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Effective Usage of SQL Server 2005 Database Mirroring

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Presentation on theme: "Effective Usage of SQL Server 2005 Database Mirroring"— Presentation transcript:

1 Effective Usage of SQL Server 2005 Database Mirroring
06th October 2007, SQL Bits Effective Usage of SQL Server Database Mirroring by Satya SK Jayanty SQL Server MVP Microsoft Campus, Reading UK October 06th 2007

2 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

3 Introduction – Speaker (Who Am I)
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Introduction – Speaker (Who Am I) Webmaster(SQLMaster) of & Been in the IT field over 15+ years (SQL Server Specialist Consultant) SQL Server DBA for over 11 years (working since ver. 4.2) Contributing Editor, Writer & Moderator for SQL Server MVP (Microsoft Most Valuable Professional) Participation in assorted forums such as MSDN, SQL Server magazine, dbforums and so on. Principal Consultant & Director – SSQA.net Limited (www.ssqa-net.co.uk) 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

4 Effective Usage of SQL Server 2005 Database Mirroring
Agenda Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

5 SQL Server 2005 High Availability features
Effective Usage of SQL Server 2005 Database Mirroring SQL Server 2005 High Availability features 06th October 2007, SQL Bits High Availability is a set of best practices and technologies designed to increase database availability through backups that help recover from catastrophic disasters, redundant database components engineered to prevent downtime, and redundant server implementation designed to minimize downtime. What it takes? How much it cost? Can you handle a disaster? Tips & Tricks! Failover clustering and Database mirroring both provide the Server & Database High Availability features. Constraints in Failover Clustering Operates at the server instance scope not to a specific database Requires certified hardware (HAL) Has no reporting on standby (either Active/Active or Active/Passive) Utilizes a single copy of the database Does not protect against disk failure High costs to implement 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

6 SQL Server 2005 High Availability features
Database Mirroring benefits for the constraints above: Operates at the database scope Uses a single, duplicate copy of the database Uses standard servers - no HAL Provides limited reporting on the mirror server by using database snapshots (Enterprise) Database Mirroring - What is the advantage over Failover Clustering? You can mix & match database mirroring with a failover clustering. Database mirroring works easily between failover clusters and also between a failover cluster and a non-clustered host. Configuration - High Protection & High Performance. No licensing cost to setup witness server. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

7 SQL Server 2005 High Availability features
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits SQL Server 2005 High Availability features Database Mirroring - What is the advantage over Failover Clustering? Database mirroring & log shipping This combination can be used to provide high availability for your notification applications, you can maintain a standby server that you can bring online after failures or during server maintenance. Benefits of – increased data protection, increased availability of databases during any hardware or software patching. It is recommended that you first learn the High Performance configuration and then move to the High Protection configuration (more about this in upcoming slides). If using witness server, two of the three servers must be able to communicate to determine principal. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

8 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

9 Effective Usage of SQL Server 2005 Database Mirroring
Feature Overview 06th October 2007, SQL Bits SQL Server 2005 Editions for Database Mirroring Enterprise Edition - Fully supported (failover partners must use the same edition) Standard Edition - Safety full only Workgroup Edition, SQL Express & SQL Express with Advanced services – Only as witness SQL Express and the Workgroup Edition can be used as a witness server, but they cannot be used as a partner server in database mirroring. When operating synchronously, provides for zero work loss through delayed commit on the principal database. Only one mirror (copy) can exist per DBM session for the purposes of failover. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

10 Effective Usage of SQL Server 2005 Database Mirroring
Feature Overview 06th October 2007, SQL Bits Database Mirroring (DBM) features Database mirroring transfers transaction log records directly from one server to another and can quickly fail over to the standby server. An originating SQL Server 2005 instance continuously sends a database's transaction log records to a copy of the database on another standby SQL Server instance. No dependency on proprietary hardware (cost and complexity), and easy to setup. SQL Server 2005 supports database mirroring between 64 bit & 32 bit editions. System databases cannot be a part of mirroring (none of High Availability features). Fast failover - with minimal data loss has traditionally involved higher hardware cost and greater software complexity. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

