Data
Guard Interview Questions & Answers
1) What are the types of Oracle Data Guard?
Oracle Data Guard classified in to two types based on way of creation
and method used for Redo Apply. They are as follows.
1. Physical standby (Redo Apply technology)
2. Logical standby (SQL Apply Technology)
3. Snapshot standby (not applied until the snapshot standby is Converted)
2) What are the advantages in using Oracle Data Guard?
Following are the different benefits in using Oracle Data Guard feature
in your environment.
1. High Availability.
2. Data Protection.
3. Off loading Backup operation to
standby database.
4. Automatic Gap detection and
Resolution in standby database.
5. Automatic Role Transition using Data
Guard Broker.
3) What are the different services available in Oracle Data
Guard?
Following are the different Services available in Oracle Data Guard of
Oracle database.
1. Redo Transport Services.
2. Log Apply Services.
3. Role Transitions.
Control the automated transfer
of redo data from the production database to one or more archival destinations.
Apply redo
data on the standby database to maintain transactional synchronization with the
primary database. Redo data can be applied either from archived redo log files,
or, if real-time apply is enabled, directly from the
standby redo log files as they are being filled, without requiring the redo
data to be archived first at the standby database.
Change the role of a database
from a standby database to a primary database, or from a primary database to a
standby database using either a switchover or a failover operation.
4) What are the different Protection modes available in Oracle
Data Guard?
Data Guard protection modes implement rules that controls how the
configuration will respond to failures, enabling you to achieve specific
objectives for data protection, availability, and performance.
Following are the different protection modes available in Data Guard of
Oracle database you can use any one based on your application requirement.
1. Maximum Protection
2. Maximum Availability
3. Maximum Performance (Default Mode)
a)
Maximum Performance
– Emphasis is on primary database performance over data protection.
– Requires ASYNC (the default method) redo transport so that the LGWR process
never waits for acknowledgment from the standby database.
– Network connection between primary and standby OR the availability of the
standby database DO NOT IMPACT the primary database performance
b)
Maximum Availability
– first emphasis is on availability and second priority is zero data
loss protection.
– requires SYNC redo transport so primary database performance may be impacted
in waiting for acknowledgment from the standby (it doesn’t mean indefinite wait
in case standby database fails, maximum wait will be equal to parameter
NET_TIMEOUT seconds).
c)
Maximum Protection
– Utmost priority is on data protection.
– Also requires SYNC redo transport.
– Unlike ‘Maximum Availability’ it does not consider the NET_TIMEOUT parameter,
which means If the primary does not receive acknowledgment from a SYNC standby
database, it will stall primary and eventually abort it, preventing any
unprotected commits from occurring.
– Highly recommended to use a minimum of two SYNC standby databases at
different locations if using ‘Maximum Protection’ to have high availability of
primary database.
5) 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
6) How to change protection mode in Oracle Data Guard setup?
By using following query your can change the protection mode in your
primary database after setting up required value in corresponding
LOG_ARCHIVE_DEST_n parameter in primary database for corresponding standby
database.
ALTER DATABASE SET STANDBY DATABASE TO MAXIMUM [PROTECTION|PERFORMANCE|AVAILABILITY];
Example:
alter database set standby database to MAXIMUM PROTECTION;
7) What are the advantages of using Physical standby database in
Oracle Data Guard?
Advantages of using Physical standby database in Oracle Data Guard are
as follows.
High
Availability.
Load
balancing (Backup and Reporting).
Data
Protection.
Disaster
Recovery.
8) What is physical standby database in Oracle Data Guard?
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.
9)
What is Logical standby database in Oracle Data Guard?
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.
10)
What is Snapshot standby database in Oracle Data Guard?
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.
11) What are the advantages of Logical standby database in Oracle Data
Guard?
Better
usage of resource
Data
Protection
High
Availability
Disaster
Recovery
12) What is Switchover and Failover?
