of new
rows until the
percentage filled by the data of that block
falls below the
value of the PCTUSED parameter.
Until the percentage of the block falls
below 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 as
utilization drops to
less than 40 percent. Inserts continue
until utilization
reaches 80 percent and the cycle repeats.
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3 of 9 12/02/2013 01:40

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 to
store rows.Pctfree is used to find out how
much percentage of the block will be used
to 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 the
limit exceeds 40%,still also u can insert
rows in the datablock till the limit reaches
80% (100%-20%) as u have kept pctfree
to 20%.Now if one goes on deleting the
rows,the block is not said to be free unless
and until pctused falls below 40%.As soon
as pctused falls below 40% from deleting
the rows, that block can be used to insert
the rows.In this way the cycle
continous.So it is recommended that u
never sum up
pctused+pctfree=100.Always have some
gap between them this helps in reducing
ur Oracle server for allocation and
disallocation of freelists.
If any further doubts write to
rohitsn@hotmail.com
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 would
like to share my experience.
In the last 3 months, I had interviewed 15
Oracle DBAs and asked the same
question. Only one person answered
correctly.
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
4 of 9 12/02/2013 01:40

Consider 8K block size. The total bytes 8 x
1024 = 8196 bytes
Each block requires approximately 117
bytes for the header. Please note that the
header size varies depending upon the
block 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 and
update = (8079 – (20 * 8079)/100 ) =
6463 Bytes.
Now user can insert new rows into this
block as long as the old rows’ total bytes
+ new row’s total byte is less than or
equal to 6463 bytes. If the new row’s total
byte cannot be put into this block, then
Oracle will get the next block from the
free list, and inserts into it.
When a row is updated and the row’s data
is expanded, then PCTFREE come into
play. The updated row’s data is placed
into PCTFREE’s area, provided the updated
row’s new data can be fit into PCTFREE
area. If it is not fit into that area, another
new block will be obtained from the
Freelist, and the row will be migrated. But
the original row info (pointer) is kept in
the old block. For subsequent access to
this row involves 2 read I/O. That is why
row migration should be avoided because
of excessive I/Os.
ROW DELETION:
The PCTUSED parameter value (in this
example 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 starts
deleting rows from the block. When a row
is deleted, Oracle does not put the block
into the FREELIST because it requires
many recursive calls to update the
FREELIST. The PCTUSED % (50)
determines when the block should be
added into FREELIST. When the total
bytes in the block is less than or equal to
4039 bytes, then the block will be added
into FREELIST.
If a table has high inserts and high
deletion, then you should decrease the
5 of 9 12/02/2013 01:40

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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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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#6
Senior Advisor
sambavan
6 of 9 12/02/2013 01:40

That was one damn good explanation. But
on reading this thread, I'm getting more
questions than that of answers.
1. How do one get to check the over head
for 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 the
first 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
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tamilselvan
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02-26-2001 12:44 PM
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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 rows
so often and fill the 80% up then it´s up
#8
Pando & Company
pando
7 of 9 12/02/2013 01:40

to you but I think it´s pretty rare
I say 10% but it can be less since
pctfree+pctused cant be more than 100
and if we set pctused 20 and pctfree 80
most probably we will face perfomance
issues because the block has to be put on
freelist 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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642
To add on to the question pool,
I understand that the pctused is used by
oracle to put the block into the free list for
future inserts.
1) Will any new blocks(extends) added will
directly 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 the
parameters for
PCT FREE = 100
and PCT USED=0?
Badrinath
#9
Advisor
badrinathn
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02-26-2001 01:11 PM
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badrinathn:
on site [url]http://www.ixora.com.au
/q+a/datablock.htm[/url] is good
discussion of that issue. from my
understanding, new blocks goes to so
called master free list.
pctused=0 and pctfree=100 would make
sense for insert only table.
#10
Member
LND
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8 of 9 12/02/2013 01:40

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