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What’s in a Full Backup?

I got asked a question today that I though was quite interesting. If my backup starts at 6pm and it runs for say 30 minutes what happens to the transactions that occur during the 30 minutes that the backup is running?

The short answer:

All committed transactions that run during the duration of the backup will be included in the full backup, open transactions will not be included.

The proof:

Please note, that your database will get around 784MB in size and the backup created will be roughly the same size, so before running these scripts, make sure you have enough space on the box you testing it on.

I will create a dummy database called buptest for the purpose of this test

USE master
go
CREATE DATABASE buptest
GO
ALTER DATABASE buptest SET RECOVERY SIMPLE





I have put the database into simple recovery mode just to keep the log manageable, I am not worried about point in time recovery for this demo.


Next we will create two tables to hold some test data

USE buptest
go

CREATE TABLE t1 (id INT, val CHAR(8000))
go

CREATE TABLE t2 (id INT, val CHAR(8000))
go



Next we will populate our table t1 with some data, 100,000 rows. I’m doing this so the backup will run for long enough to allow me to execute some transactions against the database whilst the backup is running. I don’t know of another way of doing this…If you do, please leave me a note in the comments.

DECLARE @i INT
SET @i = 0
WHILE @i < 100000
BEGIN

INSERT INTO t1
VALUES (@i, REPLICATE('a',8000))
SET @i=@i+1
END



The next step has two parts, I want to kick off a full backup in one query window and then in a second query window I want to run two transactions that each insert a row into the t2 table. The first transaction will be committed before the backup completes the second transaction will be committed after backup has finished. So I suggest if you are following along, you open two query windows, paste the backup code below into the first window, paste the insert transaction into the second window (You will see that the second commit statement in commented out), switch to the first window, start the backup, quickly switch to the second window, fire the script making sure that the commit statement is commented out. Wait for the backup to complete. When the backup has completed uncommented the commit transaction statement and commit the currently open transaction.


The script to run the backup

--We then take a backup of the database
--While this is running fire the second script below
BACKUP DATABASE buptest TO DISK = 'c:\Backup\buptest.bak'



While the backup script above is running (it took about a minute on my laptop to complete) run this:

USE buptest
GO

--Run in a new query window
BEGIN TRANSACTION
INSERT INTO t2
VALUES (1, 'gethyn')
COMMIT TRANSACTION


BEGIN TRANSACTION
INSERT INTO t2
VALUES (2,'Ian')





When the backup completes run the final commit transaction in the second window. This ensures that the second transaction was not committed prior to the backup completing.

COMMIT TRANSACTION



If you run

SELECT * FROM t2



We get two rows returned.









1Gethyn
2Ian

Next we run a restore of our database from the backup taken

USE master
go
ALTER DATABASE buptest SET SINGLE_USER WITH ROLLBACK IMMEDIATE

RESTORE DATABASE buptest FROM DISK = 'c:\Backup\buptest.bak' WITH replace



Then re-run the same select

SELECT * FROM t2



We see that only the first inserted record is present.






1Gethyn

Hence only committed transaction are included in a full database backup

 


Comments

Posted by Jason Brimhall on 4 November 2010

Thanks Gethyn

Posted by Nakul Vachhrajani on 8 November 2010

Thank-you, Gethyn.

I am now very curious - how is this managed under the hood? Assume that this is on a huge database with multiple files & filegroups. While a backup of this database is running, I make a change to file F1 on filegroup FG1. How does SQL internally manage this change? Is the backup internally split up or fragmented?

Maybe another article? :)

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