Sql-server – Sql Server 2008 R2: Simple recovery model with transaction log backups

A full backup in SQL Server 2008 does not break the log chain. It only resets the differential base-lsn.

You also can restore log backups after restoring from a copy only. The following script demos that:

CREATE DATABASE BakTst13;
GO
ALTER DATABASE BakTst13 SET RECOVERY FULL;
GO
USE BakTst13;
GO
CREATE TABLE dbo.tst(id INT IDENTITY(1,1));
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Full_1' WITH INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP LOG BakTst13 
TO DISK = 'BakTst13_Log_1' WITH INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Full_2' WITH INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Full_C' WITH COPY_ONLY,INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP LOG BakTst13 
TO DISK = 'BakTst13_Log_2' WITH INIT,FORMAT;
GO
USE tempdb;
GO
DROP DATABASE BakTst13;
GO
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Full_1' WITH NORECOVERY;
RESTORE LOG BakTst13 FROM DISK='BakTst13_Log_1' WITH NORECOVERY;
RESTORE LOG BakTst13 FROM DISK='BakTst13_Log_2' WITH RECOVERY;
GO
SELECT * FROM BakTst13.dbo.tst;
GO
DROP DATABASE BakTst13;
GO
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Full_C' WITH NORECOVERY;
RESTORE LOG BakTst13 FROM DISK='BakTst13_Log_2' WITH RECOVERY;
GO
SELECT * FROM BakTst13.dbo.tst;
GO
DROP DATABASE BakTst13;

It creates a database and a table and inserts 50 rows into that table. In between those inserts several backups are taken in this order:

1. Full
2. Log
3. Full
4. Full Copy_Only
5. Log

Next the database is dropped and restored like this:

1. 1st Full
2. 1st Log
3. 2nd Log

The following SELECT demonstrates that the restore was successful.

This shows that neither a COP_ONLY nor a normal Full Backup break the log chain.

Then the database is dropped again and restored like this:

1. Copy_Only Full
2. 2nd Log

Afterwards the SELECT demonstrates success again.

This demonstrates that you can use a COPY_ONLY full backup as the base of your Log Restore.

Differential tests

I created a DIFFERENTIAL version too:

CREATE DATABASE BakTst13;
GO
ALTER DATABASE BakTst13 SET RECOVERY FULL;
GO
USE BakTst13;
GO
CREATE TABLE dbo.tst(id INT IDENTITY(1,1));
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Full_1' WITH INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Diff_1' WITH DIFFERENTIAL,INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Full_2' WITH INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Diff_2' WITH DIFFERENTIAL,INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Full_C' WITH COPY_ONLY,INIT,FORMAT;
GO
INSERT INTO dbo.tst DEFAULT VALUES
GO 10
GO
BACKUP DATABASE BakTst13 
TO DISK = 'BakTst13_Diff_3' WITH DIFFERENTIAL,INIT,FORMAT;
GO
USE tempdb;
GO
DROP DATABASE BakTst13;
GO
RAISERROR('------> Starting restore F1, D1, D2',0,1)WITH NOWAIT;
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Full_1' WITH NORECOVERY; 
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Diff_1' WITH NORECOVERY;
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Diff_2' WITH NORECOVERY;--<--Fails!
GO
DROP DATABASE BakTst13;
GO
RAISERROR('------> Starting restore FC, D3',0,1)WITH NOWAIT;
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Full_C' WITH NORECOVERY;
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Diff_3' WITH NORECOVERY;--<--Fails!
GO
DROP DATABASE BakTst13;
GO
RAISERROR('------> Starting restore F2, D2, D3',0,1)WITH NOWAIT;
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Full_2' WITH NORECOVERY; 
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Diff_2' WITH NORECOVERY;
RESTORE DATABASE BakTst13 FROM DISK='BakTst13_Diff_3' WITH RECOVERY;
GO
SELECT * FROM BakTst13.dbo.tst;
GO
DROP DATABASE BakTst13;

This takes backups in this order:

1. 1st Full
2. 1st Differential
3. 2nd Full
4. 2nd Differential
5. Copy_Only Diff
6. 3rd Differential

It then tries this restore route:

1. 1st Full
2. 1st Differential
3. 2nd Differential

Step 3 fails with this error:

Msg 3136, Level 16, State 1, Line 4
This differential backup cannot be restored because the database has not been restored to the correct earlier state.

This shows that a normal full backup breaks the differential chain.

Next the database is dropped and this restore flow is attempted:

1. Copy_Only Full
2. 3rd Differential

Step 2 fails with the same error as step 3 above. This shows that a copy only backup cannot be used as the base for a differential restore.

Then the database is dropped again and the following restore is executed:

1. 2nd Full
2. 2nd Differential
3. 3rd Differential

The following select proves that this restore succeeded. This shows that a COPY_ONLY Full backup does not interrupt the differential chain.

No, it it's not possible to restore a database from an ldf file. The ldf file would be restored along with the mdf files.

No, it's not redundant as they have two different purposes.

It's important to take full backups, and transaction log backups. Only having a copy of the ldf file doesn't help you restore the database.

As to what a ldf file is for, the ldf is the transaction log. Think of it as a circular buffer that records changes to your database. When you update a row, the change is immediately written to the ldf. At some point in the future (usually less than five minutes), the modified data is written to the mdf file.

If the server crashed or there was a power failure, when SQL starts, it reads the ldf and re-applies (REDO) those changes.

Additionally, if you have a transaction that hasn't been commited and the sever crashes, all changes made by that transaction have to be undone to make the database consistent. The ldf file has that task as well. (UNDO)

I mentioned above that the ldf file is circular. Taking a transaction log backup (.trn) copies out a portion of the ldf file. After a trn file is safely created, sql can reuse that portion of the ldf file. The series of trn backups create a chain that together record every modification made to the database. Of course, if you never took a tranaction log backup, the ldf file would grow and grow and grow.

In a disaster scenario, restoring the full backup gets you a copy of the database as of the time the full backup finished. You can then restore the trn files in order and bring the database current to any point in time including up to the last trn backup.

I'm glossing over some important details, but the gist is the that ldf is a working file that records recent changes to the database. The trn files are copies of parts of the ldf made under the assumption that you will keep then safe so that sql can reuse the space in the ldf and if disaster strikes, you'll have them in an alternate location.