A switchover is
a role reversal between the primary database and one
of its standby databases. A switchover ensures no
data loss. This is typically done for planned maintenance of the primary
system. During a switchover, the primary database transitions to a standby
role, and the standby database transitions to the primary role.
A failover is
when the primary database is unavailable. Failover is performed only in the
event of a failure of the primary database, and the failover results in a transition of a standby database to the
primary role. The database administrator can configure Data Guard to ensure no data loss.
13) What
is data guard in simple language?
Your primary database is running and you want to reduce downtime
because of unplanned outages. You create a replica of this primary database
(termed as standby database).
You regularly ship redo generated in the primary database to standby database
and apply it there. So that is our ‘Data Guard’ standby database and it is in a
continuous state of recovery, validating and applying redo to remain in sync
with the primary database.
14) Your standby database was out of reach because of network issue.
How will you synchronize it with primary database again?
Data Guard automatically resynchronizes the standby following network or
standby outages using redo data that has been archived at the primary.
15) What
is Redo Transport Services (RTS)?
This process takes care of the transmission of redo from a primary
database to the standby database.
Below is how Redo Transport Services
(RTS) works:
1) Log Network Server (LNS) reads redo information from the redo buffer
in SGA of PRIMARY Database
2) Log Network Server (LNS) passes redo to Oracle Net Services for transmission
to the STANDBY database
3) Remote File Server (RFS) records the redo information transmitted by the LNS
at the STANDBY database
4) Remote File Server (RFS) writes it to a sequential file called a standby
redo log file (SRL) at the STANDBY database
** For multi-standby configuration, the primary database has a separate
LNS process for each standby database.
** Two redo transport methods are supported with the LNS process: synchronous
(SYNC) or asynchronous (ASYNC).
16) What
is the difference between SYNC and ASYNC redo transport method?
Synchronous transport (SYNC)
Also known as a “zero data loss” redo transport menthod.
Below is how it works:
1) Log Network Server (LNS) reads redo information from the redo buffer
in SGA of PRIMARY Database
2) Log Network Server (LNS) passes redo to Oracle Net Services for transmission
to the STANDBY database
3) Remote File Server (RFS) records the redo information transmitted by the LNS
at the STANDBY database
4) Remote File Server (RFS) writes it to a sequential file called a standby
redo log file (SRL) at the STANDBY database
5) Remote File Server (RFS) transmits an acknowledgement back to the LNS
process on the primary database
6) Log Network Server (LNS) notifies the LGWR that transmission is complete on
the primary database.
7) Log Writer (LGWR) acknowledges the commit to the user.
Asynchronous transport (ASYNC)
Unlike SYNC, Asynchronous transport (ASYNC) eliminates the requirement
that the LGWR wait for acknowledgement from the LNS. This removes the
performance impact on the primary database irrespective of the distance between
primary and standby locations. So 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 Online Redo logs.
Once the LNS is caught up, it automatically transitions back to reading &
sending directly from the log buffer.
Below is how it works:
1) Log Network Server (LNS) reads redo information from the redo buffer
in SGA of PRIMARY Database
2) Log Network Server (LNS) passes redo to Oracle Net Services for transmission
to the STANDBY database
3) Remote File Server (RFS) records the redo information transmitted by the LNS
at the STANDBY database
4) Remote File Server (RFS) writes it to a sequential file called a standby
redo log file (SRL) at the STANDBY database
so step 5, 6 & 7 as discussed above for SYNC are not applicable
here.
The only drawback of ASYNC is the increased potential for data loss. Say
a failure destroyed the primary database before any transport lag was reduced
to zero, this means any committed transactions that were a part of the
transport lag will be lost. So it is highly advisable to have enough network
bandwidth to handle peak redo
generation rates when using ASYNC method.
17) How
Synchronous transport (SYNC) can impact the primary database performance?