11 Effective Usage of SQL Server 2005 Database Mirroring
Feature Overview 06th October 2007, SQL Bits Database Mirroring (DBM) Terms you come across: Principal – the server that holds a copy of database that is accessible to client applications at any given time. Mirror – the server that holds copy of database that is always in restoring state that is not accessible to the applications. Witness – the optional server that is useful to provide an automatic failover mechanism in case of any failure on principal server. Send Queue - a queue (principal) that builds up when the log records can’t be sent at the rate at which they are generated from principal to mirror. Redo Queue – a queue (mirror) that builds up when the log records can’t be applied at the rate at which they are received from principal server. Send queue - It is a part of the log that has not yet been sent to mirror server. Redo queue - It is a part of log that remains to be applied to the mirror database to roll it forward. Endpoint - It encapsulates a transport protocol and a port number. Transaction safety level - Two safety levels—OFF and FULL. Transaction safety is a mirroring-specific database property that determines whether a database mirroring session operates synchronously or asynchronously. There are two safety levels: FULL and OFF. SAFETY FULL Full transaction safety causes the session to operate synchronously in high-safety mode. If a witness is present, a session supports automatic failover. When you establish a session using ALTER DATABASE statements, the session begins with the SAFETY property set to FULL; that is, the session begins in high-safety mode. After the session begins, you can add a witness. SAFETY OFF Turning off transaction safety causes the session to operate asynchronously, in high-performance mode. If the SAFETY property is set to OFF, the WITNESS property should also be set to OFF (the default). For information about the impact of the witness in high-performance mode, see "The State of the Witness," later in this topic. For more information about running with transaction safety turned off, see Asynchronous Database Mirroring (High-Performance Mode). Transport security in SQL Server 2005 involves authentication and, optionally, encryption of messages exchanged between the databases. In some situations, such as when server instances are not in trusted domains or when SQL Server is running as a local service, Windows Authentication is unavailable. In such cases, instead of user credentials, certificates are required to authenticate connection requests. The mirroring endpoint of each server instance must be configured with its own locally created certificate. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

12 Effective Usage of SQL Server 2005 Database Mirroring
Feature Overview 06th October 2007, SQL Bits Database Mirroring (DBM) Terms you come across: Endpoint: A SQL Server object that enables Principal, Mirror & Witness servers to communicate over the network. Encryption - Data between the principal and the mirror is transferred encrypted by default. Transaction safety level – that determines whether the changes on the principal database are applied to the mirror database synchronously or asynchronously (FULL & OFF). Synchronous - the mirror server must synchronize the mirror database with the principal database. Speeds up the writes all of the incoming log records to disk as quickly as possible. Asynchronous – this sets up database mirroring with high performance. With safety OFF, the transaction is committed as soon as the principal server writes the log record to the local log and sends the log record to the mirror, without waiting for an acknowledgement from the mirror server. Send queue - It is a part of the log that has not yet been sent to mirror server. Redo queue - It is a part of log that remains to be applied to the mirror database to roll it forward. Endpoint - It encapsulates a transport protocol and a port number. Transaction safety level - Two safety levels—OFF and FULL. Transaction safety is a mirroring-specific database property that determines whether a database mirroring session operates synchronously or asynchronously. There are two safety levels: FULL and OFF. SAFETY FULL Full transaction safety causes the session to operate synchronously in high-safety mode. If a witness is present, a session supports automatic failover. When you establish a session using ALTER DATABASE statements, the session begins with the SAFETY property set to FULL; that is, the session begins in high-safety mode. After the session begins, you can add a witness. SAFETY OFF Turning off transaction safety causes the session to operate asynchronously, in high-performance mode. If the SAFETY property is set to OFF, the WITNESS property should also be set to OFF (the default). For information about the impact of the witness in high-performance mode, see "The State of the Witness," later in this topic. For more information about running with transaction safety turned off, see Asynchronous Database Mirroring (High-Performance Mode). Transport security in SQL Server 2005 involves authentication and, optionally, encryption of messages exchanged between the databases. In some situations, such as when server instances are not in trusted domains or when SQL Server is running as a local service, Windows Authentication is unavailable. In such cases, instead of user credentials, certificates are required to authenticate connection requests. The mirroring endpoint of each server instance must be configured with its own locally created certificate. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

