Month: February 2014

In this blog post, let’s see what SQL Server database Snapshots are and how they can be created and how to revert to a DB Snapshot and few important things to keep in mind before making use of them.

Microsoft introduced database snapshots in SQL Server 2005 as an enterprise only feature(I believe, that’s true even with SQL 2014…but I could be wrong). The basic concept of a database snapshot is to create a read-only, transactionally consistent(as of the moment of snapshot creation) static view of a given database. Even though they are named as “DB Snapshots”, they have got nothing to deal with Snapshot replication or Snapshot Isolation.

When could a DB Snapshot be useful?
Most common use case:  Let’s say your DEV/Testers are doing some testing(which makes whole bunch of changes to your database) in your test environment and they want the database to be reverted to a state how it was prior to the testing once they are done. A Snapshot works awesome in this case. Reverting is waaaay faster than restoring your database(especially when you are dealing with huge databases).

See this for more info on how they work behind the scenes, when will they grow in size, other use cases and Gotchas to remember while using Snapshots. Let me brief a few important things here.

• DB Snapshots are not substitutes for your DB backups. Irrespective of whether you’ve snapshots or not, you should have healthy restorable backups in place. Don’t treat DB Snapshots as DR or HA solution.
• Snapshots will introduce IO overhead, depending on workload on your Source Database.
• You can’t backup a DB Snapshot and can’t attach/detach a snapshot; you can’t create snapshot for System Databases.
• No GUI for creating Snapshot(Only T-SQL).
• You can’t drop Original Source Database as long as a referring snapshot exists for that database.
• No FAT32 support(Snapshots work on NTFS Sparse files technology).
• Since they are read-only, static copies…new users can’t be added.
• Make sure you’ve enough space on the drive holding your Snapshot file. If the drive runs out of space, Snapshot will be marked as “SUSPECT” by SQL Server.
• Reverting to a Snapshot will break Log chain. Take either a Full or a Diff Backup(assuming you’ve got your latest Full backup) to bridge the Gap in your log chain) immediately after you revert. If not, all the log backups will fail from now on…
• If your DB is not in SIMPLE recovery mode, Take a log backup before you perform Reversion to secure everything since your last log backup.
• Full Text Catalogs will be lost once you revert your database. Be careful, if your database uses Full Text features as this could be an unpleasant surprise.
• Both the source DB and the snapshot will be unavailable when the actual reversion process is in progress.
• Source DB should be Online,No read only file groups should be present in your Source DB to be able to revert from a snapshot.
• If your source DB gets corrupted, You can’t use Snapshot for reverting. ( See, they simply can’t replace your Backups)

Now it’s Demo time 🙂
I’ve a database called “Tst_restore” which has 3 tables and this will be my source database for this demo purpose. I’ve created Snapshot as shown in the below screenshot.

T-SQL:

IF EXISTS (SELECT name FROM sys.databases WHERE name = N'Tst_Restore_SS')
DROP DATABASE Tst_Restore_SS
GO
CREATE DATABASE Tst_Restore_SS ON (
NAME = Tst,
FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL11.SREE\MSSQL\DATA\Tst_Restore_SS.ss')
AS SNAPSHOT OF Tst_Restore;

Now you can see I’ve a snapshot created.

See below for the Properties(files section).

As you can see, it has got no ldf file. (If you’ve noticed, while creating the Snapshot I didn’t provided ldf file in my syntax. Yes, that correct…you should provide only (all of) the data files, not the log file while creating DB Snapshot)

Revert a Database to a snapshot:

Now, let’s see how to revert a database to a Snapshot. Assuming something went wrong and you want to revert to snapshot which you’ve taken in the above step, you can use below syntax.

USE master
GO
ALTER DATABASE Tst_Restore SET OFFLINE WITH ROLLBACK IMMEDIATE
GO
ALTER DATABASE Tst_Restore SET ONLINE
GO

-- Reverting Tst_Restore to Tst_Restore_SS

RESTORE DATABASE Tst_Restore
FROM
 DATABASE_SNAPSHOT = 'Tst_Restore_SS';
 GO

As you can see above, to be able to revert we need exclusive access to the database(same as restore). Hence I am taking DB offline and bringing it back online immediately.
Note: Don’t forget to bridge the gap in backup chain right after you revert to a Snapshot. As I’ve already mentioned reverting to a Snapshot will break Log chain. Log backups will fail if you forget this step.

Hope this is informative…!