Last week on one of our production servers (version 2011.110.3373.0), we’ve encountered a strange issue which is relatively new (only applicable for SQL Server 2012 and 2014 versions). Based on the symptoms observed and doing a quick Bing search found we encountered a product bug!
I observed multiple SPIDs, around 300+ running the same stored procedure which is doing an INSERT operation and are in a suspended status, they seemed to be hanging there for atleast like 45 minutes (usually they finish in few seconds). There is no blocking and I found nothing wrong with the SQL Server except for started seeing elevated values for PWAIT_MD_RELATION_CACHE and MD_LAZYCACHE_RWLOCK wait types. And, oh! I also noticed that tempdb data was at 99% full.
So I thought I found the root cause and tried mitigating the problem by adding extra tempdb space but still wasn’t any help. As the last resort I had to failover the services to another node, basically restarted SQL Server, to bring the server fully functional.
On further investigation we found the root cause as someone tried to create an index (online) to improve performance, but later he cancelled the create index and we hit this bug. However, this bug only effected the table on which index operation was attempted and so, everything else was operational except for that Stored Procedure running multiple SPIDs trying to INSERT to the table on which the user attempted CREATE INDEX command was blocked, impairing that part of functionality of the application.
Resolution: Cumulative Update 9 for SQL Server 2012 SP1; Cumulative Update 1 for SQL Server 2014
Here are more details about this problem: http://support.microsoft.com/kb/2926712
Hareesh G
Jayapal Vajrala
Suresh Raavi
DataGinger.com 