SYNC guarantees protection for every transaction that the database
acknowledges as having been committed but at the same time LGWR must wait for
confirmation that data is protected at the standby before it can proceed with
the next transaction. It can impact primary database performance and it depends
on factors like
> the amount of redo information to be written
> available network bandwidth
> round-trip network latency (RTT)
> standby I/O performance writing to the SRL.
> distance betweeen primary and standby databases as network RTT increases
with distance.
18) What
is Data Guard’s Automatic Gap Resolution?
Your database is using ASYNC transport method and the instance load is
at the peak. 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 Online Redo logs. Once the LNS is
caught up, it automatically transitions back to reading & sending directly
from the log buffer.
Now in some cases there can be two or
more log switches before the LNS has completed sending the redo information
from online redo log files and in meantime if any such required online redo log
files were archived then those redo information will be transmitted via Data
Guard’s gap resolution process “Automatic Gap Resolution”.
OR
In some other case when your network
or the standby database is down and your primary system is one busy system, so
before the connection between the primary and standby is restored, a large log
file gap will be formed.
Automatic Gap Resolution will take care of such scenarios by following below
action plan:
1) ARCH process on the primary database continuously ping the standby
database during the outage to determine its status.
2) As soon as 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.
3) Data Guard determines which log files are required to resynchronize the
standby database and immediately begins transmitting them using additional ARCH
processes.
4) LNS process at primary database will also attempt and succeed in making a
connection to the standby database and will begin transmitting current redo. So
first all the ARCH files are applied and then current redo log.
The Data Guard architecture enables gaps to be resolved quickly using
multiple background ARCH processes
19) What
is the difference between Physical standby and Logical standby database?
Data Guard Apply process in standby database can apply redo information
directly and in that case it will be called physical standby.
OR It can apply SQL and in that case it will be called Logical standby.
Physical Standby:
In this case standby database is an exact, block-by-block, physical
replica of the primary database.
The change vectors received by RFS process are directly applied to the standby
database by using media recovery.so here the apply process read data blocks,
assemble redo changes from mappings, and then apply redo changes to data blocks
directly.
Physical Standby is the best choice for disaster recovery (DR) based upon their
simplicity, transparency, high performance, and good data protection.
Logical Standby:
In this case standby database uses SQL Apply method to “mine” the redo
by converting it to logical change records, and then building SQL
transactions and applying SQL to the standby database.
As this process of replaying the workload is more complex than the
Physical Standby’s process, so it requires more memory, CPU, and I/O.
One good advantage here is that a logical standby database can be opened
read-write while SQL Apply is active which means you can update
(create/insert/delete etc) local tables and schemas in the logical standby
database.
20) How
is Data Guard Apply process works if primary and secondary database involves
Oracle RAC?
If Primary database is RAC but standby is Non-RAC:
Each primary Oracle RAC instance ships its own thread of redo that is
merged by the Data Guard apply process at the standby and applied in SCN order
to the standby database.
If both Primary
and standby databases are RAC:
If the standby is also an Oracle RAC database, only one instance (the
apply instance) will merge and apply changes to the standby database. If the
apply instance fail for any reason, the apply process will automatically
failover to a surviving instance in the Oracle RAC standby database when using
the Data Guard broker.
21) What is Active Data Guard Option (Oracle Database 11g
Enterprise Edition)?
For physical standby database, prior to 11g, the database would have to
be in the mount state when media recovery was active which means you were not
able to query the standby database during media recovery stage as there was no
read-consistent view.
Active Data Guard 11g features solve the read consistency problem by
use of a “query” SCN. The media recovery process on the standby database will
advance the query SCN after all the changes in a transaction have been applied.
The query SCN will appear to user as the CURRENT_SCN column in the V$DATABASE
view on the standby database. So Read-only users will only be able to see data
up to the query SCN, and hence guaranteeing the same read consistency as the
primary database.
This enables a physical standby database to be open as read-only while
media recovery is active, making it useful for doing read-only workloads.
