Thursday, September 16, 2010

Waiting to Kill DTC



Today I was migrating database files for Biztalk to new storage.  I wrote a simple little automated move script which I'll share at a later date.  Everything was going peachy.  I asked the Biztalk administrator to shutdown the services before I began.  I noticed that there was a spid with pending transactions, but since he assured me that he was completely offline I decided to proceed.  Every database moved cleanly except for BizTalkMgmtDb.  This database was the one with the spid.  Since my script rolled back transactions I knew that SQL Server would be trying to rollback the spid's transaction already.  Nevertheless, with the Biztalk admin's permission I tried killing the spid.  It of course gave me the famous (at least to me):

SPID X: transaction rollback in progress. Estimated rollback completion: 0%. 

This was a production environment and I was inside of business hours; so thought that I was going to be stuck.  As I studied the problem, I could see that the SPID was clearly waiting on DTC:

SELECT lastwaittype FROM sys.sysprocesses WHERE req_spid = X

lastwaittype
--------------------------------
DTC

Then I remembered that the clustered DTC service on the SQL cluster crashed the previous evening. One of the infrastructure engineers resolved the issue and brought the DTC service back online before I could take a look. I tried restarting the DTC service, in case killing the connection to the SQL Server service would release the spid. Alas, it made no difference for me.  I searched around for a solution but found lots of folks advocating restarting SQL Server.  Since I knew better, I kept on researching. 



Eventually, I found some useful material about how the DTC works.  I realized that it must be the unit of work below the hood that was keeping the SPID from being rolled back.   I used the DTC Tester Tool to validate the DTC. I could tell that the DTC was no longer accepting connections:


DtcGetTransactionManager failed: 8004d01b DtcGetTransactionManager Call Error: Indicates unable to connect to the transaction manager, or the transaction manager is unavailable.

Finally, I was getting somewhere!  This gave me an idea, the spid must be hung because it' DTC transaction is orphaned.  If that's the case, then there should be no reason why I couldn't kill it.  All I needed was the unit of work id.  It turns out that the solution is pretty simple.  


SELECT req_transactionUOW FROM syslockinfo WHERE req_spid = X

req_transactionUOW
--------------------------------
AC6B59D7-A9C8-44DE-A650-CB63B8249D04


The result will be a GUID-like string.  Use the result to kill each unit of work like this:

KILL 'AC6B59D7-A9C8-44DE-A650-CB63B8249D04'
 

That's it, after that the spid that I had previously killed finally died!  I hope this tidbit is helpful to someone else in a pinch.

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