Recently, I had replied to a MSDN post regarding an Access Violation reported by a SQL Server instance. In this post, I shall elucidate on troubleshooting memory dumps generated by SQL Server exceptions and what can be done without the availability of private symbols for debugging.
I had written a post earlier on when a memory dump is generated in SQL Server. If a single Errorlog has multiple stack dumps from multiple exceptions, then the first thing to do is to investigate the first access violation reported in the series of dumps reported. It is also possible that an exception (eg:access violation) could be followed by another exception/assertion or a non-yielding condition which may be a side effect of the earlier access violation.
When SQL encounters an access violation, there will be SQLDumpXXXX.mdmp, SQLDumpXXXX.txt and SQLDumpXXXX.log files created in the LOG folder associated with the exception. The SQL Errorlog will report the occurrence of the exception as well.
- The .mdmp file is the memory dump which would be required by CSS to troubleshoot the issue.
- The .txt file is the symptom dump file which contains environment information along with other information depending on the nature of the exception.
- The .log file contains an extract from the SQL Errorlog when the exception was encountered. This helps in looking at what messages were reported in the Errorlog prior to the issue. Helps during post-mortem analysis if the current Errorlogs have already been recycled.
There can be two types of exceptions: one that is fatal enough to cause a SQL service termination or one that encounters the exception which terminates the thread which was encountered the issue.
Now what can be done in such a situation without having to look into memory dump file. The first step would be to check if the issue is a recurring behavior due to a particular operation to establish some pattern or if there is not identifiable pattern at all. If you look into the SQL Errorlog, you might be able to identify the input buffer which cause the exception (unless it’s an exception which terminated the SQL service, one such example is shown below).
Server * BEGIN STACK DUMP:
Server * spid 5728
Server * ex_handle_except encountered exception C0000005 – Server terminating
If the above snippet contains the T-SQL query which raised the exception, then you would want to execute the same query from a Management Studio query window and check if the exception re-occurs. If yes, then you have a valid repro scenario.
Example snippet from an Errorlog where the Access Violation didn’t cause a service termination:
* Exception Address = 0042B866
* Exception Code = c0000005 EXCEPTION_ACCESS_VIOLATION
* Access Violation occurred writing address 67192000
* Input Buffer 38 bytes –
* exec sp_updatestats
Next check if you can spot any external components loaded into SQL Server address space. I shall revert back to my MSDN post where I had spotted an anti-virus filter DLL loaded in SQL Server process address space.
Server * bcrypt 000007FEFC900000 000007FEFC921FFF 00022000
Server * PCTLsp64 000000000B1E0000 000000000B251FFF 00072000
Server * mswsock 000007FEFC840000 000007FEFC893FFF 00054000
Based on a few Bing searches, I was able to determine that the PCTLsp64 was part of a virus monitoring program from PC Tools. The next step would be to check if this external component can be moved out of SQL Server process address space. The fact that the DLL is loaded in SQL process address space doesn’t make the DLL a culprit. However, when troubleshooting such issues, it is important to eliminate all unknown or third party elements that could prevent us from narrowing down to the culprit. Other DLLs which can be commonly found in SQL Server process address space would be provider DLLs when linked servers are being used on the SQL Server instance. Additionally execution of Extended Stored Procedures (XSPs) could also load external DLLs into SQL Server process address space depending on the way the XSP DLL was coded. An easy way to identifying a XSP dll is searching for the word “using” in the SQL Server Errorlog. SQL Server reports the first instance of a XSP dll being loaded into SQL Server process address space.
Using ‘xpstar.dll’ version ‘2009.100.1600’ to execute extended stored procedure ‘xp_readerrorlog’
If you do find an external component loaded in SQL process address space, check the callstack reported in the Errorlog below the list of modules printed below to determine if the functions of the exception stack are associated with external component that you identified. For the issue reported in the MSDN post, I did find this:
Module(sqlservr+00000000017A954C)
Module(kernel32+0000000000099380)
Module(ntdll+0000000000096228)
Module(ntdll+0000000000014F48)
Module(ntdll+0000000000034F6D)
Module(ntdll+0000000000015B2C)
Module(ntdll+000000000004F638)
Module(ntdll+000000000004C8F4)
Module(ntdll+000000000004C822)
Module(PCTLsp64+00000000000040AD)
Module(PCTLsp64+0000000000004701)
Module(PCTLsp64+0000000000005434)
Now this makes my story a bit more compelling of removing the external component out of SQL Server process address space. Once that is done and if you have a repro condition which causes the issue, then it would be good to re-run your repro and check if the issue is actually resolved. Not all the exceptions are the same even though they are reported in the same manner.
If all the above hasn’t helped in narrowing down the issue, then check if you are on the latest available cumulative update for the version of SQL Server that you are using. If not, then it might be a good idea to update your instance to the latest available build to ensure that you are not hitting a known issue. If this is possible, then apply the latest cumulative update package and check if the issue re-occurs. I am cognizant of the fact that this might be an easy option due to change management restrictions on production environments. For such a dependency, my recommendation would be engage CSS to assist with a thorough investigation of the root cause of the issue.
As you can see from the steps outline above, you can look into a few things before you need to start poking into the exception memory dump using a Debugging tool.
Happy debugging!
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