Also, if you need read-write access to the standby database, you can use
SQL Apply method of dataguard.
22) What
are the important database parameters related to Data Guard corruption
prevention?
On the primary database:
a) DB_ULTRA_SAFE
Values can be DATA_AND_INDEX or DATA_ONLY. Setting DB_ULTRA_SAFE at the
primary will also automatically set DB_ LOST_WRITE_PROTECT=TYPICAL on the
primary database.
In Oracle Database 11g Release 2 (11.2), the primary database automatically
attempts to repair the corrupted block in real time by fetching a good version
of the same block from a physical standby database.
On the standby database:
a) DB_BLOCK_CHECKSUM=FULL
DB_BLOCK_CHECKSUM detects redo and data block corruptions and detect
corruptions on the primary database and protect the standby database. This
parameter requires minimal CPU resources.
b) DB_LOST_WRITE_PROTECT=TYPICAL
A lost write can occur when an I/O subsystem acknowledges the completion of a
write, while in fact the write did not occur in persistent storage.
This will create a stale version of the data block. When the
DB_LOST_WRITE_PROTECT initialization parameter is set, the database records
buffer cache block reads in the redo log, and this information is used to
detect lost writes.
You set DB_LOST_WRITE_PROTECT to TYPICAL in both primary and standby
databases.
23) What
is Switchover event?
Switchover is useful for minimizing downtime during planned maintenance.
It is a planned event in which Data Guard reverses the roles of the primary and
a standby database.
The primary database runs unaffected while we are making the required
changes on our standby database (e.g. patchset upgrades, full Oracle version
upgrades, etc).
Once changes are complete, production is switched over to the standby site
running at the new release.
This means regardless of how much time is required to perform planned
maintenance, the only production database downtime is the time required to
execute a switchover, which can be less than 60 seconds
Below operations happens when switchover command is executed:
1. primary database is notified that a switchover is about to occur.
2. all users are disconnected from the primary.
3. a special redo record is generated that signals the End Of Redo (EOR).
4. primary database is converted into a standby database.
5. the final EOR record is applied to standby database, this guarantees that no
data has been lost, and it converts the standby to the primary role.
24) What
is Failover event?
The Failover process is similar to switchover event except that the
primary database never has the chance to write an EOR record as this is an
unplanned event.
Whether or not a failover results in data loss depends upon the Data
Guard protection mode:
a) Maximum Protection >> No Data Loss
b) Maximum Availability >> No Data
Loss (except when there was a previous failure (e.g. a network failure) that
had INTERRUPTED REDO TRANSPORT and allowed the primary database to move ahead
of standby)
c) Maximum Performance (ASYNC) >> may
lose any committed transactions that were not transmitted to the standby database
before the primary database failed.
Failover event can be of two types:
1)
Manual
Administrator have complete control of primary-standby role transitions.
It can lengthen the outage by the amount of time required for the administrator
to be notified and manual execution of command.
2)
Automatic
It uses Data Guard’s Fast-Start Failover feature which automatically
detects the failure, evaluates the status of the Data Guard configuration, and,
if appropriate, executes the failover to a previously chosen standby database.
25) Which tools can be used for
Data Guard Management?
1) SQL*Plus – traditional method, can prove most tedious to use
2) Data Guard broker – automates and centralizes the creation, maintenance, and
monitoring of a Data Guard configuration. Simplifies and automates many
administrative
tasks. It has its own command line (DGMGRL) and syntax.
3) Enterprise Manager – requires that the Data Guard broker be enabled. a GUI
to the Data Guard broker, replacing the DGMGRL command line and interfacing
directly with the broker’s monitor processes.
26) What
is Data Guard 11g snapshot standby?
With 11g, you can thoroughly test your changes on a true replica of your
production system and database using actual production workload.
Data Guard 11g physical standby can now be converted to a snapshot standby,
independent of the primary database, that is open read-write and able to be
used for preproduction testing. It uses Flashback Database and sets a
guaranteed restore point (GRP) at the SCN before the standby was open
read-write.
