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

Similar Messages

Most buffer gets and disk reads on table "CRMD_BRELVONAI"

hello friends,
we have to much buffer gets and disk reads on this table. se11 says:
2 indexes on this table:
first:
OBJKEY_A -
Index CRMD_BRELVONAI~1 exist on DB ORACLE
second:
OBJKEY_B-
Index not exist on DB ORACLE
Has anbody created the second index and good experience? OSS has no information.
thanks,
christoph

Ho Christioph,
If you double click on secondary index OBJECT_B you will get to index screen. Note that secondary index OBJECT_B as been set to No database index.
For your information, here is a copy of the related documentation.
Create the index in the database (selection)
Whether an index improves or worsens performance often depends on the database system. You can therefore set whether an index defined in the ABAP Dictionary should be created in the database.
This makes it easier to install a platform-specific customer system.
You can set this option as follows:
Index in all database systems: The index is always created in the database.
In selected database systems: The index is created depending on the database system used. In this option, you must specify the databases in which the indexes are to be created. You can do this either on an inclusive (list of systems on which it should be created) or an exclusinve (list of systems on which it should not be created) basis. In either case, you can list up to four different database systems.
No database index: The index is not created in the database. If you set this option for an index that already exists in the database, it is deleted when you activate the table in the ABAP Dictionary.
Note: Unique indexes have an extra function, and must therefore always be created in the database. The database system prevents entries or index fields being duplicated. Since programs may rely on this database function, you cannot delete unique indexes from the database.
Hopping this answer your question.
Sincerely,
Alain Gauthier

What causes BUFFER GETS and PHYSICAL READS in INSERT operation to be high?

Hi All,
Am performing a huge number of INSERTs to a newly installed Oracle XE 10.2.0.1.0 on Windows. There is no SELECT statement running, but just INSERTs one after the other of 550,000 in count. When I monitor the SESSION I/O from Home > Administration > Database Monitor > Sessions, I see the following stats:
BUFFER GETS = 1,550,560
CONSISTENT GETS = 512,036
PHYSICAL READS = 3,834
BLOCK CHANGES = 1,034,232
The presence of 2 stats confuses. Though the operation is just INSERT in database for this session, why should there be BUFFER GETS of this magnitude and why should there by PHYSICAL READS. Aren't these parameters for read operations? The BLOCK CHANGES value is clear as there are huge writes and the writes change these many blocks. Can any kind soul explain me what causes there parameters to show high value?
The total columns in the display table are as follows (from the link mentioned above)
1. Status
2. SID
3. Database Users
4. Command
5. Time
6. Block Gets
7. Consistent Gets
8. Physical Reads
9. Block Changes
10. Consistent Changes
What does CONSISTENT GETS and CONSISTENT CHANGES mean in a typical INSERT operation? And does someone know which all tables are involved in getting these values?
Thank,
...

Flake wrote:
Hans, gracias.
The table just have 2 columns, both of which are varchar2 (500). No constraints, no indexes, neither foreign key references are in place. The total size of RAM in system is 1GB, and yes, there are other GUI's going on like Firefox browser, notepad and command terminals.
But, what does these other applications have to do with Oracle BUFFER GETS, PHYSICAL READS etc.? Awaiting your reply.Total RAM is 1GB. If you let XE decide how much RAM is to be allocated to buffers, on startup that needs to be shared with any/all other applications. Let's say that leaves us with, say 400M for the SGA + PGA.
PGA is used for internal stuff, such as sorting, which is also used in determing the layout of secondary facets such as indexes and uniqueness. Total PGA usage varies in size based on the number of connections and required operations.
And then there's the SGA. That needs to cover the space requirement for the data dictionary, any/all stored procedures and SQL statements being run, user security and so on. As well as the buffer blocks which represent the tablespace of the database. Since it is rare that the entire tablespace will fit into memory, stuff needs to be swapped in and out.
So - put too much space pressure on the poor operating system before starting the database, and the SGA may be squeezed. Put that space pressure on the system and you may enbd up with swapping or paging.
This is one of the reasons Oracle professionals will argue for dedicated machines to handle Oracle software.

