Database Mirroring: High Availability (HA) and Disaster Recovery (DR) Technology

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1 Written by Zakir Hossain, CS Graduate (OSU) CEO, Data Group Fed Certifications: PFA (Programming Foreign Assistance), COR (Contracting Officer), AOR (Assistance Officer) Oracle Certifications: OCP (Oracle Certified Professional), OCA (Oracle Certified Associate), Oracle RAC- Admin, Oracle Backup/Recovery-Admin, Oracle Performance/Monitoring-Admin, Oracle App Server-Admin Microsoft Certifications: MCDBA (Microsoft Certified Database Admin), MCITP: SQL Database Admin, MCITP: SharePoint Admin System Admin Certifications: MCITP: Windows Server 2012 Admin, RHCSA (Red Hat Certified System Admin) Programming Certifications: SCJP (Sun Certified Java Programmer) Other Certifications: Security+, ITIL V3 Please Read License Agreement carefully.. You are allowed to use this document only for your personal and job related purposes. Under no circumstances, you are not allowed to share this document with any institute/organization, individuals, business entity, and students either current or discontinued without written permission. It also prohibited modifying or altering any parts of this document either for commercial or training purposes. It is 100% prohibited and violation of Federal Law of Intelligence Product. A serious action will be taken if it is violated. If you need our books, notes, documents, please contact directly with Zakir Hossain ([email protected], ) Introduction to High Availability Options in SQL Server: Data means everything to a business, a business cannot run without reliable data, have available data when a business need, and the way business requires the data. How we make data always available to users, vendors, employees is a very critical task. This is one of the most important, and critical task of a Database Engineer. Business needs data 99.99% of the time of a day to support their clients and customers. So, after calculation, a database can be down for about 5 minutes a year. This could be tough and very expensive. However, it is not impossible to achieve using SQL Server. Since SQL Server 2008 R2, Microsoft assures six (6) availability meaning SQL Server 2008 R2 is the only database system that is available for % of time comparing other popular databases like Oracle, DB2, MySQL, and Sybase. High availability and Disaster Recovery solutions of database is used for high uptime of a database and be able to distribute data to multiple instances running in different locations regardless of geographical locations. High Availability Technologies/Solutions in SQL Server: VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

2 1. Log Shipping 2. Replication 3. Database Mirroring 4. Failover clustering 5. AlwaysOn (AlwaysOn Availability Group) Database Mirroring: Database mirroring is a technology in SQL Server that offers high availability (HA) and Disaster Recovery (DR) solution for Databases. Database mirroring technology started from SQL Server Again, it is a HA/DR solution to maintain two copies of same database on two different servers/instances. To use Database mirroring technology a database must have Full recovery model. The database can be mirrored from one SQL Server instance to another SQL Server instance and it cannot be done within the same instance. So, it must have at least two instances. The source instance is called Principal server and the target instance is called Mirrored server. However, it can have one more server, which is called Witness server. These instances could be on the same machine or on different machines. For production environment, the instances should be on different servers/machines. The principle server sends the active transaction log record to the mirrored server. The mirrored server applies the transaction log record one by one in sequence as they come-in. Database Mirroring Topology: VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

3 VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

4 Database Mirroring Advantage: 1. If primary server fails, it can automatically failover onto secondary server (in high safety mode). It depends on operational mode 2. It Increases Database Availability and can be zero downtime 3. Database mirroring can be combined with other high available and disaster recovery technologies such as clustering, log shipping, and replication 4. Failover can be as fast as few seconds 5. Data and database on the target server is a copy of the principal server is in real time 6. May be no loss of data 7. Data synchronization in real time 8. No downtime/can minimize downtime for HD or software upgrade 9. Comparatively easy to configure and manage 10. Does not require expense hardware 11. Does not require additional licensing cost VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

