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TSQL Tuesday Misconceptions in SQL Server

I haven’t written a post for a TSQL Tuesday for a little while and I know that I’m late getting this one out too. This month’s party is being hosted by Sankar Reddy This topic  I think, maybe expect, really I know, that this topic has been covered already, I know that the Steve Jones has posted on this misconception already today and Paul Randal (Blog | Twitter) in his misconceptions series back in April covered this too much better than I have here so go and check out Paul’s post . The misconception I will look at here is - SQL Server will truncate the the transaction log after a full backup when the database is in full recovery mode. After my blog post around recovery models earlier today I think this maybe quite apt for me.

TSQL2sDay

The misconception I am going to talk about here is SQL Server truncates the log of a database in full recovery mode when a full backup is taken. It doesn’t.

This one of the biggest problems I come across, a database in full recovery mode and regular full backups being taken but  no transaction log backups being taken. The log grows and grows and eventually fills up all drive space.

I tested this on a SQL Server 2000 instance earlier today and it holds true in that version too. SQL Server does not truncate the log when a full backup of a database is taken in full recovery mode, although I believe in versions gone by it may have been the case that a full backup truncated the log. If you know the exact version please leave a comment.

The Proof

I will create a database for the purpose of this test called demodb and then create a table in that database called t1 that will hold some data.

CREATE DATABASE demodb
GO

CREATE TABLE t1 ( id int, amount int )
go




Firstly I will take a full database backup, the reason for backup is to ensures the database is in Full recovery mode and not in psuedo-simple mode.



BACKUP database demodb to disk = 'C:\BACKUP\demodb.bak'



If I then run a DBCC LOGINFO we can see that 1 of the 2 virtual log files (VLFs) are are active, they have a status of 2. I will cover VLF’s and the circular nature of the transaction log in a later posts, but for now DBCC LOGINFO shows some information on VLFs. a status of 2 means the VLF  is active, a status of 0 means that it is inactive or has been been truncated and can be used again.



TST1



I will then run some inserts on my table, all of which will be logged in the transaction log:



DECLARE @i INT
SET @i = 0

DECLARE @j INT
SET @j = 100000

WHILE @i < 10000

BEGIN
INSERT INTO dbo.t1 ( id, Amount )
VALUES ( @i, @j + @i )
SET @i = @i + 1
END
SELECT GETDATE()



Running the DBCC LOGINFO command again,  you can see from the below diagram the log has grown and the number of active VLF’s has increased, I now have 103 VLF’s in my log and 102 are active, this means they cannot be re-used until they are marked as inactive and truncated.



TST2




I will then run another full backup of my database, to prove that a full backup of a database in full recovery mode does not truncate the log.



backup database demodb to disk = 'C:\BACKUP\demodb2.bak'



Running DBCC LOG info shows that the log has not been truncated by the full backup, the same number of VLFs are still active with a status of 2.



dbcc loginfo


The following results show that we still have 103 VLFs with 102 with a status of 2 – active.



TST3



If we then run a LOG backup and then run the DBCC LOGINFO command we can see that the log has been truncated and more of the VLF’s have been marked as inactive with a status of 0 and can now be reused.







backup log demodb to disk = 'C:\BACKUP\demodblog.trn'
GO
dbcc loginfo


TST4 


You will see that I still have 103 VLFs (more on that later) but now only one of those is active with a status of 2. The rest have been truncated and can be reused in the log. I hope that is helpful. I have rushed this post to get it out in time, I didn’t realise it was that time again until a couple of hours ago, if you find any inaccuracies in the post please let me know in the comments.


Comments

Posted by Ken Lee-263418 on 17 October 2010

I have seen comments where you don't have to worry about shinking a DB log file in simple mode. I have worked a lot with simple mode. That is false. (Unless you work in a data center that consistently shrinks the log "WITH TRUNCATE ONLY" specified on all DBs as a matter of practice and you aren't aware of this being done for you. Then it is absolutely true. Must be nice working in a center like that) You have to maintain the logs by shinking them.

"WITH TRUNCATE ONLY" is the fastest way of shinking the log file.

It is also my understanding that in FULL mode that is also true, but the "WITH TRUNCATE ONLY" or even the slower more IO intensive way of shrinking a file won't help if you don't back up your log on a timely basis.

I bet the guy with the log backups every 4 hours will find his log file in the MB range right after following it with "shrink with truncate" as the next step after the backup is created. Whether 4 hours later it is back to 15 GB is a different issue.

Posted by Jason Brimhall on 18 October 2010

Thanks for the article.

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