Implementing Database Snapshot & Database Mirroring in SQL Server 2005 Presented by Tarek Ghazali IT Technical Specialist Microsoft SQL Server MVP Microsoft SQL Server MCTS Web Development MCP

Database Snapshot Recovering from Human Error Snapshot of an entire database at a point in time –Great to recover from User Error –Created instantly –Read only Works with –Single server –Database Mirroring –Failover Cluster Base database continues to change –Database Snapshot does not restrict the base database Multiple Snapshots are allowed

Database Snapshot How it really works dbSnap – Read-Only Database Snapshot USE dbSnap SELECT (pages 4, 6, 9, 10, 14) 1 Page CREATE DATABASE dbSnap AS SNAPSHOT OF mydb mydb – Database USE mydb UPDATE (pages 4, 9, 10)

Value R D B H J L Y M Space Used Command Create Northwind_SS NorthwindNorthwind_SS Update Northwind 0% Value DX Read Northwind_SS 12.5% Result: Snapshot (Copy on Write) DD

Database Snapshot How it really works (Cont) A sparse file is an essentially empty file that contains no user data and has not yet been allocated disk space for user data. As more and more pages are updated in the source database, the size of the file grows.

Constraints on the Source Database DB snapshot and source database must be on the same instance If any db snapshots exist on a source database, the source database cannot be dropped or restored –first delete all the db snapshots However, backing up the source database works normally; it is unaffected by DB snapshots

Considerations of DB Snapshots None of the specifications of the files of the DB Snapshot can be changed A DB Snapshot inherits the security constraints of its source database –DB snapshots are read-only, inherited permissions cannot be changed –If you drop a user from the source DB, it is still in the snapshot To prevent future access you can drop\disable the login Snapshots cannot be backed up (or restored) Snapshots cannot be attached (or detached) Snapshots cannot be created on FAT32 or RAW partitions Full-text indexing is not supported on Database Snapshots, and full- text catalogs are not propagated from the source database

Database Snapshot How much Space Used? Size Based on Update Patterns –View the sparse files (sys.master_files) –View the Maximum Size Possible –View the Actual Size Select the size column from sys.master_files. The size column in sys.master_files reflects the maximum space, in SQL pages, that the snapshot can ever use; this value is equivalent to the Windows Size field, except that it is represented in terms of the number of SQL pages in the file; the size in bytes is: ( number_of_pages * 8192)

Database Snapshot How much Space Used? (Cont) Use Windows Explorer to right click on the NTFS sparse file 'C:\AWDW.SNP' and select Properties Look at the “Size” and you will see it is similar to the size of the source database file However, look at the “Size on Disk” and you will see how much space is REALLY used

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High Availability Solutions Overview SQL Server provides several options for creating high availability for a server or database. –Failover clustering –Database mirroring –Log shipping –Replication

Database Mirroring New for SQL Server 2005 Instant Stand-By Fault Tolerant Virtual Database Very fast Failover (3 sec)– No Data-Loss Automatic, Transparent Client-Redirect Automatic Re-Sync after Failover Principal Client Witness Mirror Client

Database Mirroring Overview of Database Mirror Mirroring allows the user to create an exact copy of a database on a different server. The mirrored database must reside on different instance of SQL Server Database engine.

Database Mirroring SQL Server 2005 RTM,SP1,SP2 Microsoft fully supports database mirroring with SQL Server 2005 SP1,SP2 onwards. For the RTM release (prior to SP1), Microsoft support services will not support databases or applications that use database mirroring. The database mirroring feature should not be used in production environments. Prior to SP1, database mirroring is disabled by default, but can be enabled for evaluation purposes by using trace flag The following T-SQL statement can be used to achieve this: DBCC TRACEON(1400)

Transparent Client Redirect No changes to application code Client automatically redirected if session is dropped MDAC is aware of Principal and Mirror servers – Upon initial connect to Principal, MDAC caches Mirror name – When client attempts to reconnect If Principal is available, connects If not, MDAC automatically redirects connection to Mirror

Database Mirroring Synchronous Mode This is also called as high safety mode. – In this mode, every transaction applied to the principal will also be committed on the mirror server. The transaction on the principal will be released only when it is also committed on the mirror. High safety mode with/without automatic failover

Database Mirroring Asynchronous Mode This is also known as the high performance mode. –Here performance is achieved at the cost of availability. In this mode, the principal server sends log information to the mirror server, without waiting for an acknowledgement from the mirror server. This mode allows the principal server to run with minimum transactional latency and does not allow the user to use automatic failover

Failover Solutions Clustering and Mirroring Compared Both Provide –Automatic Detection and Failover –Manual Failover –Transparent Client Redirect –Zero Work Loss Database Mirroring –Database scope –Standard servers –Fastest failover –Limited reporting on standby –Duplicate copy of database Failover Clustering –System scope –Certified hardware –Fast failover –No reporting on standby –Single copy of database

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Resources & Questions Microsoft Resources: - msdn.microsoft.com/sqlserver/msdn.microsoft.com/sqlserver/ - Contact me: - Download Presentation : -