Database Mirroring in the Real
World
Craig Purnell
About Me
•Database Administrator with Baker
Hostetler
•Experience with SQL Server 7/2000
and up
•Previously consulted in the
transportation industry, programming
custom UNIX ERP solutions
•MCSE / MCSA / MCITP (DBA) etc.
Overview
• Mirroring is a high availability technology
• Continuous stream of log records are sent
to the mirror and “replayed”
• Mirror is unavailable for client connections
• Protects a single database at a time
Mirroring Demystified
• Synchronous with Witness
– No data loss
– Auto detection of Failure / Failover
• Synchronous without Witness
– No data loss
– Possibility of downtime
• Asynchronous
– Manual failover
– Possibility of data loss.
Asynchronous Mirroring
Client
1. Transaction
4. Transaction Complete
Mirror Server
2c. Mirror Acknowledge Write
Principal
Server
2. Transmit to Mirror
3. Acknowledge Write
complete
2. Write to Local Log
DB
LOG
2a. Write to Mirror
Log
2b. Redo
changes on DB
LOG
DB
Synchronous Mirroring
Client
1. Transaction
7. Transaction Complete
Mirror Server
6. Mirror Acknowledge Write
Principal
Server
2. Transmit to Mirror
2. Write to Local Log
DB
3. Acknowledge
Write complete
LOG
5. Acknowledge
Write complete
4. Write to
Mirror Log
Redo changes
on DB
LOG
DB
Synchronous Mirroring with
Witness
• Microsoft’s SAP system
Primary Data Center
Disaster Recovery Data Center
Witness
Principal
Mirror
Log Shipping Secondary
Synchronous
Database
Mirroring
Log Shipping
Mirroring with Certificates
• Different domains without a trust
(mergers, acquisitions, DMZ)
• Internet (partners, affiliates)
Mirroring at Baker Hostetler
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11 Offices; All SQL is centralized
Early adopter of the technology
Was mirroring from Cleveland to Denver
Currently in a state of transition….
Future: Eagan, MN to Cleveland
Mirroring at Baker Hostetler
Endpoints
(1)
• Required by mirroring
• SQL Server Object that maps in TCP
socket
• Authentication
• Encryption (RC-4 etc)
• See BOL for options
Limitations (32bit)
• Best Practice: 10 mirrored databases per
server.
• Effects of mirroring on the target: SQL
runs out of VAS (32 bit)
• Band-aid: -g512 (increase memtoleave)
Mirror Configuration
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Identical Hardware not required
Memory / Disk requirements
Drive Letter layouts
File growths/shrinks
Demo #1 (T-SQL)
Megadata from I1 to I2
WAN acceleration
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(1)
Riverbed Steelhead (hardware based)
Turn off encryption and compression
SQL 2008 Effects (native compression)
Trace Flag -T1462
WAN Acceleration
Index Rebuilds : 70-75% compression
(2)
MPLS Network QOS
Mirroring Timeouts
Mirroring Timeouts
Test: 2 servers / 10Mbps crossover, dedicated
endpoints
Demo #2 (GUI)
Adventureworks from I1 to I2
Mirror on a Dedicated IP
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Assign IP to 2nd NIC
Create Endpoint to listen only on that IP
Repeat for mirror
Have Network ops team configure routing
Mirroring and Licensing
(1)
• Witness can be SQL Express
• Mirror server can be unlicensed – as long
as you do not have anything running
queries against the instance.
• No snapshots allowed.
• See SQL Licensing White paper for more
details
Mirroring and Licensing
(2)
• Must use Standard or Enterprise
• Synchronous vs. Asynchronous mirroring
• Can’t mix versions.
Demo #3 (T-SQL)
Rolling Upgrade of Northwind
and Pubs
Mirroring: Lessons Learned
• Not a substitute for T-Log backups
• Network latency definitely is a factor
• Timeouts due to “design flaw” in Riverbed
dual WAN configuration
• Log Buildup on Primary
• QOS network rules
• Great way to shovel data from A to B
Index Rebuilds
• Carefully Schedule production instances
• Selective rebuild script really helps.
• Your mileage may vary.
