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Wednesday, July 11, 2012

Control File Parallel Read & Write wait Event

A control file contains information about the associated database that is required for access by an instance, both at startup and during normal operation . It is the Oracle control file(s) that records information about the consistency of a database's physical structures and operational statuses . The database state changes through activities such as adding data files, altering the size or location of datafiles, redo being generated, archive logs being created, backups being taken, SCN numbers changing, or checkpoints being taken.

Why Control File Waits Occur ?
Control File Waits Occur due to the following reason .
1.) This wait occurs when a server process is updating all copies of the controlfile i.e, the session is writing physical blocks to all control files at same time .
2.) The session commits a transaction to a controlfile
3.) Changing a generic entry in the controlfile, the new value is being written to all controlfiles
4.) Controlfile resides on such a disk which is heavily used i.e, facing lots of i/o's .

We can check the wait experience by a session using v$session_wait views as
SQL>select event, wait_time, p1, p2, p3 frpm v$session_wait wher event like '%control%';
Here wait_time is the elapsed time for reads or writes.

Possible steps to reduce this wait :
1.) Reduce the number of controlfile copies to the minimum that ensures that not all copies can be lost at the same time.
2.) Move the controlfile copies to less saturated storage locations.
3.) Reduce the frequent log switches . To find the optimal time and size for log switch check this post .

As far my experience , Control file access is governed by activities such as redo logfile switching and checkpointing . Therefore it can only be influenced indirectly by tuning . It rarely occurs in my report and it get solved automatically when i check the other wait metric especially "log sync wait" and others waits .

Enjoy :-)

Friday, June 29, 2012

Configure OEM in Oracle 11gR2 on Window

The Oracle Enterprise Manager, (OEM) is the standard monitoring tool for Oracle databases . The OEM allows the DBA to easily manage every aspect of the database, even non-traditional tasks like applying patches and scheduling jobs . Sometimes OEM doesn't work due to some issue and i face such a issues while re-configuring it and follow this method . Let's have a look on this below issue . All the steps are same for window or linux/unix platform . (just change path and variable where required)

C:\>set ORACLE_SID=orcl

C:\>emca -deconfig dbcontrol db -repos drop

STARTED EMCA at Jun 28, 2012 10:52:27 AM

EM Configuration Assistant, Version 11.2.0.0.2 Production

Enter the following information:

Database SID: orcl

Listener port number: 1521

Password for SYS user:

Password for SYS user: xxxx

Do you wish to continue? [yes(Y)/no(N)]: y

Jun 28, 2012 10:52:53 AM oracle.sysman.emcp.EMConfig perform

INFO: This operation is being logged at C:\app\Neerajs\cfgtoollogs\emca\orcl\emca_2012_06_28_10_52_24.log.

Jun 28, 2012 10:52:55 AM oracle.sysman.emcp.EMDBPreConfig performDeconfiguration

WARNING: EM is not configured for this database. No EM-specific actions can be performed.

Jun 28, 2012 10:52:56 AM oracle.sysman.emcp.ParamsManager checkListenerStatusForDBControl

WARNING: Error initializing SQL connection. SQL operations cannot be performed

Jun 28, 2012 10:52:56 AM oracle.sysman.emcp.EMReposConfig invoke

INFO: Dropping the EM repository (this may take a while) ...

Jun 28, 2012 11:03:43 AM oracle.sysman.emcp.EMReposConfig invoke

INFO: Repository successfully dropped

Enterprise Manager configuration completed successfully

FINISHED EMCA at Jun 28, 2012 11:03:44 AM

Now, creating OEM

C:\>sqlplus sys/xxxx@orcl as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Thu Jun 28 11:07:03 2012

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> drop user SYSMAN cascade ;

drop user SYSMAN cascade

ERROR at line 1:

ORA-01918: user 'SYSMAN' does not exist

SQL> drop user MGMT_VIEW ;

drop user MGMT_VIEW

ERROR at line 1:

ORA-01918: user 'MGMT_VIEW' does not exist

SQL> drop role MGMT_USER ;

drop role MGMT_USER

ERROR at line 1:

ORA-01919: role 'MGMT_USER' does not exist

Drop all the dependent synonyms of "sysman"

SQL> spool drop_synonym.sql

SQL> SELECT 'drop public synonym ' || synonym_name || ';' FROM dba_synonyms WHERE table_owner = 'SYSMAN';

no rows selected

If we have got any output here , then run the below script .

