Can we configure mirroring between Standard Edition & Enterprise Edition or Vice Versa?

Nope, its not possible, both principal and mirror should have the same edition.

What are Quorum types in Mirroring?

A Quorum is the relationship between the Witness,Principal and the Mirror.Depending on the mode of operation it is divided into 3.

Full Quorum: This is when all 3 Witness,Principal and the Mirror can communicate with each other.Since witness is present automatic failover occurs.

Quorum: This state exist if the Witness and either partner can communicate with it.

Partner-to-Partner: When only the Principal and Mirror can communicate with each other.

Can we upgrade SQL while mirror is configured? (More details)

When upgrading server instances from SQL Server 2005 or SQL Server 2008 to SQL Server 2012, you can reduce downtime for each mirrored database to only a single manual failover by performing a sequential upgrade, known as a rolling upgrade.

A rolling upgrade is a multi-stage process that in its simplest form involves upgrading the server instance that is currently acting as the mirror server in a mirroring session, then manually failing over the mirrored database, upgrading the former principal server, and resuming mirroring.

Can Log Shipping and mirroring configure together?

Yes we can combine both together, please visit this official article for more details.

Does mirroring support FILESTREAM file group?

No:

Database mirroring does not support FILESTREAM. A FILESTREAM filegroup cannot be created on the principal server. Database mirroring cannot be configured for a database that contains FILESTREAM filegroups.

Does mirroring support cross database transaction or distributed transaction?

No:

Database mirroring is not supported with either cross-database transactions or distributed transactions. This is because transaction atomicity/integrity cannot be guaranteed.

During the mirroring session, can we increase the disk size of primary database?

Yes, You can increase the size of a disk.

Can we have a different collation setting on both the database?

No, It wouldn’t work.

T-SQL Script to enable mirroring.

ALTER DATABASE [AdventureWorks] SET PARTNER = N‘TCP://databasemirror.adatum.com:5022';

Specify the partner from the principal server:

ALTER DATABASE [AdventureWorks] SET PARTNER = N‘TCP://databasemaster.adatum.com:5022';

T-SQL Script to check associate ports with DB Mirroring.

SELECT type_desc, port FROM sys.tcp_endpoints;

T-SQL Script to check the state of the DB Mirroring.

SELECT state_desc FROM sys.database_mirroring_endpoints

T-SQL Script check the service account connect permission on the DB Mirror endpoints.

SELECT 
	EndPnt.name
	,SvrPerm.STATE
	,CONVERT(nvarchar(38), suser_name(SvrPerm.grantor_principal_id)) AS GRANTOR
	,SvrPerm.TYPE AS PERMISSION
	,CONVERT(nvarchar(46),suser_name(SvrPerm.grantee_principal_id)) AS GRANTEE
FROM sys.server_permissions SvrPerm, sys.endpoints EndPnt
WHERE SvrPerm.major_id = EndPnt.endpoint_id
ORDER BY Permission, grantor, grantee;
GO

T-SQL Script to check the DB Mirror timeout and resetting the DB Mirror timeout.

SELECT mirroring_connection_timeout
FROM sys.database_mirroring;

The most interesting issue in DB Mirroring you will ever see.

Before & After configuration of Database Mirroring , What should be our checklists?

• The principal database and the mirror database should be on separate physical hardware, and ideally, in different physical locations.
• The witness server should be on separate physical hardware, and be on a separate network (best if at a third location).
• Initial database mirroring setup should be done during less busy times, as the setup process can negatively affect performance of the production database being mirrored.
• Use high availability mode whenever possible, and high performance mode only when required.
• While a fast connection is not required between mirrored servers, the faster the connection, and the better quality the connection, the better.
• You will want to optimize the performance of the mirrored database as much as possible to reduce the overhead caused by the mirroring process itself.
• Thoroughly test database mirroring before putting it into production.
• Monitor database mirroring daily to ensure that it is working properly, and is meeting performance goals.
• Develop a formal operational and recovery procedure to support mirroring. Periodically test the failover process to ensure that it works.

How to move database files of a Mirrored SQL Server Database?

What are your steps to apply a service pack or patch to Mirrored SQL Server Databases?

• Step 1: Always backup all system and user databases before applying patches.
• Step 2: Remote Desktop into the “Mirror” server (SSQL2 in our example) and download/copy the patch to the server.
• Step 3: Stop all SQL Services on the “Mirror” server.
• Step 4: Run the patch on the “Mirror” server.
• Step 5: Once the patch is complete, reboot the “Mirror” server.
• Step 6 (optional): If your database mirroring is set to asynchronous (High Performance mode), we will need to synchronize the databases first.
• Step 7: The databases might change to “synchronizing” while the transactions catch up.
• Step 8 (optional):Once the failover has taken place and all the databases have switched roles we can set the operating mode back to asynchronous.
• Step 9: Remote Desktop into the “new” “Mirror” server (SSQL1 in our example) and download/copy the patch to the server.
• Step 10: Stop all SQL Services.
• Step 11: Run the patch on the server.
• Step 12: Once the patch is complete, reboot the server (SSQL1).
• Step 13: Upon reboot and login, make sure all databases come online clean and synchronized.
• Step 14: You now have two choices, leave the databases alone and leave SSQL2 as the principal and SSQL1 as the mirror or you can fail back to return the environment back to how it was from the beginning.
• More Details

What are the basic steps to configure a Database Mirroring?

