what is the use of Standby redo log ?
The Advantage of having Standby Redo Logs is that every records written into the Online RedoLogs of the Primary Database
is transferred to the Standby database and written into the Standby Redo Logs at the same time.
therefore, you it minimizes the probability of Data Loss on the Standby Database.
For real time apply, it is mandatory to have redolog.
Without Standby Redo Logs, an Archived Redo Log is created by the RFS process and when it has completed,
this Archived Redo Log is applied to the Standby Database by the MRP process . An incomplete ArchiveLog file cannot be applied on the Standby Database and will not be used in a Failover situation. This causes a certain data loss.
If you have Standby Redo Logs, the RFS process will write into the Standby Redo Log ,when there record entered in online redolog and when a log switch occurs, the Archiver Process of the Standby Database will archive this Standby Redo Log to an Archived Redo Log, while the MRP process applies the information to the Standby Database. In a Failover situation, you will also have access to the information already written in the Standby Redo Logs, so the information will not be lost.
Oracle recommends to keep standby redologs on both primary and standby database.(Though SLR is not mandatory for primary, it will make the switchover quick without any additional DBA activity. i.e If you don’t standby redologs on primary, whenever we do switchover, we need to create the SLR
What are different protection modes in dataguard
There are three modes .
1. MAXIMUM PROTECTION
This mode provides maximum protection. It guarantees zero data loss. In this mode the redo/transaction data must be written to both primary redo log and standby redo log. For any reason(mostly N/W issue) if it is unable to write to standby, Then primary database will get shutdown.
2. MAXIMUM AVAILABILITY:
It provides the highest level of data protection that is possible without affecting the availability of the primary database.Transactions are not allowed to commit until all redo data are written to the online redo logs and propagated to at least one synchronized secondary database. If for any reason, the primary database cannot propagate its redo stream to one secondary database, the primary will NOT shutdown and operates as it it were in maximum performance mode until issues are fixed.
3. MAXIMUM PERFORMANCE:(default one)
This is the default protection mode. With this protection mode, a transaction is committed as soon as the redo data needed to recover the transaction is written to the local (online) redo log.
What are the advantages in using Oracle Data Guard?
Following are the different benefits in using Oracle Data Guard feature in your environment:
High Availability.
Data Protection.
Off loading Backup operation to standby database.
Automatic Gap detection and Resolution in standby database.
Automatic Role Transition using Data Guard Broker.
What is the difference between switchover and failover:
A switchover means just switching roles between the primary database and standby db.
nswitchover, the primary database chnaged to a standby role, and the standby database changed to the primary role.
This is typically done for planned maintenance of the primary db server.
A failover is when the primary database fails and one of the standby databases is transitioned to take over the primary role. Failover is performed only in the event of a catastrophic failure of the primary database, and there is no possibility of recovering the primary database in a timely manner. Failover may or may not result in data loss depending on the protection mode in effect at the time of the failover.
What are different types of dataguard and their difference.
PHYSICAL STANDBY:
Provides a physically identical copy of the primary database, with on disk database structures that are identical to the primary database on a block-for-block basis. The database schema, including indexes, are the same. A physical standby database is kept synchronized with the primary database, through Redo Apply, which recovers the redo data received from the primary database and applies the redo to the physical standby database.As of Oracle Database 11g release 1 (11.1), a physical standby database can receive and apply redo while it is open for read-only access. A physical standby database can therefore be used concurrently for data protection and reporting.
LOGICAL STANDBY:
Contains the same logical information as the production database, although the physical organization and structure of the data can be different. The logical standby database is kept synchronized with the primary database through SQL Apply, which transforms the data in the redo received from the primary database into SQL statements and then executes the SQL statements on the standby database.
A logical standby database can be used for other business purposes in addition to disaster recovery requirements. This allows users to access a logical standby database for queries and reporting purposes at any time. Also, using a logical standby database, you can upgrade Oracle Database software and patch sets with almost no downtime. Thus, a logical standby database can be used concurrently for data protection, reporting, and database upgrades.
SNAPSHOT STANDBY:
A snapshot standby database is a fully updatable standby database. Like a physical or logical standby database, a snapshot standby database receives and archives redo data from a primary database. Unlike a physical or logical standby database, a snapshot standby database does not apply the redo data that it receives. The redo data received by a snapshot standby database is not applied until the snapshot standby is converted back into a physical standby database, after first discarding any local updates made to the snapshot standby database.
A snapshot standby database is best used in scenarios that require a temporary, updatable snapshot of a physical standby database. Note that because redo data received by a snapshot standby database is not applied until it is converted back into a physical standby, the time needed to recover from a primary database failure is directly proportional to the amount of redo data that needs to be applied.
What are the different types of Apply services in dataguard.
REDO APPLY:
With redo apply, apply services waits for a standby redo log file to be archived before applying the redo to standby.
REAL TIME APPLY:
If the real-time apply feature is enabled, then apply services can apply redo data as it is received, without waiting for the current standby redo log file to be archived.
What are the different types of redo transport services in dataguard.
SYNC and ASYNC.
SYNC(SYNCHRONOUS):
This mode is used for maximum protection and maximum availability protection mode. The synchronous redo transport mode transmits redo data synchronously with respect to transaction commitment. A transaction cannot commit until all redo generated by that transaction has been successfully sent to every standby destinations.
ASYNC(ASYNCHRONOUS):
This mode is used for maximum performance mode. A transaction can commit 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.
How to check what protection mode of primary database in your Oracle Data Guard?
By using following query you can check protection mode of primary database in your Oracle Data Guard setup:
SQL> select protection_mode from v$database;
PROTECTION_MODE
MAXIMUM PERFORMANCE
Which process is responsible for Redo log information transmitting?
LGWR or ARCH.
What is fal_client and fal_server parameter?
FAL Means – Fetch Archive log. FAL_CLIENT and FAL_SERVER parameters are used on standby database for archive gap resolution.
FAL_SERVER and FAL_CLIENT parameters are required on standby database only .
FAL_SERVER: Specify an Oracle Net Service Name (TNS-Alias or Connect Descriptor) that points to the Database from where the missing ArchiveLog(s) should be requested i.e the tns alias of the primary db.
FAL_CLIENT: Specify an Oracle Net Service Name (TNS-Alias or Connect Descriptor) that points from FAL_SERVER TO standby db. i.e the tns alias of the standby db.
fal_server = ‘primdb’
fal_client = ‘stdbydb’
Which parameter user for GAP Resolution?
FAL_SERVER and FAL_CLIENT
When MRP finds that an archive log is missing during media recovery, it sends the fal_client information to the server identified by fal_server and requests fal_server to resend the file again. The fal_client and fal_server init.ora (spfile) parameters are set on the standby instance
What is standby_file_management parameter oracle.
Standby_file_management parameter can be have two values. MANUAL or AUTO(Recommended).
AUTO – > Datafiles created on primary will be create automatically on standby .
MANUAL – > Datafiles created on primary , will not be replicated to standby. The datafile need to be created manually on Standby
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