Delayed Durability while Purging Data
Delayed Durability Background
It is January, which means a new year, a new you, time to purge everything from your closet to your pantry, and perhaps your database, too! This whole data purge related topic at the beginning of the year is nothing more than a happy little accident, but I thought I would roll with it just now.
Microsoft introduced the Delayed Durability feature in SQL Server 2014 as a potential way to reduce latency and increase performance, but with an increased risk of data loss (up to about 7MB in a perfect storm, worst-case scenario). If you are not familiar with this feature, I invite you to read Aaron Bertrand’s detailed blog post regarding Delayed Durability before continuing with this one.
Despite Delayed Durability having been available for a couple of years, I personally have not talked about it with anyone who is actually using it in production. I suspect this is because people are uncomfortable with an increased risk of data loss, which is understandable, since data loss has been associated with an increased risk of job loss. To be fair, my perception of it as a rarely-used feature could easily be because I do not talk to that many people. Have I mentioned that my social network consists mostly of squirrels?
Recently, while considering possible use cases for Delayed Durability, it occurred to me that data loss might be entirely acceptable in cases where the data would not truly be lost. I have worked with a number of applications that have processes that purge old information from the database. If a purge process failed in these applications, data would simply live a little bit longer, and be purged the next time the process was successful – they have a recovery mechanism built in as it is. I decided to test Delayed Durability in a database with a long-running purge to observe the potential performance impact. I chose a process that was clearly contributing to transaction log waits, because that is where the real performance impact comes from when delaying durability. If you do not have notable waits or some level of a bottleneck there, you are not likely to improve anything simply by turning on this feature.
Now, because I wanted to look at the impact of utilizing this on a specific purge process, I set Delayed Durability to Allowed within the database options, and updated the specific transaction to commit with Delayed Durability set to On. You can view the specifics of how to do this in Aaron's post as well as the Control Transaction Durability entry on MSDN.
Testing with SQL Sentry Performance Advisor
Testing this process was rather simple. I had a test server where I executed the stored procedure, and monitored that server with SQL Sentry Performance Advisor. I was able to watch my stored procedure execute within the Top SQL tab to get some statistics, including the basics like start/end time and duration.
On the Performance Advisor Dashboard, I used the start and end times from Top SQL to set the window for my baseline:
Figure 1 showing start and end times on Performance Advisor Dashboard
I highlighted that time period on the Dashboard, and right-clicked to create both a Quick Report and a Baseline:
Figure 2 showing how to create reports and baselines from the Performance Advisor Dashboard
I used the values calculated in the Baselines and Quick Reports for my comparisons:
Figure 3 showing values calculated for the Baseline
Figure 4 showing values calculated for the wait stats via Quick Report
You can see how quickly you can collect and compare your performance metrics when using Performance Advisor.
Delayed Durability Test Results
The purge process that I tested in this example had multiple transactions that required a change to commit with Delayed Durability set to on. As shown below, the process took about ninety minutes to complete when Delayed Durability was Disabled in the database and approximately sixty-six minutes to complete when Delayed Durability was Allowed and turned on for specific transactions.
Figure 5 showing Delayed Durability providing a 26% savings in total duration
As one would probably expect, there is a very similar percentage of savings between duration and disk-related waits when using Delayed Durability. If you have these types of waits during your purge process, then it is possible that this change will assist in reducing them.
Figure 6 showing Delayed Durability providing a 25% savings in disk-related waits
The disk waits related specifically to the transaction log show a 48% improvement when delaying durability.
Figure 7 showing Delayed Durability providing a 48% savings in transaction log related waits
There is also a 48% decrease in the average number of transaction log flushes per second when delaying durability.
Figure 8 showing Delayed Durability providing a 48% savings in average log flushes per second
In this specific process, environment, and overall scenario that I have tested, I saw a noteworthy performance improvement when delaying durability during a significant purging of data. If you have a purge or even an archive-type process where you would not actually lose data when delaying durability, and could restart the process if it should fail, I would encourage you to try this in your test environment. What do you have to lose? ;-)
While one might expect to see a performance improvement by using this SQL Server feature, it still falls under the omnipresent umbrella of "it depends." Trust that it works in at least one scenario for me, but verify that it also works in yours because that is what really matters, for you anyway. If you are fortunate enough to have SQL Sentry Performance Advisor, then testing this out will be particularly easy for you with the baseline feature to collect those metrics and calculate the averages for you. It certainly makes my life easier when performing these types of tests.