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

Consistent gets and physical reads

Hi all,
I am tuning a DM SQL query, by comparing execution plans with STAR TRANSFORMATION enabled or disabled. I got the following results:
STAR TRANSFORMATION ON
74889 consistent gets
254365 physical reads
STAR TRANSFORMATION OFF
1945892 consistent gets
168028 physical reads
I thought a physical read would be counted as a logical read as well, because the data block would be read from disk (1 physical IO), placed in the buffer cache and then read from there (1 more logical IO or consistent get).
So, one physical IO does not cause a logical IO?
Thanks!
Edited by: user10634835 on 12-Jul-2011 08:40

But shouldn't consistent gets be >= physical reads (Since, as per my understanding, 1 PIO causes at least 1 LIO)? In this case it is not.
74889 consistent gets
254365 physical readsJust clarifying for my knowledge.
regards

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....

Understanding Statistics io and Logical reads - is logical reads information correct

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

Physical reads, writes and logical reads,writes

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.
If you are on <=9i, then, use, v$sysstat and look for 'physical reads', 'logical reads'.
The values dervied from v$sysstat are cummulative values. i.e. values counted since the instance started.
Jaffar

Buffer app and Digg Reader not loading

The websites bufferapp.com and digg.com/reader will not load. All I see is a white screen. The blog sites for both of these sites load, only the sites above do not work. I have no issue loading these pages on Chrome.

I believe the store is overloaded, and will clear itself when people calm down loding OSX Lion.
Try a different time of day, don't forget usa is in the afternoon evening and they will be hammering Lion downloads about now after a hard day at work...

Buffer_gets and 'session logical reads'

When I compare the following values:
select sum(buffer_gets) from v$sqlarea
and
select value from v$sysstat where name='session logical reads'
it usually happens that sum(buffer_gets) > 'session logical reads'.
Does buffer_gets include logical and physical reads? As long as the SGA keeps all the SQL statments, I guess sum(buffer_gets) will be bigger than 'session logical reads'. Is this correct?
Thxs

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

Need help in understanding why so many gets and I/O

Hi there,
I have a sql file somewhat similar in structure to below:
delete from table emp;-- changed to Truncate table emp;
delete from table dept;--changed to Truncate table dept;
insert into emp values() select a,b,c from temp_emp,temp_dept where temp_emp.id=temp_dept.emp_id
update emp set emp_name=(select emp_name from dept where emp.id=dept.emp_id);
commit --only at the end
the above file takes about 9-10 hrs to complete its operation. and
the values from v$sql for the statement
update emp set emp_name=(select emp_name from dept where emp.id=dept.emp_id);
are as below:
SHARABLE_MEM PERSISTENT_MEM RUNTIME_MEM SORTS LOADED_VERSIONS OPEN_VERSIONS USERS_OPENING FETCHES EXECUTIONS PX_SERVERS_EXECUTIONS END_OF_FETCH_COUNT USERS_EXECUTING LOADS FIRST_LOAD_TIME INVALIDATIONS PARSE_CALLS DISK_READS DIRECT_WRITES BUFFER_GETS APPLICATION_WAIT_TIME CONCURRENCY_WAIT_TIME CLUSTER_WAIT_TIME USER_IO_WAIT_TIME PLSQL_EXEC_TIME JAVA_EXEC_TIME ROWS_PROCESSED COMMAND_TYPE OPTIMIZER_MODE OPTIMIZER_COST OPTIMIZER_ENV OPTIMIZER_ENV_HASH_VALUE PARSING_USER_ID PARSING_SCHEMA_ID PARSING_SCHEMA_NAME KEPT_VERSIONS ADDRESS TYPE_CHK_HEAP HASH_VALUE OLD_HASH_VALUE PLAN_HASH_VALUE CHILD_NUMBER SERVICE SERVICE_HASH MODULE MODULE_HASH ACTION ACTION_HASH SERIALIZABLE_ABORTS OUTLINE_CATEGORY CPU_TIME ELAPSED_TIME OUTLINE_SID CHILD_ADDRESS SQLTYPE REMOTE OBJECT_STATUS LITERAL_HASH_VALUE LAST_LOAD_TIME IS_OBSOLETE CHILD_LATCH SQL_PROFILE PROGRAM_ID PROGRAM_LINE# EXACT_MATCHING_SIGNATURE FORCE_MATCHING_SIGNATURE LAST_ACTIVE_TIME BIND_DATA TYPECHECK_MEM
18965 8760 7880 0 1 0 0 0 2 0 2 0 2 2011-05-10/21:16:44 1 2 163270378 0 164295929 0 509739 0 3215857850 0 0 20142 6 ALL_ROWS 656 E289FB89A4E49800CE001000AEF9E3E2CFFA331056414155519421105555551545555558591555449665851D5511058555155511152552455580588055A1454A8E0950402000002000000000010000100050000002002080007D000000000002C06566001010000080830F400000E032330000000001404A8E09504646262040262320030020003020A000A5A000 4279923421 50 50 APPS 0 00000003CBE5EF50 00 1866523305 816672812 1937724149 0 SYS$USERS 0 01@</my.sql -2038272289 -265190056 0 9468268067 10420092918 00000003E8593000 6 N VALID 0 2011-05-11/10:23:45 N 5 0 0 1.57848E+19 1.57848E+19 5/12/2011 4:39 0
1) how do i re-write this legacy script? and what should be done to improve performance?
2) Should i use PL/sql to re-write it?
3) Also help in understanding why a simple update statement is doing so many buffer gets and reading , Is this Read consistency Trap as i'm not committing anywhere in between or it is actually doing so much of work.
(assume dept table has cols emp_name and emp_id also)

