SQL Server 2008 Database Mirroring

Here are some notes on “SQL Server 2008 Database Mirroring” I took while attending an advanced class on SQL Server taught by Greg Low (from http://sqlblog.com/blogs/greg_low/ and http://www.sqldownunder.com/).

Please note that, although these notes were taken during the class, I might have added some of my own (mis)interpretation :-). Always check your facts on Books Online (I try to provide links when applicable). As with anything else you get from a blog, never use any of this in production before you thoroughly validate it a test environment. Please keep in mind that some of those will be hard to follow without some pre-requisite knowledge and the right context. Reading the post from top to bottom will help.

Database Mirroring

• Software solution for high availability.
• Per database and not per server.
• Increase data protection, availability, upgrade availability.
• Careful – A lot of things live outside the database
• SQL HA options for the masses, without the high-end hardware requirement.
• Very popular option.
• See http://msdn.microsoft.com/en-us/library/ms189852.aspx

Requirements and Limitations

• Full recovery model
• Express (witness only), Standard (can’t do async) or Enterprise
• Logins (logins are not in the database, you need to make sure they are on the other side)
• Cannot have multiple mirrors – use log shipping if you need this
• Not intended for a large number of databases – Depends on traffic
• Careful – In multi-database applications, failover behavior
• Cannot mirror master, msdb, tempdb, model

Endpoints

• Options for name, port, encryption
• Careful – Names, FQDN, IP addresses
• Endpoint encryption
• See http://msdn.microsoft.com/en-us/library/ms181591.aspx
• Authentication (domain accounts, certificates)
• See http://msdn.microsoft.com/en-us/library/ms190456.aspx
• DNS/WINS – name resolution is important, use FQDN
• See http://msdn.microsoft.com/en-us/library/ms189921.aspx
• Careful – domain suffix issues
• Consider the LISTENER_IP option in endpoint configuration – private network for mirroring
• Example: Setting Up Database Mirroring Using Certificates (Transact-SQL)
• See http://msdn.microsoft.com/en-us/library/ms191140.aspx  

Modes

• Roles: Principal, Mirror, Witness
• Modes: high performance (async), high safety (sync), high safety with HA (sync with witness)
• Async: DR scenarios, long distrances, no need for witness – Consider log shipping instead
• See http://msdn.microsoft.com/en-us/library/ms187110.aspx
• Sync: Avoid data loss, performance impact  (wait for both logs to write before ack to client)
• See http://msdn.microsoft.com/en-us/library/ms179344.aspx
• Witness: third system to be able to do failover (any edition, including Express)
• See http://msdn.microsoft.com/en-us/library/ms189902.aspx
• Careful – Keep the witness as a third site. One witness can work with multiple mirrors.
• Encryption: Can be used, encrypted data is usually not larger than original data

Clients

• Clients: ADO.NET v2 required for automatic failover: Failover partner in connection string.
• Careful – Client will ask the primary upon connect. Can try the partner if primary not there.
• In SQL Server 2008: Failover partner cached in registry upon first connect to primary (no change to connection string required if primary available on first connect)
• See http://msdn.microsoft.com/en-us/library/ms366348.aspx

Reporting on Mirror

• Mirror can be access as read-only.
• Database snapshots can be created on mirror, you can use snapshot for reports.
• See http://msdn.microsoft.com/en-us/library/ms175511.aspx
• Snapshot cannot be restored to mirror. Snapshot performance implications.
• Careful – refreshing snapshot, finding the correct snapshot, failover situations
• Consider using snapshot as source for a separate reporting database.
• Consider having a process that keeps track of which is the latest snapshot.

Clustering and Mirroring

• Using database mirroring combine with failover clustering.
• Commonly used as a DR solution.
• Do not use auto failover.
• See http://msdn.microsoft.com/en-us/library/ms191309.aspx

SQL Server 2008

• Logstream compression
• See http://technet.microsoft.com/en-us/library/cc645581.aspx
• Automated page repair
• See http://msdn.microsoft.com/en-us/library/bb677167.aspx
• Incoming log stream processing improvements
• Transparent client redirection

Failover

• Manual failover – Must be in sync, use SSMS or ALTER DATABASE...
• See http://msdn.microsoft.com/en-us/library/bb522476.aspx
• Automated failover – Need witness, must avoid “split brain” scenario
• Careful – Lack of synchronization
• Careful – Option to “Force Service” when not in sync, allows data loss
• See http://msdn.microsoft.com/en-us/library/ms189977.aspx
• After failover, if mirroring isn’t disabled, original principal becomes mirror

Upgrade Steps

• Switch to high safety (sync). Make sure it’s in sync.
• Disable the witness.
• Perform a rolling upgrade (upgrade mirror, manual failover to mirror, upgrade original principal, manual failover to original principal).
• Switch back to high performance (async)
• Re-enable the witness.
• See http://msdn.microsoft.com/en-us/library/bb677181.aspx
• Careful – Some data loss may occur

Technet Best practices

• Looking at Database Mirroring Best Practices and Performance Considerations
• http://download.microsoft.com/download/4/7/a/47a548b9-249e-484c-abd7-29f31282b04d/DBM_Best_Pract.doc
• Comparing Workload1 and Workload2 – OLTP vs. batch-type workload
• Looking at performance with no mirroring vs. Safety OFF vs. Safety FULL
• Looking at time to index creation, index rebuild – impact with ONLINE=ON due to waiting on mirror

