9.1 Oracle Data Guard Configuration Best PracticesData Guard is the Oracle optimized solution for Data availability and protection. It excels at simple, fast, and reliable one-way replication of a complete Oracle Database to provide High Availability and Disaster Recovery.
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Data Guard offers various deployment options that address unplanned outages, pre-production testing, and planned maintenance. Active Data Guard, an extension of basic Data Guard capabilities, further enables production offload of read-only workload to a synchronized physical standby database, automatic repair of corrupt blocks, and offload of fast incremental backups.The focus of Data Guard is High Availability and Data Recovery.
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Data Guard design principles are simplicity, high performance, and application transparency.Data Guard is not intended to be a full-featured replication solution. Oracle GoldenGate is the solution recommended for advanced replication requirements, such as multi-master replication, granular replication of a subset of a database, many to one replication topologies, and data integration. Oracle GoldenGate also provides additional options for reducing downtime for planned maintenance and for heterogeneous platform migrations.Depending upon your requirements, the most efficient solution to use may be using Data Guard alone, using Data Guard with Oracle GoldenGate in a complementary manner, or just using Oracle GoldenGate.For more information about Data Guard and Oracle GoldenGate see the Product Technical Brief on Oracle Active Data Guard and Oracle GoldenGate atprovides a summary of the Data Guard deployment options that are appropriate, depending on your requirements. Two or more options may be used in combination to address multiple requirements. This chapter also presents the Best practices for implementing each option.
Note:Standby-First Patch allows you to apply a patch initially to a physical standby database while the primary database remains at the previous software release (this applies for certain types of patches and does not apply for Oracle patch sets and major release upgrades; use the Data Guard transient logical standby method for patch sets and major releases). Once you are satisfied with the change, then you perform a switchover to the standby database. The fallback is to switchback if required. For more information, see 'Oracle Patch Assurance - Data Guard Standby-First Patch Apply' in My Oracle Support Note 1265700.1 at. 9.2 Determine Protection Mode and Data Guard TransportOracle Data Guard Zero Data Loss protection provides both a guarantee of that data is protected and the simplest recovery. For these reasons a Zero Data Loss protection mode, either Oracle Data Guard Maximum Protection or Maximum Availability, is recommended. While both modes use Oracle Data Guard synchronous redo transport by default, there are differences in the rule-sets used to govern behavior at failover time that must be considered, as described below.
Oracle Data Guard synchronous redo transport, however, can impact primary database performance if round-trip network latency between primary and standby databases is too great (latency is a function of distance and how 'clean' the network is). If this is the case (testing is easy to do, a DBA may change protection modes and transport methods dynamically), then use Oracle Data Guard Maximum Performance. Maximum Performance uses Oracle Data Guard asynchronous transport services and does not have any impact on primary database performance regardless of network latency. In an environment with sufficient bandwidth to accommodate redo volume, data loss potential is measured in single-digit seconds when using Maximum Performance.To determine the appropriate data protection mode for your application, consult Oracle Data Guard Concepts and Administration.Best practices for the protection mode:.Maximum Protection mode guarantees that no data loss will occur if the primary database fails, even in the case of multiple failures (for example, the network between the primary and standby fails, and then at a later time, the primary fails). This is enforced by never signaling commit success for a primary database transaction until at least one synchronous Data Guard standby has acknowledged that redo has been hardened to disk. Without such an acknowledgment the primary database will stall and eventually shut down rather than allow unprotected transactions to commit.
To maintain availability in cases where the primary database is operational but the standby database is not, the best practice is to always have a minimum of two synchronous standby databases in a Maximum Protection configuration. Primary database availability is not impacted if it receives acknowledgment from at least one synchronous standby database.Maximum Availability mode guarantees that no data loss will occur in cases where the primary database experiences the first failure to impact the configuration. Unlike the previous protection mode, Maximum Availability will wait a maximum of NETTIMEOUT seconds for an acknowledgment from a standby database, after which it will signal commit success to the application and move to the next transaction. Primary database availability (thus the name of the protection mode) is not impacted by an inability to communicate with the standby (for example, due to standby or network outages).
