When change tracking is enabled for a table, some administration operations are affected. The following table lists the operations and the effects you should consider.

Effects on DML

Change tracking has been optimized to minimize the performance overhead on DML operations. The incremental performance overhead that is associated with using change tracking on a table is similar to the overhead incurred when an index is created for a table and needs to be maintained.

For each row that is changed by a DML operation, a row is added to the internal change tracking table. The effect of this relative to the DML operation depends on various factors, such as the following:

• The number of primary key columns
  
• The amount of data that is being changed in the user table row
  
• The number of operations that are being performed in a transaction
  

Snapshot isolation, if used, also has an effect on performance for all DML operations, whether change tracking is enabled or not.

Here we will see how Data Storage happens with Change Tracking and what effect does it carry. For Each User Table enabled for Change Tacking there is one internal change table and Internal Transactional Table.

There is one internal transaction table for the database.These internal tables affect storage requirements:

Internal change table for each table enabled for Change Tracking and Internal Transactional Table.

I have enabled a Table for CT; let’s see what the space is used for by internals tables. Initially Internal change Table will be of Zero Size.

Now have some transactions done on the table (Delete or Insert) and check the space used by internal tables.

For each change to each row in the user table, a row is added to the internal change table. This row has a small fixed overhead plus a variable overhead equal to the size of the

Primary key columns. The row can contain optional context information set by an application. And, if column tracking is enabled, each changed column requires 4 bytes in the tracking table.

For each committed transaction, a row is added to an internal transaction table.

sys.syscommittab :

Displays one row for each transaction that is committed for a table that is tracked by SQL Server change tracking. The sys.dm_tran_commit_table management view, which is provided for supportability purposes and exposes the transaction-related information that change tracking stores in the sys.syscommittab system table. The sys.syscommittab table provides an efficient persistent mapping from a database-specific transaction ID to the transaction’s commit log sequence number (LSN) and commit timestamp. The data that is stored in the sys.syscommittab table and exposed in this management view is subject to cleanup according to the retention period specified when change tracking was configured.

sys.change_tracking_databases: The view list the database’s enabled for Change Tracking.

sys.change_tracking_tables        : The view list the table’s information enabled for Change tracking.

sys.dm_tran_commit_table       : Displays one row for each transaction that is committed for a table that is tracked by SQL Server change tracking. The sys.dm_tran_commit_table management view, which is provided for supportability purposes and exposes the transaction-related information that change tracking stores in the sys.syscommittab system table. The sys.syscommittab table provides an efficient persistent mapping from a database-specific transaction ID to the transaction’s commit log sequence number (LSN) and commit timestamp. The data that is stored in the sys.syscommittab table and exposed in this management view is subject to cleanup according to the retention period specified when change tracking was configured.

Also, the sys.internal_tables catalog view lists the internal tables that are created when change tracking is enabled for a user table

Change tracking in SQL Server 2008 enables applications to obtain only changes that have been made to the user tables, along with the information about those changes. With change tracking integrated into SQL Server, complicated custom change tracking solutions no longer have to be developed.

Change tracking is an important building block for applications that synchronize and replicate data in scenarios in which end-to-end replication solutions do not work and a custom solution is required.

For example, a scenario that requires synchronizing data with data stores that are not SQL Server databases or in which the store schemas are very different.`

Enable Change tracking at Database Level.

Change Tracking

Use to enable or disable change tracking for the database.

To enable change tracking, you must have permission to modify the database.

Setting the value to True sets a database option that allows change tracking to be enabled on individual tables.

You can also configure change tracking by using ALTER DATABASE.

Retention Period

Specifies the minimum period for keeping change track information in the database. Data is removed only if the Auto Clean-Up value is true.

The default value is 2.

Retention Period Units

Specifies the units for the Retention Period value. You can select Days, Hours, or Minutes. The default value is Days.

The minimum retention period is 1 minute. There is no maximum retention period.

Auto Clean-Up

Indicates whether change tracking information is automatically removed after the specified retention period.

Enabling Auto Clean-Up resets any previous custom retention period to the default retention period of 2 days.

Enable Change tracking at Object Level.

Change Tracking

Indicates whether change tracking is enabled for the table. The default value is False.

This option is available only when change tracking is enabled for the database. For more information, see Database Properties (ChangeTracking Page).

To enable change tracking, the table must have a primary key, and you must have permission to modify the table.

You can also configure change tracking by using ALTER TABLE.

Track Columns Updated

Indicates whether the SQL Server Database Engine tracks which columns were updated.

Will cover about internals of Change Tracking with important catalog views in next post.

Developed the script to get installed SQL Server Instance on a Server. To run this script user should have admin privileges or minimum permission to read OS registry on the server.

If we configure Database mirroring for Synchronous mode where principal waits for an acknowledgement from the mirror server before settling a commit or rollback, what would happen if the mirror server fails?

When transaction safety is set to FULL, the database mirroring session runs in high-safety mode and operates synchronously after an initial synchronizing phase.

