AWR's buffer cache reads and logical reads

In AWR report under "Segments by Logical Reads" section, there is a total logical reads, I assume it is in unit of block. Under "IOStat by Function summary" section, it has buffer cache reads in unit of bytes. Shouldn't the number of logical reads x 8k (if the block size is 8k) = the number of buffer cache reads?
They are not equal, not even close, does anybody know why? Thanks

Hi,
buffer gets = number of times a block was requested from buffer cache. A buffer get always request in a logical read. Depending on whether or not a copy of the block is available in the buffer cache, a logical read may or may not involve a physical read. So "buffer gets" and "logical reads" are basically synonyms and are often used interchangeably.
Oracle doesn't have a special "undo buffer". Undo blocks are stored in rollback segments in UNDO tablespace, and are managed in the same way data blocks are (they're even protected by redo). If a consistent get requires reading from UNDO tablespace, then statistics counters will show that, i.e. there will be one more consistent get in your autotrace.
For more information and some examples, see a thread at askTom:
http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:549546900346542976
Best regards,
Nikolay

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Find physical reads and logical reads ?

Hi,
how will find out physical reads and logical reads ?

Well I would suggest you read the report from statspack/awr.They as suggested by Amit,have a load profile section.That would be helpful for you in finding the details of this thing. Also which version you are? If you are in 10g than the EM is able to give you a compare period report where you can compare 2 different days's periods information and can check which particular part has changed.
In addition to this , look for the information of the metrics in the documentation.As the physical read and logical reads are statistics which are happening in the system.So from 10g onwards, oracle is keeping a track in the deflectionin the statistics. So if you see that than it willbe easy for you to manage and monitor it.
I shall try to findthe name of some views related to it and post.
Aman....

Buffer gets and logical reads

Hello !
Does the counter of event buffer gets include the logical reads ?
Does the mertic buffer gets include the event of reading from undo buffer ?
Thanks and regards,
Pavel
Edited by: Pavel on Jun 27, 2012 3:08 AM
Edited by: Pavel on Jun 27, 2012 3:35 AM
Edited by: Pavel on Jun 27, 2012 4:13 AM

Hi,
buffer gets = number of times a block was requested from buffer cache. A buffer get always request in a logical read. Depending on whether or not a copy of the block is available in the buffer cache, a logical read may or may not involve a physical read. So "buffer gets" and "logical reads" are basically synonyms and are often used interchangeably.
Oracle doesn't have a special "undo buffer". Undo blocks are stored in rollback segments in UNDO tablespace, and are managed in the same way data blocks are (they're even protected by redo). If a consistent get requires reading from UNDO tablespace, then statistics counters will show that, i.e. there will be one more consistent get in your autotrace.
For more information and some examples, see a thread at askTom:
http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:549546900346542976
Best regards,
Nikolay

What is the difference of public-read and public-read protected

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class FatherClass{
public-read var a;
public-read protected var b;
init{
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    b=1;
    println(b);
class SubClass extends FatherClass{
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var subClass=SubClass{};

As you see in my code, subclass can write the variable with either public-read and public-read protected. What write access do the public-read protected widen? That's because your subclass is in the same script as the variable. Move the subclass to a separate script and it would not be able to write when the access is public-read.
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Guys, I am very confuse about the definitions of Nonrepeatable Reads and Phantom Reads when I read the BEA Documentaion in http://e-docs.bea.com/workshop/docs81/doc/en/wls/guide/advanced/EJBsAndTransactions.html
The definition was queit confusing to me because they seems to describe the same thing, but it is obviously not the same thing. The problem is to see the difference among then.
What is that difference? Could I have an practical example of that difference?
Thanks everyone

Non-Repeatable reads: When a row is updated in database. 2 reads may not return same data.
Phantom reads is where a new row is inserted into the database, hence the 2 selects may not return same set of data.
Chekc the link below for example queies. hope it helps.
http://en.wikipedia.org/wiki/Isolation_(computer_science)
#Phantom_Reads

Difference between db file sequential read and scattered read

Hi,
Oracle Version : 10.2.0.1
Operating system: Linux
Can any one please help me what is the difference between db file sequential read and scattered read or please give any best related links .
Thanks & Regards,
Poorna Prasad.

