Before You Begin in VS .NET

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Before You Begin
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To complete the lessons in this chapter, you must have
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SQL Server 2005 installed. A connection to a SQL Server 2005 instance. A copy of the AdventureWorks sample database.
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Managing Database Snapshots
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Real World
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Michael Hotek Database administrators (DBAs) are frequently called upon to manipulate data within operational systems. When an organization needs to bulk load data or make mass changes, DBAs are called upon to directly modify data because it is the most efficient way. Before making any change of this nature to a system, I always create a backup so that the state of the database can be reverted in the event of an error during the data change operation. Creating a backup can account for a significant portion of the maintenance time required to make the changes. With the new Database Snapshot technology, I can quickly save the state of the database before making changes. By eliminating the need to make a full backup, I now have a lot more time to sleep while other DBAs spend endless nights waiting for a backup to complete.
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Lesson 1: Creating a Database Snapshot
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Lesson 1: Creating a Database Snapshot
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A Database Snapshot encompasses a set of new technologies in SQL Server 2005 that allow a DBA to quickly create a data snapshot. This lesson will introduce the key concepts of Database Snapshots along with how to create and use them.
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After this lesson, you will be able to:
Explain the structure of a Database Snapshot. Explain how copy-on-write works. Create a Database Snapshot. Query a Database Snapshot.
Estimated lesson time: 10 minutes
Database Snapshot Structure
You build a Database Snapshot against a source database. Because a Database Snapshot is a point-in-time, read-only copy, it does not contain a transaction log and cannot be written to. At the time of creation, the files underneath a Database Snapshot do not contain any data and consume very little space on disk. This is accomplished by defining the files as sparse files within the file system. By leveraging sparse file technology, you can create Database Snapshots nearly instantaneously, even for multiterabyte databases. All database objects, along with users and permissions, are the same between the source database and the Database Snapshot. Because a Database Snapshot is readonly, you cannot change permissions, add users, or modify any objects or data.
Copy-On-Write Technology
To maintain the Database Snapshot as a point-in-time, read-only copy of a database, as well as satisfy the condition that a Database Snapshot be very fast to create, a mechanism had to be created to maintain the data state. At initial creation, a Database Snapshot does not contain any pages of data nor does it contain any information from the source database. The file structure is created as a sparse file and the Database Snapshot links back to the source database.
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Managing Database Snapshots
When a data page changes in the source database, SQL Server writes the original image of the page into the Database Snapshot, which preserves the state of the data page at the instant in time that the Database Snapshot was created. Any subsequent changes to the data page in the source database are ignored by the Database Snapshot. This write of the original image of the data page from the source database is called copy-on-write.
Quick Check
What is a Database Snapshot and what information does it contain A Database Snapshot provides a point-in-time, read-only copy of the source database. The only information contained within a Database Snapshot are the pages that changed in the source database since the Database Snapshot was created.
Quick Check Answer
A Database Snapshot contains a unique structure called a catalog of changed pages, which is a bitmap that contains a list of the pages within the source database that have changed since the point in time of the creation of the Database Snapshot. When SQL Server writes the original image of a data page to the Database Snapshot, it changes the bit corresponding to that page from 0 to 1 within the catalog of changed pages. By writing the original image of a data page to the Database Snapshot, it allows the Database Snapshot to maintain the state of the data at the point in time of the Database Snapshot creation. Because a Database Snapshot contains only the original image of pages that were changed since creation, very little overhead is incurred on the system.
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