• Configuring security and communication between instances.
• Configuring endpoint.
• Creating logins for other servers service accounts.
• Grant connect permission to this logins on endpoints.
• Create mirror database by taking full and Transaction Log backup from principle server and restore it in mirror server with NORECOVERY.
• Establish mirroring session using ALTER DATABASE command.

What are the important performance monitor counters for Database Mirroring?

For Principal server:

• Log Bytes Sent/sec: Number of bytes of the log sent to the mirror per second.
• Log Send Queue KB: Total kilobytes of the log that have not yet been sent to the mirror server.
• Transaction Delay: Delay (in milliseconds) in waiting for commit acknowledgement from the mirror.
• Log Bytes Flushed/sec: The rate at which log records are written to the disk.
• Disk Write Bytes/sec: The rate at which the disk is written to.

For Mirror Server:

• Redo Bytes/sec: Number of bytes of the transaction log applied on the mirror database per second.
• Redo Queue KB: Total kilobytes of hardened log that remain to be applied to the mirror database to roll it forward.

T-SQL Script to find basic information about the instances which are involved in Database Mirroring.

SELECT
	DB_NAME(database_id) AS 'DatabaseName'
	, mirroring_role_desc
	, mirroring_safety_level_desc
	, mirroring_state_desc
	, mirroring_safety_sequence
	, mirroring_role_sequence
	, mirroring_partner_instance
	, mirroring_witness_name
	, mirroring_witness_state_desc
	, mirroring_failover_lsn
FROM sys.database_mirroring
WHERE mirroring_guid IS NOT NULL;

T-SQL Script to find information about the witness server.

SELECT
	Database_name
	, safety_level_desc
	, safety_sequence_number
	, role_sequence_number
	, is_suspended
	, is_suspended_sequence_number
	, principal_server_name
	, mirror_server_name
FROM sys.database_mirroring_witnesses;

What are the limitations of Database Mirroring?

• Limited number of databases can be mirrored per SQL instance.
• Mirrored database cannot be queried, backed up, or basically touched in any way (snapshot can be taken however)
• Only supports one to one server pairings.
• Cannot mirror a database more than once.
• Synchronous mirroring is intolerant to latency and bandwidth constraints (Asynchronous mirroring has no problems with latency or bandwidth).

What is alternative of Database Mirroring after SQL Server 2012?

Microsoft has now declared the database mirroring feature to be deprecated. Users of SQL Server are advised not to use this feature and not to develop new applications for it. As a substitute, Microsoft recommends AlwaysOn High Availability Groups.

Can we configure mirroring on 32 bit version or only on 64 version?

Yes, you can move SQL Server data back and forth between x64, x86, and IA64 architectures. The data and log files themselves do not store anything that indicates the architecture and work the same on either 32-bit or 64-bit.

Can we configure mirroring between 32 and 64 bit machine?

A database mirroring session can combine server instances that run in a 32-bit environment and server instances that run in a 64-bit environment.

Due to hard errors, What are the possible database mirroring failures?

• A broken connection or wire
• A bad network card
• A router change
• Changes in the firewall
• Endpoint reconfiguration
• Loss of the drive where the transaction log resides
• Operating system or process failure
• DNS is not working.
• Cables are unplugged.
• Microsoft Windows has a firewall that blocks a specific port.
• The application that is monitoring a port fails.
• A Windows-based server is renamed.
• A Windows-based server is rebooted.

Due to soft errors, What are the possible database mirroring failures?

• Network errors such as TCP link time-outs, dropped or corrupted packets, or packets that are in an incorrect order.
• A hanging operating system, server, or database state.
• A Windows server timing out.
• Insufficient computing resources, such as a CPU or disk overload, the transaction log filling up, or the system is running out of memory or threads.

T-SQL Script to to change time-out value.

ALTER DATABASE database_name SET PARTNER TIMEOUT 120;

T-SQL Script to Resume or Pause the Database mirroring session.

Script to Pause:

ALTER DATABASE Database_Name SET PARTNER SUSPEND;

Script to Resume:

ALTER DATABASE Database_Name SET PARTNER RESUME;

What is High Protection operating mode?

It is pretty similar to High Availability mode except that Witness is not available, as a result failover is manual. It also has transactional safety FULL i.e. synchronous communication between principal and mirror. Even in this mode if the network is poor it might cause performance bottleneck.

What are End Points and its usages?

An endpoint is a network protocol which is used to communicate Principal, Mirror and Witness servers over the network.