SQL> @drop_synonym.sql

Let's Configure EM

C:\>set ORACLE_SID=orcl

C:\>set ORACLE_HOME=C:\app\Neerajs\product\11.2.0\dbhome_1

C:\>emca -config dbcontrol db -repos create

STARTED EMCA at Jun 28, 2012 11:08:59 AM

EM Configuration Assistant, Version 11.2.0.0.2 Production

Enter the following information:

Database SID: orcl

Exception in thread "main" oracle.sysman.emcp.exception.DatabaseUnavailableException: Database instance unavailable.

at oracle.sysman.emcp.DatabaseChecks.throwDBUnavailableException(DatabaseChecks.java:151)

at oracle.sysman.emcp.DatabaseChecks.checkDbAvailabilityImpl(DatabaseChecks.java:144)

at oracle.sysman.emcp.DatabaseChecks.checkDbAvailability(DatabaseChecks.java:163)

at oracle.sysman.emcp.DatabaseChecks.getDbServiceName(DatabaseChecks.java:582)

at oracle.sysman.emcp.EMConfigAssistant.performConfiguration(EMConfigAssistant.java:1272)

at oracle.sysman.emcp.EMConfigAssistant.statusMain(EMConfigAssistant.java:574)

at oracle.sysman.emcp.EMConfigAssistant.main(EMConfigAssistant.java:522)

and the emca log contains the below details :

oracle.sysman.emcp.util.GeneralUtil initSQLEngineLoacly

CONFIG: ORA-01031: insufficient privileges

oracle.sysman.assistants.util.sqlEngine.SQLFatalErrorException: ORA-01031: insufficient privileges

at oracle.sysman.assistants.util.sqlEngine.SQLEngine.executeImpl(SQLEngine.java:1655)

at oracle.sysman.assistants.util.sqlEngine.SQLEngine.connect(SQLEngine.java:978)

at oracle.sysman.emcp.util.GeneralUtil.initSQLEngineLoacly(GeneralUtil.java:445)

at oracle.sysman.emcp.util.GeneralUtil.initSQLEngine(GeneralUtil.java:637)

at oracle.sysman.emcp.DatabaseChecks.checkDbAvailabilityImpl(DatabaseChecks.java:118)

From the above logs, we find two hints i.e, database instance is down and ORA-01031 . We search on meta-link and find some useful information . So, some possible reasons may be :

1.) Database may not be up.

2.) Database is started setting environment variable ORACLE_HOME with trailing '/'. Reset ORACLE_HOME and bounce the database.

3.) Confirm that the database is up. Connect to the database via sql*plus and do:-

C:\>sqlplus sys/xxxx@orcl as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Thu Jun 28 17:25:00 2012

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select instance_name,status,version from v$instance;

INSTANCE_NAME STATUS VERSION

---------------- ------------ -----------------

orcl OPEN 11.2.0.1.0

4.) Confirm whether the $ORACLE_HOME environmental variable is correct. This involves comparing the current ORACLE_HOME environmental setting, with the one that was set the last time the database was started.

a.) To check the current ORACLE_HOME :-

set ORACLE_HOME (windows)

b.) To check the $ORACLE_HOME which was set at the time when the database was last started .Connect to sqlplus as SYS as SYSDBA and issue the following:-

SQL> var OHM varchar2(100);

SQL> EXEC dbms_system.get_env('ORACLE_HOME', :OHM) ;

PL/SQL procedure successfully completed.

