Oracle 11g -Snapshot Standby and Active Data Guard - oracle-info

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Oracle 11g -Snapshot Standby
and Active Data Guard
HSBC TECHNOLOGY AND SERVICES
Topic Coverage
2

Concepts and Basics




Redo logs, archive logs and standby redo logs
Attributes of log_archive_dest for standby
Standby Protection modes
Redo transport services, Redo apply services, Gap resolution

10g/11g Snapshot standby

11g Active data guard
Concepts and Basics
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History of Standby database
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Online Redo Log files, Archive log files and Standby Redo Logfiles
Online redo log file (ORL)
- Contains persistently stored changed redo records.
- Redo records in the log files are stored in SCN sequential order—that is, the order in
which redo was written.
- When ORL are full, they become archived to the archive redo logs.
Archived redo log file
- Archived versions of online redo logs.
- These files are archived by the archive processes to one or more defined log archive
destinations.
Standby redo log file (SRL)
- are used to store redo data received from another database.
- are structurally identical to redo logs, and are created and managed using the same
SQL statements used to create and manage redo logs.
- Less data loss in failover situations than without Standby Redo Logs.
- Are only used if you have the LGWR activated for archival to the Remote Standby
Database. Hence are necessary for Real time log apply.
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Standby redo log file (SRL)… cont
Assume
Primary database - two redo log groups with one member each. Logfile size 500 MB
Then in this case,
Standby Database - at least 3 standby redo log groups to satisfy the requirement that a
standby redo log must have at least one more redo log group than the redo log groups
at the primary database.
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Synchronous
 A transaction cannot
Redo
commit until all redo
generated by that transaction
has been successfully sent
to every enabled redo
transport destination that
uses the synchronous redo
transport mode.
 Used by the Maximum
Protection and Maximum
Availability modes
Asynchronous (default)
 A transaction can commit
Transport Modes
without waiting for the redo
generated by that transaction
to be successfully sent to
any redo transport
destination that uses the
asynchronous redo transport
mode.
 Used by the Maximum
Performance mode
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Synchronous Redo Transport Mode
 The user commits a transaction creating a redo record. The LGWR reads the redo record
from the log buffer, writes it to the online redo log file, and waits for confirmation from the
LNS.
 The LNS reads the same redo record from the log buffer and transmits it to the standby
database using Oracle Net Services. The RFS receives the redo at the standby database
and writes it to a standby redo log file.
 When the RFS receives a write-complete from the disk, it transmits an acknowledgment
back to the LNS process on the primary database, which in turn notifies the LGWR that
transmission is complete. The LGWR then sends a commit acknowledgment to the user.
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Myth Buster: LGWR Transmits Redo to Standby Databases
A common misconception is that the LGWR is the process that transmits data to a standby
database. This is not the case. The LNS process manages all synchronous and
asynchronous redo transmissions. Eliminating this perception is the reason why the Data
Guard 11g documentation simply refers to the redo transport methods as SYNC or
ASYNC, rather than LGWR SYNC or LGWR ASYNC as was done in previous releases.
LNS
The LogWriter Network Service (LNS) reads the redo being flushed from the redo buffers
by the LGWR and performs a network send of the redo to the standby site. The main
purpose of the LNS process is to alleviate the LGWR process from performing the redo
transport role.
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Asynchronous Redo Transport Mode
 LGWR does not wait for acknowledgment from the LNS i.e. LGWR will continue to
acknowledge commit success even if the redo of previous transactions is delayed/not
sent (due to some reasons) to the standby database immediately .
 If the LNS is unable to keep pace and the log buffer is recycled before the redo can be
transmitted to the standby, the LNS automatically transitions to reading and sending
from the ORL (Data Guard 11g onward). Once the LNS has caught up, it automatically
transitions back to reading/sending directly from the log buffer.
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Few attributes of log_archive_dest_n attributes
 AFFIRM and NOAFFIRM:
AFFIRM - specifies that a redo transport destination acknowledges received redo data
after writing it to the standby redo log.
NOAFFIRM - specifies that a redo transport destination acknowledges received redo
data before writing it to the standby redo log.
 VALID_FOR
VALID_FOR=(redo_log_type,database_role)
redo_log_type keyword can assume following values:
– ONLINE_LOGFILE—This destination is valid only when archiving ORL files.
– STANDBY_LOGFILE—This destination is valid only when archiving SRL files.
– ALL_LOGFILES— This destination is valid when archiving either online redo log
files or standby redo log files.
database_role keyword identifies the role in which this destination is valid for archiving:
– PRIMARY_ROLE—This destination is valid only when the database is running in
the primary role.
– STANDBY_ROLE—This destination is valid only when the database is running in
the standby role.
– ALL_ROLES—This destination is valid when the database is running in either the
primary or the standby role.
Default: (ALL_LOGFILE,ALL_ROLES)