Which are the system stored procedure to monitor database mirroring?

• sp_dbmmonitoraddmonitoring
• sp_dbmmonitorchangemonitoring
• sp_dbmmonitorhelpmonitoring
• sp_dbmmonitordropmonitoring

Which are the other ways to monitor database mirroring?

Using profiler, performance counter, SQL Server Management Studio, Database Mirroring Monitor tool and SQL Server log.

What is Database Mirroring?

Database mirroring is a solution for increasing the availability of a SQL Server database. Mirroring is implemented on a per-database basis and works only with databases that use the full recovery model.

Database mirroring is the creation and maintenance of redundant copies of a database. The purpose is to ensure continuous data availability and minimize or avoid downtime that might otherwise result from data corruption or loss, or from a situation when the operation of a network is partially compromised.

What are the benefits of Database Mirroring?

• It increases the availability of a database. In the event of a disaster, in high-safety mode with automatic failover, failover quickly brings the standby copy of the database online.
• It Increases the data protection.Database mirroring provides complete or almost complete redundancy of the data, depending on whether the operating mode is high-safety or high-performance.
• It Improves the availability of the production database during upgrades.To minimize downtime for a mirrored database, you can sequentially upgrade the instances of SQL Server that are hosting the failover partners.

How Database Mirroring works in SQL Server?

Database mirroring maintains two copies of a single database that must reside on different server instances of SQL Server Database Engine. Typically, these server instances reside on computers in different locations. Starting database mirroring on a database, initiates a relationship, known as a database mirroring session, between these server instances.

One server instance serves the database to clients (the principal server). The other instance acts as a hot or warm standby server (the mirror server), depending on the configuration and state of the mirroring session.

The principal and mirror servers communicate and cooperate as partners in a database mirroring session. The two partners perform complementary roles in the session: the principal role and the mirror role. At any given time, one partner performs the principal role, and the other partner performs the mirror role.

Database mirroring involves redoing every insert, update, and delete operation that occurs on the principal database onto the mirror database as quickly as possible. Redoing is accomplished by sending a stream of active transaction log records to the mirror server, which applies log records to the mirror database, in sequence, as quickly as possible.

What is a automatic failover?

The process by which, when the principal server becomes unavailable, the mirror server to take over the role of principal server and brings its copy of the database online as the principal database.

What is a principal database principal server?

Principal database is a read-write database whose transaction log records are applied to mirror database and Principal server is an instance on which the principal database resides.

What is a mirror database and mirror server?

The copy of the database that is typically fully synchronized with the principal database and the mirror server is an instance on which the mirror database resides.

What is a failover partners?

The two server instances (the principal server or the mirror server) that act as role-switching partners for a mirrored database.

What is a forced service?

A failover initiated by the database owner upon the failure of the principal server that transfers service to the mirror database while it is in an unknown state.

What is a High-performance mode of the Mirroring?

The database mirroring session operates asynchronously and uses only the principal server and mirror server. The only form of role switching is forced service.

What is a High-safety mode of the Mirroring?

The database mirroring session operates synchronously and, optionally, uses a witness, as well as the principal server and mirror server.

What is a redo queue?

It receives transaction log records that are waiting on the disk of a mirror server.

What is a send queue?

It holds the unsent transaction log records that have accumulated on the log disk of the principal server.

What is role switching?

Within the context of a database mirroring session, the principal and mirror roles are typically interchangeable in a process known as role switching. Role switching involves transferring the principal role to the mirror server. In role switching, the mirror server acts as the failover partner for the principal server. When a role switch occurs, the mirror server takes over the principal role and brings its copy of the database online as the new principal database. The former principal server, if available, assumes the mirror role, and its database becomes the new mirror database. Potentially, the roles can switch back and forth repeatedly.

What is Transaction safety?

A mirroring-specific database property that determines whether a database mirroring session operates synchronously or asynchronously. There are two safety levels: FULL and OFF.

What is Witness server?

To support automatic failover, a database mirroring session must be configured in high-safety mode and also possess a third server instance, known as the witness.Unlike the two partners, the witness does not serve the database. Supporting automatic failover is the only role of the witness.

What happened, when a DBA paused the session?

Pausing preserves the session state while removing mirroring. When a session is paused, the principal server does not send any new log records to the mirror server. All of these records remain active and accumulate in the transaction log of the principal database. As long as a database mirroring session remains paused, the transaction log cannot be truncated.

What are the Prerequisites to set up a Database Mirroring?

• The partners and the witness, if any, must be running on the same version of SQL Server.
• The two partners, that is the principal server and mirror server, must be running the same edition of SQL Server. The witness, if any, can run on any edition of SQL Server that supports database mirroring.
• The database must use the full recovery model. The simple and bulk-logged recovery models do not support database mirroring.
• Verify that the mirror server has sufficient disk space for the mirror database.
• When you are creating the mirror database on the mirror server, make sure that you restore the backup of the principal database specifying the same database name WITH NORECOVERY.