13 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

14 Database Mirroring Setup
Effective Usage of SQL Server 2005 Database Mirroring Database Mirroring Setup 06th October 2007, SQL Bits Before setup of Database Mirroring – make sure to restore all of the log backups done since the full database backup. Stop transaction log backup jobs on original principal database Restore database and all transaction logs on original mirror database with NORECOVERY option DBM endpoints need to be created with same authentication and encryption methods on each server involved in DBM session. (next slide) Start DBM session. Need to use fully qualified domain names (FQDN) for server names as DBM uses DNS name resolution. If transaction log backups are not stopped, there will be a transaction log gap between the partners and the mirror database will not be able to synchronize with the principal database. Step 3 is performed by the Configure Security button in the SQL Server Management Studio’s (SSMS) Database Properties – Mirroring dialog box. Step 4 is performed by the Configure Security and Start Mirroring buttons in the SSMS Database Properties – Mirroring dialog box. All errors will occur after the Start Mirroring button is selected. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

15 Database Mirroring Setup ENDPOINT configuration & Security
Effective Usage of SQL Server 2005 Database Mirroring Database Mirroring Setup ENDPOINT configuration & Security 06th October 2007, SQL Bits SSMS - Configure Security button creates DBM endpoints and sets up connection rights to the endpoints. The following queries are executed on all servers part of the DBM session. CREATE ENDPOINT if it does not already exist GRANT CONNECT ON ENDPOINT if service accounts are entered Encryption: CREATE MASTER KEY ENCRYPTION BY PASSWORD = '<1_Strong_Password!>' Start Mirroring button sets up the DBM session between the partner servers. The following queries executed that are reported as FQDN followed by the TCP port the endpoint is listening on. ALTER DATABASE .. SET PARTNER = mirror server ALTER DATABASE .. SET PARTNER = principal server ALTER DATABASE .. SET WITNESS = principal server It is assumed that the initial mirror database is in the restoring state with the latest transaction log from the principal database restored. The Configure Security button can not setup the DBM Endpoints to use Certificate Authentication. The <partner server> and <witness server> options of the ALTER DATABASE command should be reported as Fully Qualified Domain Names (FQDN) followed by the TCP port the endpoint is listening on. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

16 Database Mirroring Setup Modes
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits High Availability Supports automatic detection and automatic failover in the event of a disaster by requiring a third instance to act as a Witness server. FULL SAFETY form If the Principal Server database fails then the Witness server will co-ordinate with the Mirror server to have the Mirror database come online automatically to service client requests (normally within 10 seconds). Suitable for automatic failover scenario such as no application downtime is desired. Automatic failover requires the database is already synchronized. The failure occurs while all three server instances are connected, and the witness and mirror server remain connected. In learning how to setup and configure Database Mirroring, there are many new and important concepts to master. If possible, you may want to follow a very specific process to setting up and learning Database Mirroring. Starting with High Performance allows you to test mirroring with the least impact to your production environment. The High Performance configuration allows you to setup a Mirror database that has very little latency behind your Principal database, effectively setting up a form of real-time log shipping. Once you feel comfortable with the setup and configuration of Database Mirroring in the asynchronous form, you can then move to a synchronous form. Moving to a synchronous form will allow you to test the actual performance impact of Database Mirroring in your environment. There are many factors which impact performance: network speed and distance, speed of the mirror database/server, and transaction load. By moving to the synchronous High Protection configuration next, you can test the impact to performance without the possibility of an automated (and possibly unexpected) failover. Once the performance is deemed acceptable and you determine that automatic detection/failover is desired, you can add the required witness to the High Protection configuration to convert to the High Availability configuration. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

