1. Database Mirroring with SQL Server
(An Overview)

2. About me Matt Denton Worked with SQL Server since version 6.5
Worked with SQL Server since version 6.5
Worn many hats
DBA
Developer
Architect
Tended to focus on Dev vs Admin
Currently working at SouthwestRe

3. Why Database Mirroring?
High Availability
Disaster Recovery (with asynchronous mode)
Generally cheaper than clustering
Relatively easy to configure
Supported from Version 2005 thru ?
At least through version 2016
Often deployable to existing hardware

4. Database Mirroring Shortcomings
Objects external to mirrored database are not synchronized
Logins, Operators, Jobs, Linked Servers, windows shares, etc
System databases cannot participate
Only 1 mirror (target) can exist per principal database
Log Shipping, Avail Groups allow multiples targets
Mirrored database is not accessible
Failover can be a challenge for legacy applications

5. How does it work? Principal server serves database to clients
Mirror Server acts as hot or warm standby
All transactions on principal are “redone” to mirror
Transactions streamed from principal to mirror
Works at level of physical log record
(Replication works at logical level)
Stream compressed starting with Version 2008
If using Witness in High-Safety mode, mirror server is brought up as principal automatically

6. How does it work?

7. Operating Modes High Safety (synchronous)
Transactions sync’d immediately on mirror
Transactions not committed on principal until committed on mirror
Most often used for High Availability
High Performance (asynchronous)
Transactions sent to mirror asynchronously
Transactions committed on principal immediately (no waiting on mirror)
Requires Enterprise Edition
Most often used for DR

8. Prerequisites All servers must be on same version of SQL Server
Principal/Mirror must be on same edition
Witness can be on any edition (including Express, as long as same version)

9. DEMO

10. Resources msdn.microsoft.com Search for “Database Mirroring” Google
You’ll find more resources and examples than you would ever need Me
Feel free to me with questions