NOTE: Primary database redo continues to be shipped to a snapshot
standby, and while not applied, it is archived for later use.
You can convert this snapshot database back into a synchronized physical
standby database when testing is complete. Redo Apply process at standby will
take care that all
primary database redo archived while a snapshot standby is applied until it is
caught up with the primary database.
27) What
is the difference between Recovery Point Objective(RPO) and Recovery Time
Objective (RTO)?
A)
Recovery Point Objective(RPO)
RPO concerns with data. It is the amount of data you are willing to lose
when the failure occurs in your database system. Usually people define data
loss in terms of time, so possible values can be 5 seconds of data loss, 2
hours of data loss etc.
Remember that each standby database has its own set of attributes and
parameters. It means you can mix zero data loss standby databases with minimal
data loss standby
databases in the same Data Guard configuration
If you have decided that you want to implement zero data loss strategy,
then you should really focus on Networks and Data Loss
B)
Recovery Time Objective (RTO)
RTO is defined as how fast you can get back up and running (whereas RPO
is concerned with data loss)
So with your RPO strategy you lost say only about 6 seconds of data as
you committed to your client but with RTO you need to formulate how fast
clients can connect back to the database system after the data loss has
occurred.
28) What
are Standby Redo Log (SRL) files?
The SRL files are where the Remote File Server (RFS) process at your
standby database writes the incoming redo so that it is persistent on disk for
recovery. SRL files are important for better redo transport performance and
data protection.
SRL are MUST in Maximum Availability or Maximum Protection mode and
OPTIONAL (but recommended) in Maximum Performance mode.
If there are no Standby Redo Log (SRL) files, then at each log switch in
the primary database, the RFS process on the standby database that is serving
an asynchronous standby destination has to create an archive log of the right
size. While the RFS is busy doing creating the archive log file, the LNS
process at the primary database has to wait, getting further and further behind
the LGWR (in case of Maximum Performance mode). That is why it recommended to
have Standby Redo Log (SRL) files in Maximum Performance mode also.
We generally configure them on our primary database as well in
preparation for a role transition b/w primary-standby.
Also, do not multiplex SRLs. Since Data Guard will immediately request a
new copy of the archive log if an SRL file fails, there is no real need to have
more than one copy of each.
Active Data
Guard - Answers to Frequently Asked Questions
The Active Data Guard
Option is an evolution of Data Guard technology that incorporates significant
innovation (multiple patents pending) designed for a specific purpose - to improve
production database performance for critical transactions. Active Data
Guard enables read-only access to a physical standby database while Redo Apply
is active. Queries and reports can be offloaded from the production
system to a synchronized physical standby database - all queries at the standby
database return up-to-date results. An Active Data Guard Option license must be
purchased in addition to Oracle Enterprise Edition in order to utilize these
new capabilities.
Active Data Guard and Oracle Data Guard 11g are related technologies, each
having a different area of focus. Oracle Data Guard provides a
comprehensive set of capabilities for data availability and protection that are
included with Oracle Database Enterprise Edition.
Licensing requirements for Active Data Guard and Oracle Data Guard 11g features
are summarized in the following table:
Capabilities and corresponding
license requirements
|
Oracle Enterprise Edition 11g
|
Active Data Guard Option
|
Advanced Compression Option
|
All Data Guard 10g features
|
X
|
|
|
New Data Guard 11g features (excludes
Active Data Guard and Advanced Compression features)
|
X
|
|
|
Real-time Query, enables a physical
standby to be open read-only while Redo Apply is active
|
|
X
|
|
The ability to enable RMAN block change
tracking on a physical standby database
|
|
X
|
|
Data Guard Redo Transport
compression
|
|
|
X
|
Additional frequently asked questions
include:
Must I purchase Active Data Guard if I
am not using Real-time Query or RMAN block change tracking on my standby
database?
