Working out of India has its own challenges and I enjoy here despite these challenges thrown at me. One of the biggest advantage I have working with Pluralsight is, I can still get my job done by working-from-home occasionally. And this is one of the perks I wish most of the companies give their employees. You might be thinking why I am doing this, well the obvious answer to this question relies on the fact how the previous day went. If it rained heavily, which is does in Bengaluru in July, then the chances are that roads would have a build-up of traffic the next day morning. Taking traffic away from your life is never so easy, but with technology improvements like Maps on the phone, I still manage to get an alternate route to reach my destination. This is what makes life interesting and the exploration into new places always fun.
I just wish SQL Server had some way of achieving the same. Blocking and Locking are fundamental to keeping databases in sync and consistent. This blog is all about Blocking Transactions report from the instance level.
To access the report, get to Server node -> Reports -> Standard Reports -> Activity – All Blocked Transactions.
From this node, if there are no apparent blocking happening in the system at the point this report was run, we will be presented with a “Blank” output as shown below.
The ideal situation for us to be in this state, even for a transitional system, but this will never be the case in reality. For a highly transactional systems which try to modify / insert data in same table, SQL Server will respect the order in which the request came and will not allow incompatible locks to exist at the same time. So this behaviour creates a queue automatically and this is what we call as Blocking.
This brings us to the next output, where we are having multiple transactions running. To show some data in report from my non-production-workload system, I have simulated a blocking scenario using two statements. In such a scenario you can see there are two regions to look at: the Session ID of 52, 53 and 54. From the hierarchy, we know that 52 is blocking both 53 and 54. We can also know there are 2 “#Directly Blocked Transactions” in the system currently from the top row for SPID 52. If there are additional transactions trying to insert or delete, then this will show the complete chain of tractions currently blocked.
We also get to see the type of statement that is waiting in this blocking scenario. In the diagram below we see the two statements involved are – INSERT and DELETE.
Various DMVs which have been used to get this information are sys.dm_tran_locks, sys.dm_tran_active_transactions, sys.dm_tran_session_transactions, sys.dm_tran_database_transactions and sys.dm_exec_requests. Along with above, report also uses DMF sys.dm_exec_sql_text to convert the SQL handle to more meaningful text.
If that was not enough then we can also head to the Activity Monitor and expand the Processes tab to get similar information. It is evident that the head of blocking is 52 whereas 53 and 54 are waiting on 52. It is completely up to us to decide what we need to do. We can Kill process 52 and the other transactions will go through.
As a small note, the Task States can give us vital information of what is happening in the system. Some of the states are worth mentioning:
|Sleeping||This shows the SPID is waiting for a command or nothing is currently executing.|
|Running||SPID is currently running.|
|Suspended||SPID is waiting for locks or a latch.|
|Rollback||Connection is in rollback state of a transaction.|
You can use the state information to take an informed decision of killing a process if required.
At this moment, yet another blog post that is worth a mention is Blocked Process Threshold post. This option makes sure there is a profiler event raised when a request is blocked beyond a predefined period of time. So do take a look at that too if you are interested in that behaviour.
The reports series is catching up and the learnings are multi-fold for me personally. Subsequent posts I will get into the other reports and give you my learnings.
Reference: Pinal Dave (http://blog.sqlauthority.com)