update emp set emp_name=(select emp_name from dept where emp.id=dept.emp_id);I guess that these are masked table names ? Nobody would have emp_name in a dept table.
Can you re-format the output, using "code" tags ( [ or { }
Hemant K Chitale
Edited by: Hemant K Chitale on May 12, 2011 12:44 PM

Scan count 17 logical reads 176543 showing in execution plan

Hello,
I am getting scan count 17 and logical reads 145634 and some time the query takes 2 minutes and sometime 5 seconds.Thelogical reads showing against a big table(Product and Orders) that is having 10 milions records.After procedure executed it gives only 7008
records as out put.
SELECT Cust.Name
FROM dbo.Customers Cust WITH (NOLOCK)
INNER JOIN Products Prod WITH (NOLOCK) ON Cust.ID = Prod.CustID
INNER JOIN dbo.Orders Ords WITH (NOLOCK) on Cust.RepID = prod.ProdId
INNER JOIN dbo.[Address] Adds WITH (NOLOCK) on Prod.id = Adds.Id
WHERE ords.pickeddate between @startdate and @enddate
Please do the needful.

>>>>How to reduce logical reads against big tables?
Have a useful index/s . But please show the execution plan of the query as David pointed.
Best Regards,Uri Dimant SQL Server MVP,
http://sqlblog.com/blogs/uri_dimant/
MS SQL optimization: MS SQL Development and Optimization
MS SQL Consulting:
Large scale of database and data cleansing
Remote DBA Services:
Improves MS SQL Database Performance
SQL Server Integration Services:
Business Intelligence

Physical read & logical read

Hi
What is ment by Physical-Read and Logical-Read in oracle?Can anybody explain me clearly.
Aqueel.