SQL> PRINT OHM

OHM

<Oracle_Home which was set at the time that the database was last started will be returned here>

c.) If the ORACLE_HOME was set wrongly at the time that the database was started (as determined by above steps), then the database will need to be shut down and re-started. To shut the database down, it will be necessary to set the 'wrong' ORACLE_HOME (the one which is returned in the OHM output). After the database has been successfully shut down, then the correct ORACLE_HOME should be set, and the database can be re-started.

5.) Confirm whether it's possible to connect to the database as the SYSDBA user remotely, using sqlplus. As part of the repository creation/drop process, the SYS user needs to be able to connect as SYSDBA . The ORA-01031: insufficient privileges error is returned when the SYS user is unable to connect as SYSDBA. To test this outside of dbconsole, test a sqlplus remote connection (one which connects via a tnsnames.ora alias) as sysdba. eg:-

$ORACLE_HOME/bin>sqlplus sys/<password>@<alias> as sysdba

If the above returns the ora-1031 error, check the following :-

a.) Check whether the REMOTE_LOGIN_PASSWORDFILE initialization parameter is set correctly in the database. For the purposes of configuring dbconsole, it must be set to 'EXCLUSIVE'. This value can be checked by logging into the database via sql*plus and typing "show parameter remote".

SQL> show parameter remote

NAME TYPE VALUE

---------------------------- --------- --------------

remote_login_passwordfile string EXCLUSIVE

If this parameter is not set to 'EXCLUSIVE', the following command can be run .

SQL>alter system set remote_login_passwordfile=EXCLUSIVE scope=spfile;

b.) For dbconsole 11.2.0.1 and 11.2.0.2 check whether the %ORACLE_HOME%/network/admin/sqlnet.ora (or %TNS_ADMIN%/sqlnet.ora) contains the entry .

SQLNET.AUTHENTICATION_SERVICES = (NTS)

In my case the value of SQLNET.AUTHENTICATION_SERVICES is NONE . So, i changed to NTS and finally able to configure the OEM . The rest are as

C:\>set ORACLE_SID=orcl

C:\>set ORACLE_HOME=C:\app\Neerajs\product\11.2.0\dbhome_1

C:\>emca -config dbcontrol db -repos create -SID ORCL -PORT 1521 -ORACLE_HOME C:\app\Neerajs\product\11.2.0\dbhome_1

STARTED EMCA at Jun 28, 2012 11:51:08 AM

EM Configuration Assistant, Version 11.2.0.0.2 Production

Enter the following information:

Listener ORACLE_HOME [ C:\app\Neerajs\product\11.2.0\dbhome_1 ]: C:\app\Neerajs\product\11.2.0\dbhome_1

Password for SYS user:

Password for DBSNMP user:

Password for SYSMAN user:

Email address for notifications (optional):

Outgoing Mail (SMTP) server for notifications (optional):

-----------------------------------------------------------------

You have specified the following settings

Database ORACLE_HOME ................ C:\app\Neerajs\product\11.2.0\dbhome_1

Local hostname ................ ramtech-199

Listener ORACLE_HOME ................ C:\app\Neerajs\product\11.2.0\dbhome_1

Listener port number ................ 1521

Database SID ................ ORCL

Email address for notifications ...............

Outgoing Mail (SMTP) server for notifications ...............

-----------------------------------------------------------------

Do you wish to continue? [yes(Y)/no(N)]: y

Jun 28, 2012 11:51:48 AM oracle.sysman.emcp.EMConfig perform

INFO: This operation is being logged at C:\app\Neerajs\cfgtoollogs\emca\orcl\emca_2012_06_28_11_51_07.log.

Jun 28, 2012 11:51:51 AM oracle.sysman.emcp.EMReposConfig createRepository

INFO: Creating the EM repository (this may take a while) ...

Jun 28, 2012 12:55:16 PM oracle.sysman.emcp.EMReposConfig invoke

INFO: Repository successfully created

Jun 28, 2012 12:55:53 PM oracle.sysman.emcp.EMReposConfig uploadConfigDataToRepository

INFO: Uploading configuration data to EM repository (this may take a while) ...