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Few attributes of log_archive_dest_n attributes… cont
 REOPEN
Specifies the minimum number of seconds before redo transport services should
try to reopen a failed destination.
 COMPRESSION
Specifies whether redo data is transmitted to a redo transport destination in
compressed form or uncompressed form when resolving redo data gaps.
NOTE: Redo transport compression is a feature of the Oracle Advanced Compression
option. You must purchase a license for this option before using the redo transport
compression feature.
 SYNC and ASYNC
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Why Isn’t ARCH Redo Transport mentioned in the Data Guard 11g Documentation?
Three redo transport methods were documented prior to Data Guard 11g: SYNC, ASYNC,
and ARCH. ARCH refers to traditional archive log shipping, in which Data Guard would wait
for an ORL to be archived before the contents of the resulting archive log file where shipped
by an ARCH process.
Data Guard 11g ASYNC performance enhancements have led Oracle to deprecate ARCH as
a documented redo transport method. Though deprecated, the functionally still exists to use
ARCH for redo transport and provide backward compatibility for previous customer
installations.
The ARCH transport infrastructure also continues to be used transparently by Data Guard
11g when automatically resolving archive log gaps between primary and standby databases.
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Modes
 Max Protection
Primary
ORL
COMMIT
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Standby
SRL
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 Max Availability
Standby
Primary
ORL
SRL
COMMIT
Fault preventing rdo to be written to standby
Max Availability
Max Performance
fault corrected and archive gaps resolved
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 Max Performance
Primary
ORL
Standby
SRL
COMMIT
Command to put standby database in one of the protection modes:
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The protection modes in the order from highest (most data protection) to the
lowest (least data protection):
Yes
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Redo apply services at standby
Real time redo apply services
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Automatic Gap resolution
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Automatic Gap resolution
 ARCH process on primary continuously pings the standby during outage to determine its
status.
 When communication with the standby is restored, the ARCH ping process queries the
standby control file (via its RFS process) to determine the last complete log file that the
standby received from the primary database.
 It determines which log files are required to resynchronize the standby database and
immediately begins transmitting them using additional ARCH processes. Simultaneously, at
the very next log switch, the LNS will attempt and succeed in making a connection to the
standby database and will begin transmitting current redo while the ARCH processes resolve
the gap in the background.
 Once the standby apply process is able to catch up to current redo records, the apply
process automatically transitions out of reading from archived redo logs, and into reading
from the current SRL (assuming the user has configured Data Guard real-time apply).
 Beginning with Data Guard 10g, one ARCH process at the primary database is always
dedicated to local archival to ensure that remote archival during gap resolution does not
impact the ability of the primary to recycle its ORLs.
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Dataguard components (Recap)
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Redo transport Security Enhancements in 11g
 Redo Transport Authentication Using a Password File
- By default password of SYS is used to authenticate redo transport sessions
- 11g onwards, we can also specify a user other than SYS for redo transport
authentication using REDO_TRANSPORT_USER init parameter
- This user should have SYSOPER privileges
- Requires password for this user to be the same at primary and all standbys
- REDO_TRANSPORT_USER parameter be set to the same value on both primary and
all standbys
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Snapshot standby
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Snapshot standby
Features:
 Fully updateable standby
 Provides disaster recovery and data
protection
 Requires Flashback database
 Simplified Single command in 11g
Differences
f
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Steps to convert Physical Standby Database to the Snapshot Standby Database
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Steps to convert the Snapshot Standby Database to the Physical Standby Database
Limitations
Flashback Database is used to convert a snapshot standby database back into a physical
standby database. Any operation that cannot be reversed using Flashback Database technology
will prevent a snapshot standby from being converted back to a physical standby.
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Using Grid control
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Benefits
 Reduced database licensing costs
Since we can now easily transform a physical standby database from its disaster
recovery mode into application testing mode, this translates into one less Oracle 11g
database that needs to be licensed.
 Reduced hardware costs
One less Oracle database also means one less database server
 Real Application Testing
A snapshot standby database is a perfect complement to Oracle 11g’s new Real
Application Testing suite because it’s an excellent platform on which to replay a
previously-recorded workload.
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Active standby database
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Difference: 11g Active and Snapshot standby
Active Standby