Monitoring Mirroring
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System Monitor Counters
T-SQL
Mirroring Monitor GUI
DMVs:
sys.dm_db_mirroring_connections
sys.database_mirroring
sys.database_mirroring_witnesses
sys.database_mirroring_endpoints
sys.tcp_endpoints
sys.server_principals
sys.database_recovery_status
(1)
Demo #4
Index Rebuilds and Monitoring
SQL Agent Alerting
• Configure SQL Agent to alert you of
mirroring problems
• See Technical Article: Alerting on Mirroring
Events (escape macros are wrong).
Error messages: T-SQL vs. GUI
Msg 1418, Level 16, State 1, Line 1
The server network address
"TCP://MYMIRROREDSERVER.domainname.com:5022" can not be
reached or does not exist. Check the network address name and that the
ports for the local and remote endpoints are operational.
Same error condition (mixing SQL versions)
Troubleshooting
• netstat –ano
• tcping hostname 5022
• Verify endpoints are using the same
encryption, authentication
• Are you running SQL as LocalSystem?
• Use the GUI to configure the endpoints for
you
• Try and recreate the problem with the GUI
Troubleshooting
• Verify the service accounts are Logins
• Verify both service accounts has CONNECT
rights on the endpoint(s)
• You must make a Backup and a T-Log
backup AFTER the DB is in full recovery
mode (watch for extraneous Tlog
backups)
• Look in the Error log
• Windows Firewall must be configured
• IPV6!
Other Issues
• Logins are NOT transferred
• Use sp_help_revlogin to get an exact sid /
SQL login list
• SQL Agent Jobs are not transferred
• Extended Stored Procedures
• Linked servers
Key Takeaways
• Database Mirroring can eliminate the single
point of failure of clustering (aka the shared disk
problem)
• Database Mirroring, coupled with clustering can
raise your business database uptime an order of
magnitude.
• This technology can be implemented without the
need of expensive SAN…just a little creativity on
YOUR part
Any Questions?
Contact Information
cpurnell@bakerlaw.com or craig.purnell@gmail.com
References
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(1)
Tcping http://www.elifulkerson.com/projects/tcping.php
SQL Server Licensing White Paper
http://download.microsoft.com/download/e/c/a/ecafe5d1-b514-48ab-93eb61377df9c5c2/SQLServer2005Licensingv1.1.doc
Asynchronous Database Mirroring with Log Compression in SQL Server 2008
http://sqlcat.com/technicalnotes/archive/2007/12/17/asynchronous-database-mirroringwith-log-compression-in-sql-server-2008.aspx
Database Mirroring White Paper
http://download.microsoft.com/download/4/7/a/47a548b9-249e-484c-abd729f31282b04d/DatabaseMirroring.doc
Database Mirroring: Best Practices and Performance Considerations
http://download.microsoft.com/download/4/7/a/47a548b9-249e-484c-abd729f31282b04d/DBM_Best_Pract.doc
Mirroring with Certificates: http://msdn2.microsoft.com/en-us/library/ms191140.aspx
Transferring Logins and Passwords between instances:
http://support.microsoft.com/kb/918992
References
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Alerting on Database Mirroring Events
http://www.microsoft.com/technet/prodtechnol/sql/2005/mirroringevents.mspx
How to: Minimize Downtime for Mirrored Databases When Upgrading
Server Instances http://msdn.microsoft.com/enus/library/bb677181.aspx
Asynchronous Database Mirroring with Log Compression in SQL
Server 2008
http://sqlcat.com/technicalnotes/archive/2007/12/17/asynchronou
s-database-mirroring-with-log-compression-in-sql-server-2008.aspx
High Availability and Disaster Recovery for Microsoft’s SAP Data Tier:
A SQL Server 2008 Technical Case Study
http://download.microsoft.com/download/d/9/4/d948f981-926e40fa-a026-5bfcf076d9b9/MSIT_SAP_Data_Tier_HA_DR.docx
Monitoring Mirroring
(3)
• EXEC [msdb].[sys].[sp_dbmmonitorresults] @dbname,
@interval
WHERE:
@dbname – the mirrored database
@interval - 0 = last row, 1 last two hours, 2 last four, 3 last
eight, 4 last day, 5 last two days, 6 last 100, 7 last 500,
8 last 1000, 9 ALL