>
A sequential read is a single-block read. Single block I/Os are usually the result of using indexes.
A db file scattered read issues a scattered read to read the data into multiple discontinuous memory locations. A scattered read is usually a multiblock read. It can occur for a fast full scan (of an index) in addition to a full table scan.
>
See Performance Tuning Guide:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/instance_tune.htm#i20526
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/instance_tune.htm#i15958
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Hi,
This question arises during a performance test - on SQL Server 2012 with SP2.
In the following example, table has only column and that is of data type INT.
When inserted 592 records of data type INT it is doing only 1 logical read but as soon another record is inserted SP is reporting 2 logical reads. Why?
In the code, i have highlighted difference between
statistics io - logical reads and sys.dm_exec_procedure_stats.total_logical_reads
to understand the difference between these 2 information.
set nocount on
GO
create table dbo.test_storage_and_logical_reads
employee_number int --primary key
GO
go
CREATE procedure dbo.test_sp_logical_reads
as
begin
select
employee_number
from dbo.test_storage_and_logical_reads
order by employee_number desc
end
go
insert into dbo.test_storage_and_logical_reads
(employee_number)
VALUES (1)
GO 592
EXEC sp_spaceused 'dbo.test_storage_and_logical_reads'
--set statistics io on
--GO
exec dbo.test_sp_logical_reads
GO
----Table 'test_storage_and_logical_reads'. Scan count 1, logical reads 1, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
---- But sys.dm_exec_procedure_stats.total_logical_reads is reporting 3 instead.
truncate table dbo.test_storage_and_logical_reads
GO
insert into dbo.test_storage_and_logical_reads
(employee_number)
VALUES (1)
GO 593
EXEC sp_spaceused 'dbo.test_storage_and_logical_reads'
exec dbo.test_sp_logical_reads
GO
----Table 'test_storage_and_logical_reads'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
---- But sys.dm_exec_procedure_stats.total_logical_reads is reporting 4 instead.
--drop procedure dbo.test_sp_logical_reads
--drop table dbo.test_storage_and_logical_reads
GO
NB: I do understand the logical and physical reads. Thanks.
For quick review of new features, try virtual labs: http://msdn.microsoft.com/en-us/aa570323

Hi.
I still need to test the scenario but if you read definition of this DMV it says 'Returns aggregate performance statistics for cached stored procedures' so I guess, I am not sure 3 can be due to this aggregated output given by this DMV. Will test it
on SS 2012 SP2 will get back to you.
Please mark this reply as answer if it solved your issue or vote as helpful if it helped so that other forum members can benefit from it.
My TechNet Wiki Articles
Hi Shanky / Sean Gallardy
i think Sean Gallardy created the "test_sp_logical_reads" table in master DB, i have that same issue when i accidentiatly created the table "test_sp_logical_reads" in master, but when i create the same table in USER DB like "sample1"
it only allocate 1 page for 592,
hi Asam,
Let me narrow down the your question, if my understanding is correct ...
<<When you have the free space in a page of a Heap, why SQL Server is assigning New page for a New Record>>
Answer is in PFS bytes
The answer is that PFS bytes are not fully reset until the page is reallocated. On deallocation, the only bit in the PFS byte that's changed is the allocation status bit - this makes it very easy to rollback a deallocation
--Before inserting the 593 record plese execute the below query
--Note Replace "database1" with your DBName
DBCC TRACEON (3604);
DBCC IND ('database1', 'test_storage_and_logical_reads', 1);
--Result
PageFID PagePID IAMFID IAMPID ObjectID IndexID PartitionNumber PartitionID iam_chain_type PageType IndexLevel NextPageFID NextPagePID PrevPageFID PrevPagePID
1 2770 NULL NULL 517576882 0 1 72057594039828480 In-row data 10 NULL 0 0 0 0
1 2769 1 2770 517576882 0 1 72057594039828480 In-row data 1 0 0 0 0 0
--your intrested in the second record(page)
DBCC PAGE ('database1', 1, 2769,3) WITH TABLERESULTS;
--you can find 38th row as
--PFS (1:1) = 0x64 MIXED_EXT ALLOCATED 100_PCT_FULL
--which means your page is full
--you can try inserting 300 rows it will show you PFS (1:1) = 0x61 MIXED_EXT ALLOCATED 50_PCT_FULL
please refer in the below link
http://blogs.msdn.com/b/sqlserverstorageengine/archive/2006/07/08/under-the-covers-gam-sgam-and-pfs-pages.aspx
http://aboutsqlserver.com/2013/12/17/sql-server-storage-engine-heap-tables/
"This page has a PFS byte value of 0x04 - how can it be full when its not allocated?"
The answer is that PFS bytes are not fully reset until the page is reallocated. On deallocation, the only bit in the PFS byte that's changed is the allocation status bit - this makes it very easy to rollback a deallocation.
Here's an example. Using a database with a simple table with one row.
A DBCC PAGE of the IAM page includes:
PFS (1:1) = 0x70 IAM_PG MIXED_EXT ALLOCATED 0_PCT_FULL
If I run the following:
BEGIN
TRANSACTION
DROP
TABLE T1
GO
And then do the DBCC PAGE again, the output now includes:
PFS (1:1) = 0x30 IAM_PG MIXED_EXT 0_PCT_FULL
And if I rollback then transaction, the DBCC PAGE output reverts to:
PFS (1:1) = 0x70 IAM_PG MIXED_EXT ALLOCATED 0_PCT_FULL
Thanks
Saravana Kumar C

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hi there,
is there any way to find the physical reads and writes, logical reads and writes
other than statspack report?

It depends which version of Oracle are you using.
If you are using 10g, you can get very comprehensive information from the dictionary views.
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Attachments:
Multicounter_Synchronization_App_Note.doc ‏55 KB

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