SQL SERVER – ReadOnly Databases and Notes Using Snapshots

This seems to be the third week that I am writing about ReadOnly databases, this blog is in continuation of last weeks notes. When I wrote the notes from last week, one of my blog reader said if it is possible to get the ReadOnly error without making the database into a ReadOnly database. This got me thinking and I couldn’t sleep that day. And a quick call to my friend to get some ideas – the answer was simple – using Snapshots.

I couldn’t control my excitement and this triggered me to write this special case scenario. As we execute the script, we will learn some of the behavior.

Let us start by creating our database which will be used for testing.

CREATE DATABASE [ReadOnlyDB]
CONTAINMENT = NONE
ON  PRIMARY
( NAME = N'ReadOnlyDB', FILENAME = N'C:\Temp\ReadOnlyDB.mdf' , SIZE = 4024KB , FILEGROWTH = 1024KB )
LOG ON
( NAME = N'ReadOnlyDB_log', FILENAME = N'C:\Temp\ReadOnlyDB_log.ldf' , SIZE = 20480KB , FILEGROWTH = 10%)
GO
USE ReadOnlyDB
GO
-- Creating our table
CREATE TABLE tbl_SQLAuth (id INT, Longname CHAR(8000))
GO

Next let us go ahead and create a snapshot database out of our ReadOnlyDB.

-- Create a snapshot on our DB
CREATE DATABASE ReadOnlyDB_SS ON
( NAME = ReadOnlyDB, FILENAME =
'C:\Temp\ReadOnlyDB_data_1800.ss' )
AS SNAPSHOT OF ReadOnlyDB;
GO

Now that our snapshot database (ReadOnlyDB_SS) is created, it is important to know that snapshot databases are created as ReadOnly databases. The behavior is similar to creating ReadOnly Databases. To test this, let us try to insert some data into our Snapshot DB.

USE ReadOnlyDB_SS
GO
INSERT INTO tbl_SQLAuth VALUES (1, 'SQLAuth');
GO

Now we will be presented with the same familiar error (3906):

Msg 3906, Level 16, State 1, Line 25

Failed to update database “ReadOnlyDB_SS” because the database is read-only.

Even though we have not explicitly marked the snapshot database as Readonly we are getting this error. I was curious to know, what will happen if we try to mark the snapshot database as ReadOnly again. The error is self-explanatory.

-- Let us set the Snapshot DB as READ_ONLY
USE MASTER
GO
ALTER DATABASE [ReadOnlyDB_SS] SET READ_ONLY
GO

Msg 5093, Level 16, State 1, Line 31

The operation cannot be performed on a database snapshot.

Msg 5069, Level 16, State 1, Line 31

ALTER DATABASE statement failed.

If that was easy, what will happen if we try to make the snapshot database to read_write? The error is self-explanatory again here:

USE [master]
GO
ALTER DATABASE [ReadOnlyDB_SS] SET READ_WRITE WITH NO_WAIT
GO

Msg 5093, Level 16, State 1, Line 37

The operation cannot be performed on a database snapshot.

Msg 5069, Level 16, State 1, Line 37

ALTER DATABASE statement failed.

Now that gives us two learnings, we cannot mark a Snapshot database as ReadOnly and nor can we mark a snapshot database as Read_Write enabled.

With these learnings in place, let us do the cleanup. Make sure to drop the Snapshot database before deleting the main database. So our cleanup script for this blog looks:

-- Clean up time
USE MASTER
GO
DROP DATABASE ReadOnlyDB_SS
GO
DROP DATABASE ReadOnlyDB
GO

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