Oracle Data Guard will continue to ping the standby and automatically re-establish connection and resynchronize the standby database when possible, but during the period when primary and standby have diverged there will be data loss should a second failure impact the primary database. For this reason, it is a best practice to monitor protection level (simple to do using Enterprise Manager Grid Control) and quickly resolve any disruption in communication between primary and standby before a second failure can occur.Maximum Performance mode (the default mode) provides the highest level of data protection that is possible without affecting the performance or the availability of the primary database. This is accomplished by allowing a transaction to commit as soon as the redo data needed to recover that transaction is written to the local online redo log at the primary database (the same behavior as if there were no standby database).
Oracle Data Guard transmits redo to the standby database directly from the primary log buffer asynchronous to the local online redo log write. There is never any wait for standby acknowledgment. Similar to Maximum Availability, it is a best practice to monitor protection level (simple to do using Enterprise Manager Grid Control) and quickly resolve any disruption in communication between primary and standby before a second failure can occur. 9.2.1 Use Redo Transport Services Best PracticesAt a high level, the Redo Transport best practices for planning and implementing redo transport services for Oracle Data Guard are as follows:.Use the SYNC redo transport mode for a high degree of synchronization between the primary and standby databases.
Use SYNC redo transport for zero data loss protection where performance service levels can tolerate the impact caused by network latency.Use the ASYNC redo transport mode for minimal impact on the primary database, but with a lower degree of synchronization. Use ASYNC redo transport when zero data loss protection is not required or when the performance impact caused by network latency makes it impractical to use SYNC.Optimize network throughput following the best practices described in. 9.2.2 Assess Performance with Proposed Network ConfigurationOracle recommends that you conduct a performance assessment with your proposed network configuration and current, or anticipated, peak redo rate.
The network effect between the primary and standby databases, and the effect on the primary database throughput must be understood. Because the network between the primary and standby databases is essential for the two databases to remain synchronized, the infrastructure must have the following characteristics:.Sufficient bandwidth to accommodate the maximum redo generation rate.If using the SYNC transport, then minimal latency is necessary to reduce the performance impact on the primary database.Multiple network paths for network redundancyIn configurations that use a dedicated network connection the required bandwidth is determined by the maximum redo rate of the primary database and the efficiency of the network.
Depending on the data protection mode, there are other recommended practices and performance considerations. Maximum protection mode and maximum availability mode require SYNC transport.The maximum performance protection mode use ASYNC redo transport. Use ASYNC redo transport when data loss can be tolerated or when the performance impact caused by network latency makes it impractical to use SYNC (use SYNC redo transport for zero data loss protection).Unlike the ASYNC transport mode, the SYNC transport mode can affect the primary database performance due to the incurred network latency. Distance and network configuration directly influence latency, while high latency can slow the potential transaction throughput and quicken response time.
The network configuration, number of repeaters, the overhead of protocol conversions, and the number of routers also affect the overall network latency and transaction response time. 9.3.1 Use Oracle Data Guard Broker with Oracle Data GuardUse Oracle Data Guard broker to create, manage, and monitor a Data Guard configuration. You can perform all Data Guard management operations locally or remotely through the Oracle Data Guard broker's easy-to-use interfaces: the Data Guard management pages in Oracle Enterprise Manager, which is the broker's graphical user interface (GUI), and the Data Guard command-line interface called DGMGRL.The broker's interfaces improve usability and centralize management and monitoring of the Data Guard configuration. 9.3.4 Use FORCE LOGGING ModeWhen the primary database is in FORCE LOGGING mode, all database data changes are logged. FORCE LOGGING mode ensures that the standby database remains consistent with the primary database. If this is not possible because you require the load performance with NOLOGGING operations, then you must ensure that the corresponding physical standby data files are subsequently synchronized.