17 Database Mirroring Setup Modes
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Still uses “full safety” mode (similar to High Availability configuration) only with the exception of the setup of the Witness server. Suitable when automatic failover not required and for more control on the failover scenario. The witness should be set only when you intend to use high-safety mode with automatic failover. High Protection No support for automatic detection or automatic failover in the event of a disaster and it does not require or use a Witness. In learning how to setup and configure Database Mirroring, there are many new and important concepts to master. If possible, you may want to follow a very specific process to setting up and learning Database Mirroring. Starting with High Performance allows you to test mirroring with the least impact to your production environment. The High Performance configuration allows you to setup a Mirror database that has very little latency behind your Principal database, effectively setting up a form of real-time log shipping. Once you feel comfortable with the setup and configuration of Database Mirroring in the asynchronous form, you can then move to a synchronous form. Moving to a synchronous form will allow you to test the actual performance impact of Database Mirroring in your environment. There are many factors which impact performance: network speed and distance, speed of the mirror database/server, and transaction load. By moving to the synchronous High Protection configuration next, you can test the impact to performance without the possibility of an automated (and possibly unexpected) failover. Once the performance is deemed acceptable and you determine that automatic detection/failover is desired, you can add the required witness to the High Protection configuration to convert to the High Availability configuration. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

18 Database Mirroring Setup Modes
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits High Performance Uses an asynchronous (“full safety off”) form of mirroring, as transaction changes are not required to be sent immediately to the Mirror Database. High-performance mode, supports only one form of role switching: forced service (with possible data loss), which uses the mirror server as a warm standby server. Does not support automatic detection or automatic failover or use a Witness. That needs WITNESS property to OFF. Mode allows the Principal Database to focus on client requests (performance) without the need to immediately and synchronously send the changes. High-performance mode can be useful in a disaster-recovery scenario in which the principal and mirror servers are separated by a significant distance and where you do not want small errors to impact the principal server. In learning how to setup and configure Database Mirroring, there are many new and important concepts to master. If possible, you may want to follow a very specific process to setting up and learning Database Mirroring. Starting with High Performance allows you to test mirroring with the least impact to your production environment. The High Performance configuration allows you to setup a Mirror database that has very little latency behind your Principal database, effectively setting up a form of real-time log shipping. Once you feel comfortable with the setup and configuration of Database Mirroring in the asynchronous form, you can then move to a synchronous form. Moving to a synchronous form will allow you to test the actual performance impact of Database Mirroring in your environment. There are many factors which impact performance: network speed and distance, speed of the mirror database/server, and transaction load. By moving to the synchronous High Protection configuration next, you can test the impact to performance without the possibility of an automated (and possibly unexpected) failover. Once the performance is deemed acceptable and you determine that automatic detection/failover is desired, you can add the required witness to the High Protection configuration to convert to the High Availability configuration. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

19 Database Mirroring Setup
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Another configuration mode asynchronous mode with no witness server – although this is possible to setup it is not recommended because it combines the risk of data loss(Forced Service) and split-brain scenario. Operating Mode Transaction Safety Transfer Mechanism Quorum Required Witness Server Failover type High Availability FULL Synchronous Yes Automatic or Manual High Protection No Manual High Performance OFF Asynchronous Forced Configuration must be chosen depending upon the business requirement. Depending on the transaction safety level and whether you have a witness, you can have automatic failover, manual failover, or both. The mode is dependant on setting of Transaction Safety and Witness Server. The impact of the transaction safety level on performance is application dependent. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

20 Database Mirroring Setup
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits When service is forced, whatever transaction logs the principal has not yet sent to the mirror server are lost. Therefore, you should limit forced service to situations where possible data loss is acceptable and immediate database availability is critical. Use forcing service only if you must restore service to the database immediately and are willing to risk losing data. Standard Edition allows only the FULL transaction safety level. It is recommended that you first learn/setup the High Performance configuration and then move to the High Protection configuration. Then, if desired, you can move to the High Availability configuration. Configuration must be chosen depending upon the business requirement. Depending on the transaction safety level and whether you have a witness, you can have automatic failover, manual failover, or both. The mode is dependant on setting of Transaction Safety and Witness Server. The impact of the transaction safety level on performance is application dependent. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