Performance counters

• Log send queue rate
• Log bytes redone from cache/sec
• Log bytes sent from chace/sec
• Log compressed bytes rcvd/sec (2008)
• Log compressed bytes sent/sec (2008)
• Log harden times (ms)
• Log remaining for undo KB
• Log scanned for undo KB
• Mirror write transactions/sec
• See http://technet.microsoft.com/en-us/library/ms189931.aspx
• See http://msdn.microsoft.com/en-us/library/ms408393.aspx

Failover process

• Failover occurs
• Some time to detect the failure (hard failures (network outage) are quicker than soft failures)
• Some time to coordinate with the witness
• Decision to failover
• Some time (order of a second) to actually fail over
• Database available on new principal
• Database Undo continues
• Time to failover typically just a few seconds (varies depending on case – pull network cable, power off principal, stop sql server service, shutdown principal, manual failover)

Network

• Log send flow control time (ms)
• Looking at typical network latency (LAN, MAN, WAN)
• Looking at whitepaper numbers on network latency effects on sync mirroring
• SQL Server 2008 – Log stream compression, trade off: data volume vs. CPU, affects both sides
• Looking at long stream compression effect on transactions/sec (SQL CAT team test)
• See http://sqlcat.com/technicalnotes/archive/2007/09/17/database-mirroring-log-compression-in-sql-server-2008-improves-throughput.aspx

Automatic page repair

• Pages on both principal and mirror can be repaired using each other’s copy
• Detection and repair of 823 and 824 errors
• Standard or Enterprise edition
• See http://msdn.microsoft.com/en-us/library/bb677167.aspx
• Looking at suspect_pages
• See http://msdn.microsoft.com/en-us/library/ms191301.aspx

Demo

• Database in full recovery mode
• Backup original database
• Restore both backups with NO RECOVERY
• Careful – Consider options to migrate and keep other objects in sync (Logins, SQL Server Agent jobs (disabled), SSIS packages, linked servers, backup devices, maintenance plans, database mail profiles, etc.)
• In SSMS, Database “Recovering…”, click on Tasks, Mirror…
• Witness – can be enabled from principal or partner, but keep in separate server
• Principal – Listener port, encryption, endpoint
• Partner – Listener port, encryption, endpoint
• Security – Service accounts for principal, partner
• Review screen – Principal and Mirror endpoints – format is TCP://computer:port
• Option to start mirroring right away…
• SELECT * FROM sys.endpoints
• SELECT * FROM sys.dm_db_mirroring_connections
• Notice that the wizard made a few calls on your behald, like encryption protocol
• Careful – If status is “Synchronizing…” all the time, you’re running behind (not keeping up)
• Adding a witness
• Launching “Database Mirroring Monitor”

Demo with TDE 

• Redoing from start – using a database with TDE
• Backup database, backup certificate and private key
• Restore certificate and private key, Restore database with no recovery
• Configuring mirroring via TSQL.
• If not already there, would need to do CREATE ENDPOINT … on each side.
• Then run on each side of the partnership
• ALTER DATABASE … SET PARTNER=’ TCP://computer1:port’
• Then to fail over
• ALTER DATABASE… SET PARTNER FAILOVER
• See http://blogs.msdn.com/sqlserverfaq/archive/2009/03/31/setting-up-database-mirroring-in-sql-server-2008-using-t-sql-when-the-database-is-encrypted-using-transparent-data-encryption.aspx

Mirroring Details

• Backup: Can’t backup mirror, can’t backup snapshot, Consider log shipping for those
• Monitoring: Check if partner is keeping up
• Monitoring GUI: GUI tool, dm_monitor monitor role
• Monitoring SP:  sp_dbmonitorupdate, once per minute by default, updates internal msdb table
• See http://msdn.microsoft.com/en-us/library/ms403827.aspx
• States: Synchronized, Synchronizing, Suspended (pause),  Disconnected
• See http://msdn.microsoft.com/en-us/library/ms189284.aspx
• Cross domain security: Use certificate security
• See http://msdn.microsoft.com/en-us/library/ms191140.aspx
• Client design: Consider the added latency when working with high safety
• Network issues: No set limits for async, compression helps, consider the consequences
• Currently no support for combining filestream and mirroring (consider Clustering)
• Careful - Mirroring and multi-dabase apps. Mirror is per database.

Mirror and other technologies

• Mirroring and Replication – Supported
• Mirroring and Database Snapshots – Supported
• Mirroring and Clustering – Supported. Typical: Cluster local, async mirroring to remote
• Mirroring and Log Shipping – Supported. Consider carefully
• See http://msdn.microsoft.com/en-us/library/bb500117.aspx

Related blog posts

• http://blogs.technet.com/josebda/archive/2009/04/02/sql-server-2008-log-shipping.aspx
• http://blogs.technet.com/josebda/archive/2009/04/02/sql-server-2008-database-snapshots.aspx
• http://blogs.technet.com/josebda/archive/2009/04/02/sql-server-2008-failover-clustering.aspx
• http://www.sqlservercentral.com/blogs/robert_davis/archive/2008/11/18/How-many-mirrored-databases-can-I-have-on-a-server_3F00_.aspx
• http://blogs.msdn.com/john_daskalakis/archive/2009/01/15/9320483.aspx
• http://www.sqlskills.com/BLOGS/PAUL/category/Conference-Questions-Pot-Pourri.aspx