To synchronize the physical standby data files, either apply an incremental backup created from the primary database or replace the affected standby data files with a backup of the primary data files taken after the nologging operation. Before the file transfer, you must stop Redo Apply on the physical standby database.You can enable force logging immediately by issuing an ALTER DATABASE FORCE LOGGING statement. If you specify FORCE LOGGING, then Oracle waits for all ongoing unlogged operations to finish. Table 9-2 Archiving Recommendations RecommendationDescriptionStart archiving on the primary and standby databasesMaintaining a standby database requires that you enable and start archiving on the primary database, as follows:SQL SHUTDOWN IMMEDIATESQL STARTUP MOUNT;SQL ALTER DATABASE ARCHIVELOG;SQL ALTER DATABASE OPEN;Archiving must also be enabled on the standby database to support role transitions. To enable archiving on the standby database:SQL SHUTDOWN IMMEDIATE;SQL STARTUP MOUNT;SQL ALTER DATABASE ARCHIVELOG;Use a consistent log format ( LOGARCHIVEFORMAT).The LOGARCHIVEFORMAT parameter should specify the thread, sequence, and resetlogs ID attributes, and the parameter settings should be consistent across all instances. For example: LOGARCHIVEFORMAT=arch%t%S%r.arcNote: If the fast recovery area is used, then this format is ignored.Perform remote archiving to only one standby instance and node for each Oracle RAC standby database.All primary database instances archive to one standby destination, using the same net service name.
9.3.6 Use Standby Redo Logs and Configure Size AppropriatelyYou should configure standby redo logs on all primary and standby databases for improved availability and performance.For each redo log thread (a thread is associated with an OracleRAC database instance), number of Standby Redo Logs = number of Redo Log Groups + 1The additional standby redo log eliminates the possibility of a standby database waiting on standby redo log. For example, if a primary database has two instances (threads) and each thread has three online log groups, then you should pre-configure 8 standby redo logs on the primary database and each standby database. Furthermore, if the primary or standby databases are not a symmetrical Real Application Cluster (example 8-node primary Oracle RAC cluster compared to 2-node standby Oracle RAC cluster), then the primary and standby databases should still have an equal number of standby redo logs and all threads should be represented.The statements in create three standby logs per thread. 9.3.7.1 Set the LOGARCHIVEMAXPROCESSES ParameterIn most cases the default for LOGARCHIVEMAXPROCESSES is sufficient. However, in a Data Guard configurations that have multiple standby databases it may be necessary to increase the number of archive processes. The value of the LOGARCHIVEMAXPROCESSES initialization parameter must be at least one greater than the total number of all remote destinations. Use the following equation when setting the LOGARCHIVEMAXPROCESSES parameter for highly available environments:LOGARCHIVEMAXPROCESSES = sum( remotedestinations) + count( threads)You can adjust these parameter settings after evaluating and testing the initial settings in your production environment.
9.3.7.2 Set the Network Configuration and Highest Network Redo RatesTo set the network configuration and highest network redo rates:.Properly Configure TCP Send / Receive Buffer SizesTo achieve high network throughput, especially for a high-latency, high-bandwidth network, the minimum recommended setting for the sizes of the TCP send and receive socket buffers is the bandwidth-delay product (BDP) of the network link between the primary and standby systems. Settings higher than the BDP may show incremental improvement. For example, in the MAA Linux test lab, simulated high-latency, high-bandwidth networks realized small, incremental increases in throughput when using TCP send and receive socket buffer settings up to three times the BDP.BDP is product of the network bandwidth and latency. Socket buffer sizes are set using the Oracle Net parameters RECVBUFSIZE and SENDBUFSIZE, so that the socket buffer size setting affects only Oracle TCP connections. The operating system may impose limits on the socket buffer size that must be adjusted so Oracle can use larger values. For example, on Linux, the parameters net.core.rmemmax and net.core.wmemmax limit the socket buffer size and must be set larger than RECVBUFSIZE and SENDBUFSIZE.Set the send and receive buffer sizes at either the value you calculated or 10 MB (10,485,760 bytes), whichever is larger.
For example, if bandwidth is 622 Mbits and latency is 30 ms, then you would calculate the minimum size for the RECVBUFSIZE and SENDBUFSIZE parameters as follows: 622,000,000 / 8 x 0.030 = 2,332,500 bytes. Then, multiply the BDP 2,332,500 x 3 for a total of 6,997,500.In this example, you would set the initialization parameters as follows:RECVBUFSIZE=10485760SENDBUFSIZE=10485760Increase SDU SizeWith Oracle Net Services it is possible to control data transfer by adjusting the size of the Oracle Net setting for the session data unit (SDU). Oracle internal testing has shown that setting the SDU to its maximum value of 65535 can improve performance for the SYNC transport.