5 12. It is not limited by geographical distance 13. Database mirroring supports full-text catalogs Database Mirroring Disadvantage/ Limitations: 1. Database mirroring is per database only solution, which means that logins and jobs from the principal Server must be manually recreated on the mirror. That is can have only one Mirrored Database copy of a database 2. If needed to mirror other databases, every database requires separate mirroring configuration 3. There might be a possibility for delay, which can only be reduced with better hardware and network bandwidth 4. Mirrored database is inaccessible to clients/users. It means clients/users cannot use the Mirrored database while the principal database is up and running. That means only one database is accessible at a time 5. Cannot read data from the Mirrored Database Can create Snapshot of Mirrored Database for Reporting/Reading Purposes 6. Does not support FileStream 7. Can have only 1 copy of the primary database 8. If witness fails, the automatic failover will not work 9. Potential data lost is possible in asynchronous operation mode Highlights of Database Mirroring: 1. At least two instances are required to configure Database Mirroring. However, it can use three instances 2. Source server is called principal server 3. Source database is called Principal database 4. Target server is called mirrored server 5. Target database is called Mirrored database 6. The instance is called a witness instance 7. The second instance hosts the copy of the live database 8. Users can access only the principal database and they can read, write, delete, and update records in this database 9. Users cannot access mirrored database. So there are no options for users to be able to read, write, delete, update records in this database VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

6 10. The 3rd instance monitors the other two instances like activities, performance, applied or not applied logs and that is why it is called witness server 11. If the principal database goes down, 3rd instance will catch it. This will bring the mirrored database online and make it principal database 12. If you do not use the 3rd instance, you can still setup database mirroring. However you need to failover the database manually 13. Witness Server helps for automatic database Failover, Monitor Principal and Mirrored Server and database for mirroring and helps to form quorum/majority Requirements to setup database mirroring: 1. Principal database must have full recovery model 2. Does not support Simple or Bulk-logged Recovery Model 3. Restore the Principal database on the secondary server with no recovery 4. Take full and transaction log backup of the Principal database and restore on the secondary server 5. Database name must be same on both Principal and Target server 6. Service Accounts: Same service accounts for all servers involve in Mirroring (AD: Active Directory) Example: DataGroupUSA1 (AD) Service Account: SQLDBA Complete Account Name: DataGroupUSA1\SQLDBA 7. Database mirroring verification/testing: It is very to verify after setting up a database mirroring. Here are the different ways to verify the setup is working: 1. Verify or count the total number of records in one of the table in principal database Select COUNT (*) from transactions Select * from transactions 2. Insert some records into one of the table in principal database insert into transactions values (20, 59.99) 3. Shutdown the principal database 4. Refresh both principal and mirrored server and watch changes of the database status 5. Start the main sever again VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

7 6. Now mirror server is principal 7. Main is mirrored database now 8. Verify Transaction: Select * from transactions (run this on current principal db) How to setup mirroring: Ensure Primary Database has FULL Recovery Model Perform Full and Transaction Log Backup local server or network Move the Backup to Target Server/Network Server Restore on Mirror Server NORECOVERY MODE Once connected, we also notice our endpoint name is Mirroring and we are listening on port Click "Next >" and you'll see the Service Accounts screen. VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

8 When using Windows Authentication, if the server instances use different accounts, specify the service accounts for SQL Server. These service accounts must all be domain accounts (in the same or trusted domains). If all the server instances use the same domain account or use certificate-based authentication, leave the fields blank. Since my service accounts are using the same domain account, I'll leave this blank. Click "Finish" and you'll see a Complete the Wizard screen that summarizes what we just configured. Click "Finish" one more time. If you see the big green check mark that means Database Mirroring has been configured correctly. However, just because it is configured correctly doesn't mean that database mirroring is going to start... Next screen that pops up should be the Start/Do Not Start Mirroring screen: VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9

9 We're going to click Do Not Start Mirroring just so we can look at the Operating Modes we can use: Since we didn't specify a witness server we will not get the High Safety with automatic failover option, but we still get the High Performance and High Safety without automatic failover options. For this example, we'll stick with synchronous high safety without automatic failover so changes on both servers will be synchronized. Next, click "Start Mirroring" as shown below. VA Office: 5600 General Washington Dr., Suite: B 212, Alexandria, VA of 9