21 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

22 DBM - Monitoring & Performance
Effective Usage of SQL Server 2005 Database Mirroring DBM - Monitoring & Performance 06th October 2007, SQL Bits Database Mirroring Monitor (DMM) [sp_dbmmonitorresults] – best to use: What is the session status & role of a partner? Determine whether the mirror database is keeping up with the principal database. Determine how much data was lost when the principal server instance becomes unavailable during high-performance mode. Compare current performance with past performance. Within upcoming release of SQL Server 2008: Is the mirror restoring transaction log? How fast is log being restored? How far is the mirror database behind? When will the mirror database catch up? Has the ability to configure Alerts around performance To determine the cause of the mirror falling behind the principal, examine the SQL Server errorlogs on both servers for errors and check to see if the DBM Session could be suspended. Another reason that the mirror could be falling behind is if the network bandwidth can not support at least double the maximum transaction log generation rate. The SQL Server:Database\Transactions/Sec Performance Monitor counter will count both read only and read write transactions on the principal database but only read write transactions on the mirror database so they will not be the same. There is currently no way to determine the number of read only transactions in a database. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

23 DBM - Monitoring & Performance
Effective Usage of SQL Server 2005 Database Mirroring DBM - Monitoring & Performance 06th October 2007, SQL Bits Database state: SYNCHRONIZING, SYNCHRONIZED, SUSPENDED, PENDING_FAILOVER & DISCONNECTED. A major component of the database failover time can be the redo phase. So the log backups after full backups is most important to reduce the redo phase time. Another reason that the mirror could be falling behind is if the network bandwidth can not support at least double the maximum transaction log generation rate. sys.databases - view the log_reuse_wait_desc column for why space in the log cannot be reused. (View Factors That Keep Log Records Active topic in BOL ) Above sequence can also be performed by using Dynamic Management Views. Also by using performance monitor (SYSMON) counters & Profiler. From the server perspective, the database failover time is the amount of time that it takes from the time the failure of the principal is detected to the time the mirror assumes the role of the principal. To accomplish roll forward, the SQL Server Database Engine processes log backups as they are restored, starting with the log that is contained in full backups. Because active portion of the log cannot be truncated or removed by shrinking, truncation can be delayed when log records remain active for a long time. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

24 DBM - Monitoring & Performance
Effective Usage of SQL Server 2005 Database Mirroring DBM - Monitoring & Performance 06th October 2007, SQL Bits SYSMON counters on each partner server helps you to monitor the performance of database mirroring, few of them are important to watch: SQLServer:Database Mirroring Databases - Transactions/sec On Principal server Log Bytes Sent/sec Log Send Queue KB Log Bytes Flushed/sec Disk Write Bytes/sec On Mirror server Redo Bytes/sec Redo Queue KB Disk Write Bytes/sec Other Logical Disk counters Tip: When capturing counters Witness Server is best to use (if installed separately). Transactions/Sec Perfmon counter is does not show same amount on both principal and mirror databases but mirror is not running behind. One way to measure the actual failover time is to use SQL Server Profiler [Database Mirroring State Change event] on the principal and the mirror [Audit Database Mirroring Login event ]. The SQL Server:Database\Transactions/Sec Performance Monitor counter will count both read only and read write transactions on the principal database but only read write transactions on the mirror database so they will not be the same. There is currently no way to determine the number of read only transactions in a database. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

25 DBM - Monitoring & Performance
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits DBM - Monitoring & Performance Preparing (testing)the Mirror for Failover It is better to use identical servers (in terms of CPU, memory, storage, and network capacity) as principal and mirror partners. Same service pack and patch levels for the operating system and SQL Server on both partners. If used Automatic Failover ensure that both servers have the same directory structure. And also all active SQL Server logins (and their permissions) on the principal server are also present/applied on the mirror server. Ensure that all SQL Server Agent jobs, alerts, linked server definitions & SSIS packages are present in mirror server (disabled) and you may need them to enable in the event of a failover. TCP Endpoint on each server and configure authentication on endpoint with CONNECT permission appropriately. The above preparation lists are also applicable to the system databases between the principal and mirror servers. SSIS Packages can be enabled again using TSQL script to avoid maximum manual processes. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