You can set SDU on a per connection basis using the SDU parameter in the local naming configuration file ( TNSNAMES.ORA) and the listener configuration file ( LISTENER.ORA), or you can set the SDU for all Oracle Net connections with the profile parameter DEFAULTSDUSIZE in the SQLNET.ORA file.Note that the ASYNC transport uses the new streaming protocol and increasing the SDU size from the default has no performance benefit. See Also:for more information about the TCP.NODELAY parameterDetermine When to Use Redo Transport CompressionIn Oracle Database 11 g Release 2 (11.2.0.2) redo transport compression is no longer limited to compressing redo data only when a redo gap is being resolved. When compression is enabled for a destination, all redo data sent to that destination is compressed.In general, compression is most beneficial when used over low bandwidth networks. As the network bandwidth increases, the benefit is reduced. Compressing redo in a Data Guard environment is beneficial if:.Sufficient CPU resources are available for the compression processing.The database redo rate is being throttled by a low bandwidth network.Before enabling compression, assess the available CPU resources and decide if enabling compression is feasible. For complete information about enabling compression, see 'Redo Transport Compression in a Data Guard Environment' in My Oracle Support Note 729551.1 at. 9.3.8.2 Assess Recovery RateTo obtain the history of recovery rates, use the following query to get a history of recovery progress:SELECT.
FROM V$RECOVERYPROGRESS;If your ACTIVE APPLY RATE is greater than the maximum redo generation rate at the primary database or twice the average generation rate at the primary database, then no tuning is required; otherwise follow the tuning tips below. The redo generation rate for the primary database can be monitored from Enterprise Manager or extracted from AWR reports under statistic REDO SIZE. If CHECKPOINT TIME PER LOG is greater than ten seconds, then investigate tuning I/O and checkpoints.
9.3.8.4 Set DBCACHESIZE to a Value Greater than on the Primary DatabaseSet DBCACHESIZE to a value greater than that for the primary database. Set DBKEEPCACHESIZE and DBRECYCLECACHESIZE to 0.Having a large database cache size can improve media recovery performance by reducing the amount of physical data block reads.
Because media recovery does not require DBKEEPCACHESIZE and DBRECYCLECACHESIZE or require a large SHAREDPOOLSIZE, the memory can be reallocated to the DBCACHESIZE.Before converting the standby database into a primary database, reset these parameters to the primary database settings. 9.3.8.5 Assess Database Wait EventsWith the Active Data Guard option and real-time query, you can use Statspack from the primary database to collect data from a standby database that is opened read-only and performing recovery. Any tuning or troubleshooting exercise should start with collecting Standby Statspack reports. For complete details about installing and using Standby Statspack, see 'Installing and Using Standby Statspack in 11g' in My Oracle Support Note 454848.1 atIf you do not have a license for the Active Data Guard option, you can determine the top system and session wait events by querying the standby database's V$SYSTEMEVENT, V$SESSIONWAIT, and V$EVENTHISTOGRAM and looking for the largest TIMEWAITED value.
You may have to capture multiple snapshots of the query results and manually extract the difference to accurately assess a certain time period.If recovery is applying a lot of redo data efficiently, the system is I/O bound and the I/O wait should be reasonable for your system. The vast majority of wait events related to parallel recovery coordinators and slaves apply to the coordinator. Slaves are either applying changes (clocking on CPU) or waiting for changes to be passed from the coordinator.Typically, in a properly tuned system, the top wait event is db file parallel write followed by checkpoint completed.
Consult the table below for tuning advice in cases where db file parallel write is not the top wait event. The database wait events are shown in. Table 9-4 Parallel Recovery Slave Wait Events Wait NameDescriptionTuningParallel recovery slave next changeThe parallel recovery slave is waiting for a change to be shipped from the coordinator. This is in essence an idle event for the recovery slave. To determine the amount of CPU a recovery slave is using, divide the time spent in this event by the number of slaves started and subtract that value from the total elapsed time. 9.3.8.6 Tune I/O OperationsDBWR must write out modified blocks from the buffer cache to the data files. Always use native asynchronous I/O by setting DISKASYNCHIO to TRUE (default).