Physical read, reading from the disk and logical reads mean reading it from the memory.
When a session(query) required information (data blocks), it will first look into the buffer cache (SGA), if it find the required data block, it will read it from the memory, i.e. logical reads, but, if it doesn't find required data block in the memory, then it read from the disk(datafile), physical read.
Generally, people motive to reduce physical read and read as much as possible from the memory, I was also int he same wrong impression until I came across a good paper by tuning expert Cary Milsap, 'why we should focus on LIO instead of PIO'. also, search http://asktom.oracle.com 'logical reads', there are good discussion on this point.
Jaffar

Logical read Vs Physical Reads

Hi,
I want to know How should we balance the logical reads and physical reads on database.
I generaly assume logical reads is always better but what if too many session accessing the same
objects it will cause latch contention.
If it is physical read then it will take more time to read from disk than reading from SGA.
If I look at statspack report I check the
Logical reads: 59,846.54 840.03
Physical reads: 1,095.91 15.38
Is there any standards for logical reads on database. If there is latch contentions can we go for the
decreasing the SGA to avoid the contentions so that there will balance between the physical reads
and logical reads.

amitbansode wrote:
I want to know How should we balance the logical reads and physical reads on database.There is no balance as it implies some kind of "+perfect ratio+" for logical I/O vs. physical I/O.
A high percentage of physical I/O can be perfectly acceptable and normal and correct for a specific database (e.g. think of a system collecting telemetry data where 90% or more of all I/O is writing new telemetry into the database and the remaining 10% is querying the data, with old data being aged out from the database using partition drops which is negligible I/O).
A high percentage of logical I/O can be indicative of a serious application design problem - where 80GB of database data is read and read again and again and again.. resulting in over a TB of logical I/O. (actually saw this in production database some years ago)
So there is no balance (e.g. not true that physical I/O = BAD and logical I/O = GOOD). No perfect cache hit ratio figure that tells you that the database is doing the right amounts of logical and physical I/O.
And I want to emphasise what Mark said - tuning requires you to identify the performance problem first, before trying to solve it.
It is very dangerous to take one metric, like the I/O cache hit ratio, and attempt to tune that. It alone is meaningless. Just like memory utilisation alone is useless and CPU utilisation alone is useless. It does not by any means point to an actual performance problem. E.g. 100% CPU utilisation can mean hardware has insufficient horses, instead of performance issues related with application design, database setting or kernel configuration.
I often repeat the following mantra here on OTN - a fundamental concept IMO for software engineering:
A solution is only as good as the problem definition.
Identify the problem first - correctly and comprehensively. And then solve it.

Kontakt 4.1.1 and Logic 9.1.1 needs Max buffer

I posted this on the NI forum too.I have 1 instance of Kontakt 4.1.1 in Logic 9.1.1 64bit with only 4 sounds loaded and have to set the buffer to Max 1024 otherwise all I get is clicks and pops on playback.The strange part is,when I first loaded this kontakt patch, it ran great with a buffer of 128.I left my computer for a couple hours and when I returned and hit play, everything was glitching out.When I bypass Kontakt, the song plays fine at a low buffer setting.when I run Kontakt, I get one of the CPU meters in Logic hitting red.Kontakt shows very low RAM and voice usage and I have 8 GB of RAM in a Mac Pro 2.66 Duo core.I have tried Logic in 32 bit mode,enabled Kontakt memory server,reloaded a blank Kontakt then loaded the patch.switched between Digi 192 (core audio) and Motu 828 MKII,rebooted,repaired permissions.The only way to get smooth playback is to set Logic's audio buffer to 1024 which makes it virtually impossible to play a part in real time.I know this isn't right as it was working fine at first.Any thoughts?

Hello -
I have a similar set-up (noted below, with Mac OS X 10.6.4). I'm still experimenting with different configurations. Currently, I have Kontakt's memory server (don't remember its official name) engaged when in 64-bit mode (both Kontakt 4 and Logic Pro). On one project, I have a total of about 32 instruments in use including ones from Kontakt and EXS. All is well.
Now, it's my understanding that when in 64-bit mode, one doesn't need to use Kontakt's memory server. Have you tried either engaging or disengaging the server?
Also. . . I'm using MOTU's 2408 MK3. As far as I know, its drivers are updated to the latest ones. This also may be helping me out. Are the drivers to your audio drivers the "latest and the greatest"??
Finally, is your OS updated to 10.6.4? Your Info currently indicates that your using OS 10.6.3. (Don't know if this makes any difference.)
It's interesting how people can have similar set-ups with differing results.

Buffer gets and logical reads

Similar Messages

Maybe you are looking for