26 DBM - Monitoring & Performance
Effective Usage of SQL Server 2005 Database Mirroring DBM - Monitoring & Performance 06th October 2007, SQL Bits Troubleshooting Database mirroring does not regularly check the components on which Sqlservr.exe relies to verify whether they are functioning correctly or have failed. Failures in databases other than the mirrored database are not detectable in a database mirroring session. Moreover, a data disk failure is unlikely to be detected, unless the database is restarted because of a data disk failure. To determine the cause of the mirror falling behind the principal, examine the SQL Server error logs on both servers for errors and check to see if the DBM Session could be suspended. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

27 DBM - Monitoring & Performance
Effective Usage of SQL Server 2005 Database Mirroring DBM - Monitoring & Performance 06th October 2007, SQL Bits Troubleshooting If database mirroring is not working, check the following to make sure the configuration is correct: Look at the column STATE_DESC in the catalog view sys.database_mirroring_endpoints. Endpoints compatible encryption settings Look at the column IS_ENCRYPTION_ENABLED in the catalog view sys.database_mirroring_endpoints Look at the columns TYPE_DESC and ROLE_DESC respectively of the catalog view sys.database_mirroring_endpoints for the correct type and roles. Ensure port numbers of the endpoints are the same as the corresponding port numbers specified in the SET PARTNER statements. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

28 DBM - Monitoring & Performance
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits With and Without Witness Witness server assists Principal or Mirror server in forming a quorum. If Principal server cannot connect to mirror server, still the quorum consists with Witness to keep up the database in service. In case Principal server is down then Witness initiates failover where the Mirror can take on the role of new principal server. When the partner servers cannot see each other, the partner servers contact the witness to see whether the witness can contact the other partner and verify that a failure has occurred. Witness is not a single point of failure. The location of Witness is dependant upon your network availability to avoid losing two different servers, and therefore quorum, if the power system fails. Witness is not the most important member of the database mirroring session. General recommendation of Witness location is to accompany Principal. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

29 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

30 Understanding problems in Failover & Failback
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Common Errors you would get to see Errors 1412 and 1478 are restore errors requiring mirror database to be restored again. Error 1416 is a restore error requiring mirror database to be restored again. The mirror database is in the standby or recovered state. Errors 1418 and 1486 are communication errors. You will need to check both sides of the communication as the problem server can not be identified via error messages. Verify DBM endpoints are started Query sys.database_mirroring_endpoints DMV to confirm that all servers are using the same authentication and encryption settings. Use Configure Security interface in SSMS to specify the startup accounts for all servers involved in the DBM session. To use NTLM or Kerberos authentication, the SQL Server instance startup account must be running with a non-local account Error 1456 is also a communication error when adding a witness server to a DBM session - follow same workaround as per error 1418. 1412 and 1478: The mirror database must have restored through the last created transaction log backup otherwise the log truncation done by the transaction log backup will prevent the mirror database from synchronizing. Scheduled jobs may need to be temporarily stopped. 1416: The last transaction log restored to the mirror database must use the NORECOVERY restore option. 1. Verify DBM endpoints are started locally and remotely with telnet to see if SQL Server is listening on the port and it is not blocked by a firewall. If you can connect locally but not remotely then there is likely to be a firewall which needs to have an exception for the sqlservr.exe process. 2. Query sys.database_mirroring_endpoints DMV to confirm that all servers are using the same authentication and encryption settings and have the correct endpoint role (i.e. partner, witness or all). If all endpoints do not have the same settings, drop the DBM endpoints and recreate with the same settings via Configure Security interface in SSMS. 3. Use Configure Security interface in SSMS to specify the startup accounts for all servers involved in the DBM session. This will grant CONNECT permissions on the endpoints for the other server instance’s startup account. 4. Use either a domain based account or the Network Service account. To specify the Network Service account the format is: <domain name>\<machine name>$ If you need to use the Local System or any other local account then Certificate authentication must be used. Confirm all servers belong to the same DNS domain or that the DNS suffix search list contains the other server domains. This can be done using IPCONFIG /ALL on each server. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

