Today I was querying sys.databases system view in SQL 2012 and was curious to find one column named target_recovery_time_in_seconds.I was pretty sure that this column was not available before 2012.
I decided to learn more about this,and a quick search on BOL lead me to a different world altogether.This modification was part of a new enhancement/feature in SQL 2012 called ‘Indirect Checkpoint’.
Target_Recovery_Time is a new database level (Important to note that this) setting with SQL 2012,which will override the sp_configure option recovery_interval for that particular database.
Lets check the sp_configure value for recovery_interval for one of my test instance.
|recovery interval (min)||0||32767||0||0|
Now lets check what is the target_recovery_time_in_seconds for my test databases
SELECT name,target_recovery_time_in_seconds from SYS.Databases
Lets now change the Target_Recovery_Time for database AdventureWorks2012 [ Note – Please do not do this directly on a production database without proper test!]
ALTER DATABASE [AdventureWorks2012] SET TARGET_RECOVERY_TIME = 10 seconds SELECT name,target_recovery_time_in_seconds from SYS.Databases Where target_recovery_time_in_seconds <>0
What does this mean ?
Checkpoint forces all dirty pages for the current database to be written to disk. Dirty pages are data or log pages modified after entered into the buffer cache, but the modifications have not yet been written to disk.Checkpoint plays an important part in the recovery interval for a database.
Before 2012 the automatic checkpoint was purely controlled using Recovery_Interval sp_configure option,and there was no flexibility to control this at database level.The default value is zero.
There are some limitations with the checkpoint algorithm before 2012,and its very clearly explained by Microsoft PFE Parikshit Savjani (B/T) in his blog post which can be read here. Another blog post which explained the concepts really clear was from Sourabh Agarwal,yet another PFE (B) and his that post can be read here.The major drawbacks of the old algorithm is massive Disk Write activity during checkpoint and unpredictable database recovery time.
With 2012 Indirect checkpoint algorithm there is a Dirty Page list is maintained and also there is a threshold which is maintained for target dirty buffer.A background process Recovery Writer polls the dirty pages to compare the threshold and flushes them.The whole process is very clearly explained in the above mentioned blog posts and I do not want to repeat the same here.Please refer both the blogs to get a detailed explanation with videos.
We can use extended events to check the behavior of Checkpoints and you can add the events checkpoint_begin and checkpoint_end as per requirement.
I did a quick check on behavior of checkpoint for a very small database which has one table.Below are the test results.A simple statement was wrtitten to insert 80000 rows to the table.
Use[Demo_Checkpoints] INSERT INTO Test VALUES(1000000) GO 80000
It took 1 minute and 17 seconds to complete the whole insert.
Default checkpoint behavior
The extended event session came up with the below results.
For the default behavior a checkpoint was applied 5 times.
Behavior after changing the Target Recovery_Time for the database
For the purpose of testing I changed the Target_Recovery_Time for the database as 90 seconds.The event session was triggered again.
This time checkpoint was applied for 3 times and I could easily understand the behavior changes.
One of the most important facts which I observed that the same INSERT query took 1 minute and 28 seconds to complete this time.This clearly states the fact that changing the default behavior needs lot of testing and should be used with lot of caution.
Thanks for reading.