August 2012
Intermediate to advanced
1416 pages
33h 39m
English
book
Microsoft SQL Server 2012 Bible
by Adam Jorgensen, Jorge Segarra, Patrick LeBlanc, Jose Chinchilla, Aaron Nelson
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How Do Database Snapshots Work?
As discussed earlier, a database snapshot is a point-in-time, read-only, static view of the source database as it existed at the time of the database snapshot creation. Understanding the mechanics and technology behind database snapshots can help you understand how it all works.
Mechanics of Snapshots
When you create a database snapshot, SQL Server runs recovery on the database snapshot and rolls back uncommitted transactions to make the database snapshot transactionally consistent. The transactions in the source database are not affected.
A database snapshot is not the same as the source database. It is a different database and has the same number of data files as the source database, but it does not have any transaction log file. When it is initially created, the data files in the snapshot database do not contain any user data and are almost empty. That is why creating a database snapshot does not take a long time.
Copy on First Write
Database snapshots use a copy-on-first-write method for each source database page updated for the first time after you create the database snapshot . For every database snapshot, SQL Server creates an in-memory bitmap. It has a bit for each data page indicating if the page is copied to the snapshot.
Every time you make an update to the source database, SQL Server checks the bitmap to see if it has been copied to the snapshot. If it is not copied, SQL Server copies the data page from the source database to the database ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.