No. An Oracle Database Enterprise
Edition license is the only license required to use Data Guard features other
than those explicitly included with the Active Data Guard Option as described
in the table above.
I am already using Data Guard 10g, do I
need to purchase Active Data Guard when I upgrade to Oracle Database
11g ?
No - as long as you do not enable the
Real-time Query feature or RMAN block change tracking on your physical standby
database. Thus your physical standby database could be open read-only, but it
cannot be applying redo at the same time. Similarly, you can perform RMAN
incremental backups your physical standby, but you cannot perform fast RMAN
incremental backups using RMAN block change tracking. You must only purchase
the Active Data Guard Option if you wish to use either or both of these
features.
Separate from Active Data Guard -
Oracle states that Data Guard 11g continues to be an included feature of Oracle
Enterprise Edition. What are the new Data Guard features that are
included in Data Guard 11g?
I thought a Data Guard physical standby
could always be opened read-only and/or used for incremental backups - why do I
need the Active Data Guard Option ?
Previous capabilities did not allow
Redo Apply to be active while a physical standby database was open read-only,
and did not enable RMAN block change tracking on the standby database.
This resulted in (a) read-only access to data that was frozen as of the time
that the standby database was opened read-only, (b) failover and switchover
operations that could take longer to complete due to the backlog of redo data
that would need to be applied, and (c) incremental backups that could take up
to 20x longer to complete - even on a database with a moderate rate of change.
Previous capabilities are still included with Oracle Data Guard 11g, no
additional license is required to use previous capabilities.
Why would I use Active Data Guard and
not just add another node to my primary Oracle RAC cluster to enhance
performance ?
Oracle RAC offers many advantages for
scalability and high availability that are well understood and embraced by
thousands of Oracle customers. Active Data Guard is designed to address a
different requirement where customers wish to physically isolate the overhead
of processing ad-hoc queries and reports from their OLTP system by using a
completely independent, synchronized replica of the production database. If the
customer requirement can be addressed using read-only access to an up-to-date
replica of the production database, then Active Data Guard is an ideal
solution.
Why would I use Active Data Guard and
not simply use SQL Apply (logical standby) that is included with Data Guard
11g ?
If read-only access satisfies the
requirement - Active Data Guard is a closer fit for the requirement, and
therefore is much easier to implement than any other approach. Active
Data Guard supports all datatypes and is very simple to implement. An Active
Data Guard replica can also easily support additional uses - offloading backups
from the primary database, serve as an open read-write test system during
off-peak hours (Snapshot Standby), and provide an exact copy of the production
database for disaster recovery - fully utilizing standby servers, storage and
software while in standby role.
With the availability of Active Data
Guard, what role does SQL Apply (logical standby) continue to play?
Use SQL Apply for the following
requirements: (a) when you require read-write access to a synchronized standby
database but do not modify primary data, (b) when you wish to add local tables
to the standby database that can also be updated, or (c) when you wish to create
additional indexes to optimize read performance. The ability to handle
local writes makes SQL Apply better suited to packaged reporting applications
that often require write access to local tables that exist only at the target
database. SQL Apply also provides rolling upgrade capability for patchsets and
major database releases. This rolling upgrade functionality can also be
used by physical standby databases beginning with Oracle 11g using Transient
Logical Standby.
My reporting application makes some
temporary writes which require read-write access to the standby database, even
though the writes do not modify primary data. How can I use it with Active Data
Guard ?
Active Data Guard does not support any
writes to the physical standby database. That said, it is possible that limited
writes needed by the reporting application can be directed back to the primary
database or to a local database that is on the same server as the standby
database, using Oracle database links. For further details, refer to the
Active Data Guard best practices paper on the MAA OTN site.
How do I collect stats from an Active
Data Guard replica given that it is open read-only ?
This is described in Metalink Note 454848.1 that details installation and usage
of standby statspack.