The mirror server synchronizes the mirror database with the principal database to achieve synchronous mirror operation session. When the session begins, the principal server begins sending its active log to the mirror server. The mirror server writes all of the incoming log records to disk as quickly as possible. As soon as all of the received log records have been written to disk, the databases are synchronized. As long as the partners remain in communication, the databases remain synchronized.

After synchronization finishes, every transaction committed on the principal database is also committed on the mirror server, guaranteeing protection of the data. This is achieved by waiting to commit a transaction on the principal database, until the principal server receives a message from the mirror server stating that it has hardened the transaction’s log to disk. Note the wait for this message increases the latency of the transaction.

The time required for synchronization depends essentially on how far the mirror database was behind the principal database at the start of the session (measured by the number of log records initially received from the principal server), the work load on the principal database, and the speed of the mirror system. After a session is synchronized, the hardened log that has yet to be redone on the mirror database remains in the redo queue.

As soon as the mirror database becomes synchronized, the state of both the copies of the database changes to SYNCHRONIZED.

Synchronous operation is maintained in the following manner:

1. On receiving a transaction from a client, the principal server writes the log for the transaction to the transaction log.
   
2. The principal server writes the transaction to the database and, concurrently, sends the log record to the mirror server. The principal server waits for an acknowledgement from the mirror server before confirming either of the following to the client: a transaction commit or a rollback.
   
3. The mirror server hardens the log to disk and returns an acknowledgement to the principal server.
   
4. On receiving the acknowledgement from the mirror server, the principal server sends a confirmation message to the client.
   

High-safety mode protects your data by requiring the data to be synchronized between two places. All the committed transactions are guaranteed to be written to disk on the mirror server.

High-Safety Mode without Automatic Failover

The following figure shows the configuration of high-safety mode without automatic failover. The configuration consists of only the two partners.

When the partners are connected and the database is already synchronized, manual failover is supported. If the mirror server instance goes down, the principal server instance is unaffected and runs exposed (that is without mirroring the data)( When mirror fails, it switches to Asynchronous mode until mirror comes back online and catches up, Once Principal and Mirror synchronized Mirroring mode switch back to Synchronous mode.). If the principal server is lost, the mirror is suspended, but service can be forced to the mirror server (with possible data loss).

High-Safety Mode with Automatic Failover

Automatic failover provides high availability by ensuring that the database is still served after the loss of one server. Automatic failover requires that the session possess a third server instance, the witness, which ideally resides on a third computer. The following figure shows the configuration of a high-safety mode session that supports automatic failover.

Unlike the two partners, the witness does not serve the database. The witness simply supports automatic failover by verifying whether the principal server is up and functioning. The mirror server initiates automatic failover only if the mirror and the witness remain connected to each other after both have been disconnected from the principal server.

When a witness is set, the session requires quorum—a relationship between at least two server instances that allows the database to be made available.

Automatic failover requires the following conditions:

• The database is already synchronized.
  
• The failure occurs while all three server instances are connected, and the witness and mirror server remain connected.
  

The loss of a partner has the following effect:

• If the principal server becomes unavailable under the above conditions, automatic failover occurs. The mirror server switches to the role of principal, and it offers its database as the principal database.
  
• If the principal server becomes unavailable when those conditions are not met, forcing service (with possible data loss) might be possible. For more information, see Forced Service (with Possible Data Loss).
  
• If the only mirror server becomes unavailable, the principal and witness continue.
  

If the session loses its witness, quorum requires both partners. If either partner loses quorum, both partners lose quorum, and the database becomes unavailable until quorum is re-established. This quorum requirement makes sure that in the absence of a witness the database never runs exposed, that is without being mirrored.

• Internals of Database Snapshot – Part 2: Snapshot and the Buffer Pool
  

Check out our new blog post on Internals of Database Snapshot. This blog is the joint effort of Suhas and me.

http://blogs.msdn.com/b/suhde/archive/2011/07/31/internals-of-database-snapshot.aspx

It is always on ease having a backup and recovery strategies in place but few times it also important to recover a database to appoint which doesn’t require a full database restore or in shot no hard efforts. While working with one of my previous organization I was asked to do something which actually made me interested in using Database snapshot feature.

Scenario: A New modules was designed and developed for finance. Once the Module will get deployed in production excel data files need to be uploaded. At the backed few SP will execute which will do processing of data and finance related data will be updated /flag (inactive).

I have been asked to design a recovery strategy for the Database which recovers the database before the time of deployment and without downtime. I thought on all recovery models and at last decided to user database snapshot.

Database snapshot give me a flexibility to revert back to the time when the snapshot was created. As the snapshot database is read-only we can rum comparison between OLPT database and database snapshot.

Combing back to the original question, let’s say Deployment has to start at 1:00 AM, will stop the customer /user access to the application and database so that no transaction should take place. Once the application and database is inaccessible to the customer run select * from sys.sysprocesses to check what all open transaction are in place other then background processes.

Create the database Snapshot.

In case you think after the deployment there is some issue with the module or data and you need to revert back. Restore the snapshot which will revert the changes to the database till the point of snapshot.

Here is an example

Revert the database to a database snapshot (No connection should exists to the source database to apply the snapshot.)

Now to verify that database is reverted successfully, check the records