SQL Sentry Example
For a real use case, we have background purge processes in the SQL Sentry database that simply delete old data. If these transactions were lost due to a power outage or some other catastrophe, it wouldn't be the end of the world; those same rows would end up being deleted the next time the purge process ran. If you have SQL Sentry Performance Advisor, SQL Server version 2014 or higher, and would like to enable Delayed Durability in your SQL Sentry database, please follow the instructions in this example.
--Step 1 --You need to allow Delayed Durability for the SQL Sentry database ALTER DATABASE dbname SET DELAYED_DURABILITY = ALLOWED;
Altering the database options
This may also be changed or checked via Database Properties –> Options in SQL Server Management Studio.
Figure 9 showing Delayed Durability setting in SSMS
The stored procedure that we'll be altering in this example is called [dbo].[PurgePerformanceAnalysisData]. This particular purge process was chosen based on its transaction log usage pattern and observed benefits when allowing Delayed Durability. You should make a backup copy of this stored procedure before proceeding. You could do this by right-clicking on the stored procedure in SSMS, and choosing to "Script Stored Procedure as" -> "Create To" -> "File" to create a script of the original.
Figure 10 showing how to send the stored procedure to a create file
Figure 11 showing the save file dialog for original stored procedure
Once you have verified that your backup has been created, you are ready to alter the stored procedure with the Delayed Durability changes. There are four transactions in this [dbo].[PurgePerformanceAnalysisData] stored procedure which will need to be wrapped in an explicit transaction to commit with delayed durability.
Figure 12 showing how to modify the stored procedure via SSMS
For each transaction, you will be adding a line before the code block:
BEGIN TRANSACTION [DelayedDurabilityOnTransactionName]
Opening line for wrapping the transaction
As well as a line after the code block:
COMMIT TRANSACTION [DelayedDurabilityOnTransactionName] WITH (DELAYED_DURABILITY = On )
Closing line for wrapping the transaction
Below are the four blocks of code you will need to find and wrap:
--First Transaction --Located around line 260 BEGIN TRANSACTION [DelayedDurabilityOn] EXEC sp_executesql @DynamicSQL ,@SQLParams ,@Timestamp = @Timestamp ,@CounterID = @CounterID SET @DeleteRowCount = @@rowcount SET @DeleteRowCountTotal = @DeleteRowCountTotal + @DeleteRowCount SET @CountersTableRowID = @CountersTableRowID + 1 COMMIT TRANSACTION [DelayedDurabilityOn] WITH (DELAYED_DURABILITY = On )
Altering the first transaction for delayed durability
--Second Transaction --Located around line 285, if counting these comments BEGIN TRANSACTION [DelayedDurabilityOn2] EXEC (@DynamicSQL) SET @DeleteRowCount = @@rowcount SET @DeleteRowCountTotal = @DeleteRowCountTotal + @DeleteRowCount COMMIT TRANSACTION [DelayedDurabilityOn2] WITH (DELAYED_DURABILITY = On )
Altering the second transaction for delayed durability
--Third Transaction --Located around line 366, if counting these comments BEGIN TRANSACTION [DelayedDurabilityOn3] EXEC sp_executesql @DynamicSQL ,@SQLParams ,@Timestamp = @Timestamp SET @DeleteRowCount = @@rowcount SET @DeleteRowCountTotal = @DeleteRowCountTotal + @DeleteRowCount SET @TimestampsTableRowID = @TimestampsTableRowID + 1 COMMIT TRANSACTION [DelayedDurabilityOn3] WITH (DELAYED_DURABILITY = On )
Altering the third transaction for delayed durability
--Fourth Transaction --Located around line 404, if counting these comments BEGIN TRANSACTION [DelayedDurabilityOn4] EXEC sp_executesql @DynamicSQL ,@SQLParams ,@PurgeBeforeTimestamp = @PurgeBeforeTimestamp ,@DeleteRowCountTotal = @DeleteRowCountTotal ,@CategoryID = @CategoryID SET @CategoryTableRowID = @CategoryTableRowID + 1 COMMIT TRANSACTION [DelayedDurabilityOn4] WITH (DELAYED_DURABILITY = On )
Altering the fourth transaction for delayed durability
Once you execute these changes, you will be able to take advantage of Delayed Durability in your SQL Sentry database. Unfortunately, you may need to re-apply these changes after a future upgrade. Fortunately, we will be adding these changes to our product so eventually they will be built in and you won't need to make any modifications to the stored procedure to benefit from Delayed Durability.