In the rare case that asynchronous I/O is not available, use DBWRIOSLAVES to improve the effective data block write rate with synchronous I/O.Ensure that you have sufficient I/O bandwidth and that I/O response time is reasonable for your system either by doing some base I/O tests, comparing the I/O statistics with those for the primary database, or by looking at some historical I/O metrics. Be aware that I/O response time may vary when many applications share the same storage infrastructure such as with a Storage Area Network (SAN) or Network Attached Storage (NAS). 9.3.8.7 Assess System ResourcesUse system commands such as UNIX sar and vmstat commands, or use system monitoring tools to assess the system resources. Alternatively, you can monitor using Oracle Enterprise Manager, AWR reports, or performance views such as V$SYSTEMEVENT, V$ASMDISK and V$OSSTAT.If there are I/O bottlenecks or excessive wait I/O operations, then investigate operational or application changes that increased the I/O volume.
If the high waits are due to insufficient I/O bandwidth, then add more disks to the relevant Oracle ASM disk group. Verify that this is not a bus or controller bottleneck or any other I/O bottleneck. The read I/O rate from the standby redo log should be greater than the expected recovery rate.Check for excessive swapping or memory paging.Check to ensure the recovery coordinator or MRP is not CPU bound during recovery. 9.3.9 Implement Multiple Standby DatabasesYou should deploy multiple standby databases for any of the following purposes. 9.4.1 Oracle Data Guard Switchovers Best PracticesA database switchover performed by Oracle Data Guard is a planned transition that includes a series of steps to switch roles between a standby database and a primary database.
Following a successful switchover operation, the standby database assumes the primary role and the primary database becomes a standby database. Switchovers are typically completed in only seconds to minutes. At times the term switchback is also used within the scope of database role management. A switchback operation is a subsequent switchover operation to return the roles to their original state.Data Guard enables you to change these roles dynamically by:.Using Oracle Enterprise Manager.Using the Oracle Data Guard broker's DGMGRL command-line interface.Issuing SQL statements, as described in. See Also:Support notes for switchover best practices for Data Guard Physical Standby (11.2.0.2):.If using SQL.Plus, see '11.2 Data Guard Physical Standby Switchover Best Practices using SQL.Plus' in My Oracle Support Note 1304939.1 at.If using the Oracle Data Guard broker or Oracle Enterprise Manager, see '11.2 Data Guard Physical Standby Switchover Best Practices using the Broker' in My Oracle Support Note 1305019.1 at.The MAA white paper 'Switchover and Failover Best Practices' from the MAA Best Practices area for Oracle Database at. 9.4.2 Oracle Data Guard Failovers Best PracticesA failover is typically used only when the primary database becomes unavailable, and there is no possibility of restoring it to service within a reasonable period. During a failover the primary database is taken offline at one site and a standby database is brought online as the primary database.With Data Guard the process of failover can be completely automated using fast-start failover or it can be a manual, user driven process.
Oracle recommends using fast-start failover to eliminate the uncertainty inherent in a process that requires manual intervention. Fast-start failover automatically executes a failover within seconds of an outage being detected.For more on Data Guard failover best practices, see:. 9.4.2.2 Failover Best Practices (Manual Failover and Fast-Start Failover)To optimize failover processing:.Enable Flashback Database to reinstate the failed primary databases after a failover operation has completed. Flashback Database facilitates fast point-in-time recovery, if needed.Use real-time apply with Flashback Database to apply redo data to the standby database as soon as it is received, and to quickly rewind the database should user error or logical corruption be detected.Consider configuring multiple standby databases to maintain data protection following a failover.Set the LOGFILENAMECONVERT parameter. As part of a failover, the standby database must clear its online redo logs before opening as the primary database.
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The time needed to complete this I/O can add significantly to the overall failover time. By setting the LOGFILENAMECONVERT parameter, the standby pre-creates the online redo logs the first time the MRP process is started. You can also pre-create empty online redo logs by issuing the SQL.Plus ALTER DATABASE CLEAR LOGFILE statement on the standby database.Use fast-start failover. The MAA tests running Oracle Database 11 g show that failovers performed using Oracle Data Guard broker and fast-start failover offer a significant improvement in availability. For more information, see.For physical standby databases, do the following:.When transitioning from read-only mode to Redo Apply (recovery) mode, restart the database.Go directly to the OPEN state from the MOUNTED state instead of restarting the standby database (as required in releases before Oracle Database 11 g release 2).See the MAA white paper 'Oracle Data Guard Redo Apply and Media Recovery' to optimize media recovery for Redo Apply from the MAA Best Practices area for Oracle Database at. 9.4.2.3 Fast-Start Failover Best PracticesFast-start failover automatically, quickly, and reliably fails over to a designated standby database if the primary database fails, without requiring manual intervention to execute the failover.