Jun 28, 2012 1:07:37 PM oracle.sysman.emcp.EMReposConfig invoke

INFO: Uploaded configuration data successfully

Jun 28, 2012 1:07:58 PM oracle.sysman.emcp.util.DBControlUtil configureSoftwareLib

INFO: Software library configured successfully.

Jun 28, 2012 1:07:58 PM oracle.sysman.emcp.EMDBPostConfig configureSoftwareLibrary

INFO: Deploying Provisioning archives ...

Jun 28, 2012 1:10:46 PM oracle.sysman.emcp.EMDBPostConfig configureSoftwareLibrary

INFO: Provisioning archives deployed successfully.

Jun 28, 2012 1:10:47 PM oracle.sysman.emcp.util.DBControlUtil secureDBConsole

INFO: Securing Database Control (this may take a while) ...

Jun 28, 2012 1:11:26 PM oracle.sysman.emcp.util.DBControlUtil secureDBConsole

INFO: Database Control secured successfully.

Jun 28, 2012 1:11:27 PM oracle.sysman.emcp.util.DBControlUtil startOMS

INFO: Starting Database Control (this may take a while) ...

Jun 28, 2012 1:14:55 PM oracle.sysman.emcp.EMDBPostConfig performConfiguration

INFO: Database Control started successfully

Jun 28, 2012 1:15:05 PM oracle.sysman.emcp.EMDBPostConfig performConfiguration

INFO: >>>>>>>>>>> The Database Control URL is https://ramtech-199:5500/em <<<<<<<<<<<

Jun 28, 2012 1:15:48 PM oracle.sysman.emcp.EMDBPostConfig invoke

WARNING:

************************ WARNING ************************

Management Repository has been placed in secure mode wherein Enterprise Manager data will be encrypted. The encryption key has been placed in the file: C:/app/neerajs/product/11.2.0/dbhome_1/ramtech-199_orcl/sysman/config

/emkey.ora. Please ensure this file is backed up as the encrypted data will become unusable if this file is lost.

***********************************************************

Enterprise Manager configuration completed successfully

FINISHED EMCA at Jun 28, 2012 1:15:48 PM

Enjoy :-)

Tuesday, May 29, 2012

Interested Transaction List ( ITL ) in Oracle

Few days back, I received an e-mail from a reader , who is having doubt on locking mechanism i.e, related to ITL . This post will give some good concept about the ITL . Let's start from the beginning i.e, from the transaction .

Transaction : A transaction starts when an update to data such as insert, update or delete occurs (or the intention to do so, e.g. SELECT FOR UPDATE) and ends when the session issues a commit or rollback. Like everything else, a specific transaction should have a name or an identifier to differentiate it from another one of the same type. Each transaction is given a transaction ID.

Locks : In multi-user systems, many users may update the same information at the same time. Locking allows only one user to update a particular data at a time , another person cannot modify the same data. When any user start the transaction then oracle automatically locks the data . Oracle uses a different locking method then most other databases, Oracle locking policy consists of the following :

Oracle locks data at the row level on modification only. There is no lock escalation to a block or table level, ever.
Oracle never locks data just to read it. There are no locks placed on rows of data by simple reads.
A writer of data does not block a reader of data .
A writer of data is blocked only when another writer of data has already locked the row it was going after.

What is Interested Transaction List (ITL) ?

It is a simple data structure called "Interested Transaction List" (ITL), a list that maintains information on transaction. The transaction identifier will be stored as an entry in the ITL in the header of the data block. The ITL contains several placeholders (or slots) for transactions. When a row in the block is locked for the first time, the transaction places a lock in one of the slots. In other words, the transaction makes it known that it is interested in some rows (hence the term "Interested Transaction List").

When a different transaction locks another set of rows in the same block, that information is stored in another slot and so on. When a transaction ends after a commit or a rollback, the locks are released and the slot which was used to mark the row locks in the block is now considered free .

