Delayed Transaction Durability

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Starting with SQL Server 2014, they added a feature called delayed transaction durability. This is a very interesting concept that could give you some speed improvements in your database. In this post, I will try to explain what this feature is and what some advantages/drawbacks are.

 
Transaction Log Durability
In 2014, SQL Server gives us the ability to control how the transaction log files are written to the disk. It gives us 2 options. Full transaction durability and Delayed transaction durability.

  • Full Transaction Durability – This is the default level. This means that as soon as a SQL transaction is executed, the transaction log is written to disk BEFORE control is sent back to the client/user.
  • Delayed Transaction Durability – This setting means that the SQL transactions will be written to a memory buffer instead of the disk. The control is then sent back to the client/user after writing to the memory buffer.

 
Advantages
The delayed transaction durability has a few distinct advantage over full transaction durability.

The first advantage is that it doesn’t need to write to disk before giving control back to the client. In many databases, writing to the disk provides a lot of contention. Log file writes to the disk can sometimes slow queries down quite a bit. Writing to memory is much faster and should speed up your queries in general if your IO to the disk is slow.

Another advantage is that the delayed transaction durability buffers the writes to disk. SQL Server can get optimizations by buffering the write operations. Instead of writing each transaction to the disk, it will fill a buffer and then flush that buffer all at once.

 
Disadvantages
Everything up to this point sounds great, right? Delayed transaction durability comes with one major pitfall. Delayed transaction durability could cause data loss if SQL Server crashes. The data that isn’t written to disk can be lost if SQL crashes.

When Should I Use Delayed Transaction Durability?
Only use delayed transaction durability if you can afford some data loss if SQL Server crashes. If you cannot afford any data loss, do not use delayed transaction durability.

 
How To Turn On/Off Delayed Transaction Durability
Okay, now that you’re an expert on delayed transaction durability, let’s quickly cover the 3 levels of transaction durability and how to activate them.

There are 3 kinds of transaction durability. Disabled, Allowed, and Forced.
DISABLED – This means delayed transaction durability is turned off. All transaction writes will go to disk and there will be no potential for loss.
FORCED – This means that delayed transaction durability is turned on for every query. All transaction writes will go to memory and there is a potential for data loss.
ALLOWED – This is kinda in the middle. It basically means that you can turn on the delayed transaction durability in the query or stored procedure.

You will configure the durability with one of these statements:

If you use ALLOWED, you will need to add this hint to your commit transaction in in your sql query or stored procedure to activate the delayed transaction durability for it:

 
 
Reference: https://msdn.microsoft.com/en-us/library/dn449490.aspx

 
 
NOTE: SQL Server shutdown/reboots are handled the same way as a crash. Make sure you flush your transaction buffer before rebooting!

How To Run SQL Scripts Via Command Prompt

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Usually, when we run SQL scripts, it is done on an ad hoc basis.  We usually run one file at a time just once.  From time to time there is the need to run a set of SQL script files.  One common reason I’ve seen for doing this is to rebuild a database for testing.  If you had all of your create tables/stored procedure commands stored in a series of SQL script files, then you could rebuild your database fairly easily by executing the SQL files in sequence.

 
SQL Server provides a tool to do this called SQLCMD.

The sqlcmd utility lets you run entire SQL script files full of t-sql via command prompt.  Leveraging this tool, you can create a batch file to run the sql script files in sequence.

 
 
How To Execute A SQL Script File Via Command Prompt With SQLCMD

Although there are TONS of command line options for executing a sql script.  Just executing a sql script file with the basic options is very easy.

The above command uses the -i option to specify the input file of DropTables.sql.  Everything inside this file will be execute on the current database.

 
NOTE: When you create your SQL script files, I always recommend putting a USE statement at the top of your script.  This will ensure that you are using the correct database when your dropping and creating tables.

 
 
Run A Batch Of SQL Script Files

Now that you know how to run one SQL script file… running multiple should be easy.  All you need to do is create a batch file and run one script after the other.

 
Your batch file will look something like this:

 
 
What About Errors?

So what happens if the SQL script file throws an error?  The above commands I showed you will just ignore any errors.  If you want to capture your errors, you need to add the command line option /b.  This flag will tell the sqlcmd utility to stop processing the script and output a DOS ERRORLEVEL value.  You can then check the DOS ERRORLEVEL value after each execution.

 
 
Reference: http://msdn.microsoft.com/en-us/library/ms162773.aspx