31 Understanding problems in Failover & Failback
Database Mirroring Session uses a default 10 “pings”. If they missed then it declares as failure: Fast Failures (nearly immediate) with SQL Server instance crashes Endpoint port closes quickly Network retry from partner quickly fails OS says that the port is closed Medium Failures (~10 sec) such as Catastrophic server failure Power supply fails, Network retry from partner waits for timeout SQL Server “ping” will most likely fail first and then Failover begins in 10 seconds Slow Failures (~40 sec) such as corruption of the log drive on principal database Pending IOs to the log drive queue up though “pings” are working fine After 20 seconds, SQL Server issues IO warning After 40 seconds, SQL Serve declares IO failure Failover begins 40 seconds after log drive crashed 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

32 Understanding problems in Failover & Failback
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Monitoring & Resolution Ensure TCP/IP is enabled from SQL Configuration Manager all the instances in database mirroring. Named pipes can also be used, but a drawback is it does not use the TCP/IP retry algorithm, in many cases, a named pipes connection attempt may time out before connecting to a mirrored database. When Database Mirroring appears to be running smoothly – don’t let yourself get a false sense of security. Things may be running smoothly but this is when you’ll want to be on the highest alert. The name of an endpoint is changed when the port changes. If SQL Server is listening on dynamic ports, the port can change, changing the endpoint name, and dropping the associated endpoint permissions. To avoid this security risk, do not set custom permissions on endpoints associated with dynamic ports, and do not change the order in which a TCP/IP endpoint occurs in the registry. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

33 Understanding problems in Failover & Failback
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Understanding problems in Failover & Failback Monitoring & Resolution The database might contain changes made by transactions that are uncommitted at the recovery point. So it is better to keep the transactions in smaller batches to avoid lengthy time during failover time for a mission-critical database. If redoing fails, the mirror server pauses the session by putting the database into the SUSPENDED state. The database owner must resolve the cause of the failure before resuming the session. Client connection string supplied by the client contains information that the data access provider uses to connect to the database. Connection string should contain the Network attribute to specify the network protocol. This ensures that the specified network protocol persists between connections to different partners. Recommended protocol for connecting to a mirrored database is TCP/IP {Network=dbmssocn;} 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

34 Understanding problems in Failover & Failback
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Monitoring & Resolution Account under which the SQL Server instance is running has the necessary CONNECT permissions. If SQL Server runs under a different user on one or more of these servers, then you need to grant CONNECT permission on the endpoint of a server to the login account of the other servers. Make sure the port that is used for database mirroring endpoints is opened through the firewall, if used. Critical - speed of error detection and, therefore, the reaction time of the mirroring session to a failure, depends on whether the error is hard or soft. Few network components and some IO subsystems, have their own time-outs to determine failures. Mirroring does not protect against problems specific to client accessing the servers, such as public network adapter on any partner server. Database mirroring failover and failback procedures are dependant on the network components and server hardware in addition to the server/database configuration. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

35 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

36 Effective Usage of SQL Server 2005 Database Mirroring
Conclusion 06th October 2007, SQL Bits To monitor: To test the automatic failover you will shutdown the principal server and watch how the mirror server automatically comes online to handle requests. If your application consists of multiple databases, develop an alert mechanism to detect if one database fails over while others don’t. In this case you might need a manual failover of other databases. If there are multiple databases in an instance, you need to mirror each database individually in the event of manual failover. In some cases (High Availability mode) failover can occur automatically while in others it must be forced. Enable ALL of the threshold warnings for BOTH the principal and the mirror instance. To understand how the system works without the option of automatically failing over, try using database mirroring without a witness for a time. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

37 Effective Usage of SQL Server 2005 Database Mirroring
Conclusion Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits High Performance is the best configuration with which to start as it has the least impact to your production workload yet still offers a secondary database that’s close to current. The performance of database mirroring is very closely associated with the type of application, transaction safety level, and network performance. The recommended limit of 10 databases in 1 SQL instance is only approximate and not an exact number. The limit (memory and threads) can vary depending upon your application and workload. When used with a high-bandwidth and low-latency network, database mirroring can provide a reliable high-availability solution against planned and unplanned downtime. Across geographically distant data centers, database mirroring can provide the foundation for a disaster-recovery solution. Do not forget to test and plan for any sort of new introduction to your High Availability ranch, even changes in Database Mirroring mode. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