ITL slots are required for every transaction, it contains the transaction ID (XID) which is a pointer to an entry in the transaction table of a rollback segment. We can still read the data but other processes wanting to change the data must wait until the lock is released (commit or rollback). The ITL entry contains a XID, undo byes address (UBA) information, flags indicating the transaction status (Flag) and lock count (Lck) showing the number of rows locked by this transaction within the block and SCN at which the transaction is updated.

How many slots are typically available in ITL ?

During the table creation, the INITRANS parameter defines how many slots are initially created in the ITL. INITRANS is a block level storage parameter which can be specified while creating an object (table). INITRANS and MAXTRANS parameters are used to control the concurrent access to the same block . When the transactions exhaust all the available slots and a new transaction comes in to lock a row, the ITL grows to create another slot. The ITL can grow up to the number defined by the MAXTRANS parameter of the table, provided there is space in the block. Nevertheless, if there is no more room in the block, even if the MAXTRANS is high enough, the ITL cannot grow. the maximum value for MAXTRANS parameter is 255.

Each ITL entry in the block transaction variable header takes 24 bytes. Though a block can have a maximum of 255 different ITL entries, the ITL allocation is limited by block size. The database block size plays an important role when allocating the number of inital ITL entries for the block. The rule is "the total size allocated for initial ITLs SHOULD be LESS THAN 50% of the database block size" .

i.e, sizeof(INITIAL ITLs) < ( 50% of the DATABASE BLOCK SIZE )

BLOCK SIZE(B) NO OF ITL entries allocated in block header

============ =============================

2048 41

4096 83

8192 169

What happens when a transaction does not find a free slot to place its lock information?

This can occur because either

1.) the block is so packed that the ITL cannot grow to create a free slot, or

2.) the MAXTRANS has already been reached. In this case, the transaction that needs to lock a row has to wait until a slot becomes available. This wait is termed as ITL waits and can be seen from the view v$session_wait, in which the session is waiting on an event named "enqueue" Since the INITRANS is one, there is only one slot for the ITL. The rest of the block is empty.

How to Diagnose the ITL Wait How do we know that a segment is experiencing ITL waits?

To check for ITL waits, set up the STATISTICS_LEVEL to TYPICAL in init.ora or via ALTER SYSTEM, then examine the segment statistics for the waits.

SQL>select owner ,object_name from v$segment_statistics where statistic_name='ITL waits' and value> 0 ;

When we suspect that a database is suffering from these waits, query the view v$session_wait. If the event on which the system is waiting is "enqueue," then the session might be experiencing ITL waits. However, enqueue is a very broad event that encompasses any type of locks, so it does not accurately specify the ITL waits. When the wait event is experienced, issue the following complex query a table tab1 (say)

SQL> Select s.sid SID, s.serial# Serial#, l.type type, ' ' object_name, lmode held, request request

from v$lock l, v$session s, v$process p

where s.sid = l.sid and s.username <> ' ' and s.paddr = p.addr and l.type <> 'TM' and

(l.type <> 'TX' or l.type = 'TX' and l.lmode <> 6)

union

select s.sid SID, s.serial# Serial#, l.type type, object_name object_name, lmode held, request request

from v$lock l, v$session s, v$process p, sys.dba_objects o

where s.sid = l.sid and o.object_id = l.id1 and l.type = 'TM' and s.username <> ' ' and s.paddr = p.addr

union

select s.sid SID, s.serial# Serial#,l.type type, '(Rollback='||rtrim(r.name)||')' object_name, lmode held,

request request

from v$lock l, v$session s, v$process p, v$rollname r

where s.sid = l.sid and l.type = 'TX' and l.lmode = 6 and trunc(l.id1/65536) = r.usn and s.username <> ' ' and s.paddr = p.addr order by 5, 6 ;

The output of the query will look something like this:

