SQL SERVER – Recovering from Snapshots – Notes from the Field #078

[Note from Pinal]: This is a 78th episode of Notes from the Fields series. What do you do when data is deleted accidentally or you are facing disaster? Well, there are plenty of the things, you can do, but when in panic quite often people make mistakes which just intensify the disaster. Database snapshot is very important but less frequently used feature.

In this episode of the Notes from the Field series database expert John Sterrett (Group Principal at Linchpin People) explains a very interesting subject of how to recover the database from snapshots. Linchpin People are database coaches and wellness experts for a data driven world. Read the experience of John in his own words.

One of the most common – and most forgotten – scenarios in disaster recovery plans is data being updated or deleted accidentally. This surprises me because, in the field, I see accidental data changes as a common disaster. If you have enterprise edition you can leverage database snapshots as a step in your disaster recovery plan to help recover faster, or at least allow you to pull back the majority of your data while you concurrently restore backups (to bring the rest of your data back). Restoring most of your data quickly can be critical with very large databases (VLDBs), especially if you have tight Recovery Time Objectives (RTOs).

Database snapshots require sparse files and store the original pages in these files when they are changed. Therefore, you will want to manage the sparse files and make sure you purge database snapshots as needed. You might also be using database snapshots without even knowing it. If you are doing database integrity checks with the default values, a special snapshot is taken in the background and your integrity checks are executed against that snapshot.

HOW DO WE CREATE A SNAPSHOT?

The following example uses the AdventureWorks2014 sample database. We need to use the CREATE DATABASE syntax, include the names of your data files, and include the file location of the sparse files. Finally, include AS SNAPSHOT OF database syntax to define the database as a snapshot.

CREATE DATABASE [AW2014_Snapshot_morning] ON
( NAME = AdventureWorks2014_Data, FILENAME =
'C:\Demo\AW_data.ss')
AS SNAPSHOT OF AdventureWorks2014

REAL-WORLD ACCIDENTAL DATA CHANGE STATEMENT

Here is a common case of a mistaken DELETE statement. We have a DELETE statement with the primary key included to delete a single row. By mistake we didn’t highlight the filter so all the rows will be deleted.

If you execute this statement, you will see an unexpected result: we deleted all rows in the table as shown below.

(19972 row(s) affected)

HOW DO WE REVERT FROM SNAPSHOT?

You have two options if you created a database snapshot earlier.

First, you could insert the data back from your snapshot database as shown below. This could be done with SSIS, BCP or many other tools. For this quick example we will do this with an INSERT INTO SELECT statement.

SET IDENTITY_INSERT Person.EmailAddress ON
INSERT INTO Person.EmailAddress (BusinessEntityID, EmailAddressID, EmailAddress, rowguid, ModifiedDate)
SELECT *
FROM AW2014_Snapshot_morning.Person.EmailAddress
SET IDENTITY_INSERT Person.EmailAddress OFF

Second, you can revert the database from the snapshot. Keep in mind this second option will revert all data changes in the database not just the data deleted in your accidental data change statement.

USE MASTER;
RESTORE DATABASE AdventureWorks2014 FROM
DATABASE_SNAPSHOT = 'AW2014_Snapshot_morning';
GO

From these examples, you can see database snapshots are a tool to help you recover data quickly. Please note that you wouldn’t want database snapshots to be your sole plan for disaster recovery and unplanned data change statements. If your snapshot or the original database suffers corruption, you wouldn’t be able to recover. So make sure you add snapshots into your existing disaster recovery plans, which should – at a minimum – include database backups to give you a better recovery point objective.

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Reference: Pinal Dave (https://blog.sqlauthority.com)