38 Effective Usage of SQL Server 2005 Database Mirroring
Conclusion Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits SP Rolling upgrade Upgrade the mirror first Failover Upgrade the new mirror Failover again back to original Principal Wait for a low activity window Switch to synchronous, and wait till SYNCHRONIZED Follow process similar to one above Switch back to asynchronous Various runtime and DBCC CHECK* errors, such as 605, 2511, etc. Unit of transfer between principal and mirror databases is a transaction log block. Database and Transaction log backups mitigate hard drive failures if they are usable meaning that they can be restored. One of the biggest mistakes is to create backups on the same drive or set of drives that make a logical partition or LUN. Asynchronous mode 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

39 How DBM provides the highest level of data protection?
Conclusion How DBM provides the highest level of data protection? Hard drive failure – there are two redundant copies of database data and log files which mitigates a hard drive failure as a possible cause of extended downtime. Mirror database transaction log has not hardened all log blocks written to the principal database transaction log - this is mitigated by the fact that the DBM session must be synchronized before an automatic or manual failover occurs. Partial or Incorrect database file writes - can cause database corruption which requires a restore operation in SQL Server 2005. Implementing the databases that should be grouped together as file groups in a single database can provide the flexibility to split up large maintenance operations. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

40 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

41 Frequently Asked Questions
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits What is the slowest network that I can use with mirroring? What and Why don’t some technologies work seamlessly after a failover? How to disable database mirroring for a particular database, is it possible? What is the usage of WITNESS instance in Database Mirroring setup? Is it recommended to use 32-bit server as Witness for a 64-bit Database Mirroring Partner server? Why client applications are not redirected to new principal database? This only works under the following conditions: Client application using ADO. Net or JDBC driver Connection string contains a database that is part of a DBM Session Connection to server never changes database context (i.e. USE <database>) Where to look for errors for Communication errors & Failover causes? Service Pack/Hot fix upgrade? Depending upon the rolling upgrades the service pack levels can be different between principal and mirror, but for steady state they must be identical. Disable DB Mirroring: Database mirroring requires AUTO_CLOSE OFF. ALTER DATABASE <database_name> SET PARTNER OFF Whenever a witness is set for a database mirroring session, quorum is required. 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. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

42 Frequently Asked Questions
Useful Catalog Views & DMVs to monitor DMM: Sys.database_mirroring, Sys.tcp_endpoints & Sys.server_principals Sys.database_mirroring_witnesses Sys.database_mirroring_endpoints Sys.dm_db_mirroring_connections Sys.database_recovery_status Telnet & Ping Using Telnet for endpoint test – better for initial port test but not useful for authentication or encryption mismatches checks. Ping – reports the FQDN name of a server DBM across Internet – it is possible however consider factors like allowing DBM endpoint to pass through firewall and additionally may need to disable IP Security. (overhead) 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

43 Effective Usage of SQL Server 2005 Database Mirroring
Agenda 06th October 2007, SQL Bits Introduction SQL Server 2005 High Availability features Feature Overview Database Mirroring Setup (overview) Monitoring & Performance Understanding problems in Failover & Failback Conclusion Frequently Asked Questions Q & A 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

44 Effective Usage of SQL Server 2005 Database Mirroring
Questions & Answers Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits I’ll do my best to answer your questions within the available time. If not we can discuss within the Ask The Experts (ATE) area. Useful links: Database Mirroring in SQL Server 2005 & FAQ Blogs - Webcast Implementing Database Mirroring by Kimberly L. Tripp, SQLskills.com – look at search for event id 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -

45 Thank you and have a great day & weekend ahead.
Effective Usage of SQL Server 2005 Database Mirroring 06th October 2007, SQL Bits Thank you and have a great day & weekend ahead. Feedback is highly appreciated. 06th October 2007, SQL Bits Effective Usage of SQL Server 2005 Database Mirroring -


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