You can use fast-start failover only in an Oracle Data Guard configuration that is managed by Oracle Data Guard broker.The Oracle Data Guard configuration can be running in either the maximum availability or maximum performance mode with fast-start failover. When fast-start failover is enabled, the broker ensures fast-start failover is possible only when the configured data loss guarantee can be upheld. Maximum availability mode provides an automatic failover environment guaranteed to lose no data. Maximum performance mode provides an automatic failover environment guaranteed to lose no more than the amount of data (in seconds) specified by the FastStartFailoverLagLimit configuration property.Use the following fast-start failover best practices in addition to the generic best practices listed in the:.Run the fast-start failover observer process on a host that is not located in the same data center as the primary or standby database.Ideally, you should run the observer on a system that is equally distant from the primary and standby databases. The observer should connect to the primary and standby databases using the same network as any end-user client. If the designated observer fails, Oracle Enterprise Manager can detect it and automatically restart the observer. If the observer cannot run at a third site, then you should install the observer on the same network as the application.
If a third, independent location is not available, then locate the observer in the standby data center on a separate host and isolate the observer as much as possible from failures affecting the standby database.Make the observer highly available by using Oracle Enterprise Manager to configure the original primary database to be automatically reinstated as a standby database when a connection to the database is reestablished. Also, Oracle Enterprise Manager enables you to define an alternate host on which to restart the observer.After the failover completes, the original primary database is automatically reinstated as a standby database when a connection to it is reestablished, if you set the FastStartFailoverAutoReinstate configuration property to TRUE.Set the value of the FastStartFailoverThreshold property according to your configuration characteristics, as described in. Table 9-6 Minimum Recommended Settings for FastStartFailoverThreshold ConfigurationMinimum Recommended SettingSingle-instance primary, low latency, and a reliable network15 secondsSingle-instance primary and a high latency network over WAN30 secondsOracle RAC primaryOracle RAC miscount + reconfiguration time + 30 secondsTest your configuration using the settings shown in to ensure that the fast-start failover threshold is not so aggressive that it induces false failovers, or so high it does not meet your failover requirements. 9.5 Use Oracle Active Data Guard Best PracticesIf you have a license for the Oracle Active Data Guard option then you can open a physical standby database for read-only access while Redo Apply on the standby database continues to apply redo data received from the primary database. All queries reading from the physical standby database execute in real time and return current results, providing more efficient use of system resources and additional assurance that the standby is healthy without compromising data protection or extending recovery time if a failover is required. Hence, this capability is referred to as. Note:A physical standby database can be open for read-only access while Redo Apply is active if a license for the Oracle Active Data Guard option has been purchased.
9.6 Use Snapshot Standby Database Best PracticesBeginning with Oracle Database release 11 g, you can convert a physical standby database into a fully updatable standby database called a.To convert a physical standby database into a snapshot standby database, issue the SQL.Plus ALTER DATABASE CONVERT TO SNAPSHOT STANDBY statement. This command causes Oracle Data Guard to perform the following actions:.Recover all available redo data.Create a guaranteed restore point.Activate the standby database as a primary database.Open the database as a snapshot standby databaseTo convert the snapshot standby back to a physical standby, issue the ALTER DATABASE CONVERT TO PHYSICAL STANDBY statement. This command causes the physical standby database to be flashed back to the guaranteed restore point that was created before the ALTER DATABASE CONVERT TO SNAPSHOT STANDBY statement was issued. Then, you must perform the following actions:.Restart the physical standby database.Restart Redo Apply on the physical standby databaseTo create and manage snapshot standby databases:.Use the Oracle Data Guard broker to manage your Oracle Data Guard configuration, because it simplifies the management of snapshot standby databases. The broker will automatically convert a snapshot standby database into a physical standby database as part of a failover operation.
Without the broker, this conversion must be manually performed before initiating a failover.Create multiple standby databases if your business requires a fast recovery time objective (RTO).Ensure the physical standby database that you convert to a snapshot standby is caught up with the primary database, or has a minimal apply lag. See for information about tuning media recovery.Configure a fast recovery area and ensure there is sufficient I/O bandwidth available. This is necessary because snapshot standby databases use guaranteed restore points.
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