 Read Only - redo from primary
received and applied
Snapshot Standby
 Read Write - redo from primary
received but not applied
 Redo applied when snapshot
standby database is converted back
to physical standby
Note: Both Active Data Guard and Snapshot standby require a physical standby database in place
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What is Active Data Guard?
 Introduced in 11g Release 1
 Available in the database server Enterprise Edition as an additional license option
 Real time replica of a production database which is open in read only mode while
changes transmitted from the primary database are being simultaneously applied to it.
 Applies to physical and not logical standby database
 Can offload reporting, queries and fast incremental backups from the primary site to the
standby site improving the performance.
 Performs its primary Data Guard objective of preventing data loss and downtime due to
data corruptions, database and site failures, human error, or natural disaster.
 Automatically repair block corruptions online from Primary and for Primary
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Active data guard
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Block corruption error
SQL> select * from monitor.emp;
select * from monitor.emp
*
ERROR at line 1:
ORA-01578: ORACLE data block corrupted (file # 5, block # 188)
ORA-01110: data file 5: '/u02/oradata/apex/monitor_data01.dbf'
Alert.log file extract
Corrupt block relative dba: 0x018005da (file 5, block 188)
Fractured block found during user buffer read
Data in bad block:
type: 6 format: 2 rdba: 0x018005da
last change scn: 0x0000.001b3ebf seq: 0x2 flg: 0x04
spare1: 0x0 spare2: 0x0 spare3: 0x0
consistency value in tail: 0x00000000
check value in block header: 0x1b14
computed block checksum: 0xb2a8
Reading datafile '/u01/oracle/testdr01.dbf' for corruption at rdba: 0x018005da (file 5, block 188)
Reread (file 5, block 188) found same corrupt data
Requesting Auto BMR for (file# 5, block# 188)
Waiting Auto BMR response for (file# 5, block# 188)
Auto BMR successful
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Active Data Guard Usage
 Offload Read-Only Queries and Reporting
- Read-only queries and reports can be offloaded from the primary to the standby
database, reducing I/O and CPU consumption and improving quality of service for
read-write transactions on primary
- Should the primary database fail, data is protected at the standby and failover is
immediate because the standby database is completely up-to-date.
 Offload Backups
- Includes ability to use RMAN block change tracking and perform fast, online,
incremental backups of your physical standby database.
- Because backups taken on a physical standby can be used to restore either the
primary or standby databases, it is no longer necessary to perform backups on the
primary, freeing system resources to process critical transactions. (assuming same
storage structure)
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Active Data guard – Operations Permitted
We can do
 Issue SELECT statements
 Issue complex queries such as grouping SET queries and WITH clause queries
 Call stored procedures
 Use database links to write to remote databases
 Use stored procedures to call remote procedures via dblinks
Active Data guard – Operations Permitted
We cannot do
 Any DML excluding SELECT
 Any DDL
 So no additional indexes allowed
 AWR tool not supported for Active DG standby
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ON ACTIVE STANDBY
SQL> select getsal('KING') from dual;
GETSAL('KING')
---------------5000
SQL> select getsal_remote ('KING') from dual; -- getsal_remote is synonym for [email protected]
GETSAL_REMOTE('KING')
--------------------5000
SQL> create table mytab
2 (col_a number);
create table mytab
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-16000: database open for read-only access
SQL> delete from scott.emp;
delete from scott.emp
*
ERROR at line 1:
ORA-16000: database open for read-only access
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SQL> select database_role from v$database;
DATABASE_ROLE
-------------------------PHYSICAL STANDBY
SQL> select open_mode from v$database;
OPEN_MODE
-------------------READ ONLY WITH APPLY
SQL> INSERT INTO [email protected] --remote database
2
VALUES (50,'I.T','HOUSTON');
1 row created.
SQL> commit;
Commit complete.
SQL> UPDATE [email protected] -- remote database
2
SET LOC='DALLAS‘
3
WHERE LOC='HOUSTON';
1 row updated.
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Enabling Active data guard
If the standby database is not running (note in 11g, STARTUP command will start standby in
read-only mode)
If standby database is running and managed recovery in operation
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Guaranteed SLA’s for read-only users
 New with Data Guard 11g Release 2
 Active Data Guard service level agreements (SLA) can be implemented using the
session parameter, STANDBY_MAX_DATA_DELAY (NONE, NON-ZERO, ZERO)
 Specifies a limit for the amount of time (in seconds) allowed to elapse between when
changes are committed on the primary and when they can be queried on an active
standby database
 The active standby will return an ORA-3172 error code if the limit is exceeded.
 Applications can handle this error and redirect queries to the primary database
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Guaranteed SLA’s for read-only users…cont
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Guaranteed SLA’s for read-only users…cont
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Guaranteed SLA’s for read-only users…cont
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Monitoring Active Data Guard
 Query SCN on Standby is equivalent to CURRENT SCN on primary
 View V$DATAGUARD_STATS to monitor “apply lag” as well as “transport lag”
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