SID SERIAL# TY OBJECT_NAME HELD REQUEST

----- ---------- ------- ------------------- ------- -------------

36 8428 TX 0 4

36 8428 TM TAB1 3 0

52 29592 TM TAB1 3 0

52 29592 TX (Rollback=RBS1_6) 6 0

Note how the sessions 36 and 52 both have a TM (DML) lock on the table TAB1 of type 3 (Row Exclusive), but session 52 also holds a TX (Transaction) lock on the rollback segment of mode 6 (Exclusive) and Session 36 is waiting for a mode 4 (Share) lock. If this combination of locking occurs, we can be sure that session 36 is waiting for ITL on the table TAB1. Beware of a similar but different diagnosis when two sessions try to insert the same key value (a real locking – primary key violation). In that case, we would also see an additional TX lock on a rollback segment from the session that is waiting; for ITL waits, this TX lock on the rollback segment would not be seen. To know more about working of ITL click here

Examining ITL allocation :

1.) Create a table with INITRANS 4.

SQL> create table test1 (I number) INITRANS 4 ;

2.) Insert few record for testing purpose. A blockdump can be taken later.

SQL> insert into test1 values (10) ;

SQL> commit ;

now updated the table from sessions too these transaction are in hang state .

3.) Find out the block number and the file id for dumping the block :

SQL> select dbms_rowid.rowid_relative_fno(rowid), dbms_rowid.rowid_block_number(rowid) from test;

or use the below command

SQL> select header_file,header_block from dba_segments where owner='HR' and segment_name='TEST' ;

4.) Dump the data block:

SQL> alter system dump datafile 4 block min 636 block max 636 ;

5.) Open the dump trace file located in USER_DUMP_DEST directory and check the following:

Block dump from disk:

buffer tsn: 4 rdba: 0x0100027c (4/636)

scn: 0x0000.002a1814 seq: 0x02 flg: 0x04 tail: 0x18140602

frmt: 0x02 chkval: 0x1561 type: 0x06=trans data

Hex dump of block: st=0, typ_found=1

Block header dump: 0x0100027c

Object id on Block? Y

seg/obj: 0x1277b csc: 0x00.2a1372 itc: 10 flg: E typ: 1 - DATA

brn: 0 bdba: 0x1000278 ver: 0x01 opc: 0

inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc

0x01 0x0001.001.00000380 0x00c01117.0204.07 --U- 2 fsc 0x0000.002a1376

0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

0x04 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

bdba: 0x0100027c

data_block_dump,data header at 0xe511924

itl = interested transaction list (list of transactions on a block that have changes on that block)

xid = transaction id (it is a set of three numbers – undo segment#, undo slot# and undo record# - separated by periods.)

uba = undo block address (pointer to a transactions list of undo blocks)

scn/fsc = system change/commit number of the current transaction

tsn = Tablespace Number

rdba = Relative data block address.Its value is "0100027c" in hexa decimal & 16777852 in decimal

seg/obj = is the object_id. 0x 1277b = 75643 in 75643 in decimal.

itc: 10 = Number of interested transaction list(ITL) entry in the block.

typ: 1 = data –> Indicates this is an data block

Flag = State for current transaction (C – Commited) U- indicates that the particular ITL was used.

Lck = Number of Lock held by the transacion

csc: 0x00.2a1372 = This is the comit SCN number of the block

If we use locally managed tablespaces with automatic segment space management , we can more or less forget about these parameters as Oracle takes care of it for us . In Oracle 10g and above, this is now more to concern . In the past they were much more important that they are now .

Note :

Lck” column shows a value. It shows “2”, meaning two rows in the blocks is locked by a transaction. But, which transaction? To get that answer, note the value under the “Xid” column. It shows the transaction ID - 0x0001.001.00000380 . These numbers are in hexadecimal (as indicated by the 0x at the beginning of the number).
A transaction is identified by a transaction ID (XID) which is a set of three numbers – undo segment#, undo slot# and undo record# - separated by periods.
We can also check all the active transactions in the view v$transaction, where the columns XIDUSN, XIDSLOT and XIDSQN denote the undo segment#, undo slot# and undo rec# - the values that make up the transaction ID.
The v$transaction view also contains the session address under SES_ADDR column, which can be used to join with the SADDR column of v$session view to get the session details

References:- http://www.rampant-books.com/

Enjoy :-)