SQL Server: Duplicate Index Question We recently has a massive performance problem on our system, which we narrowed down to a problem with I/O on the LUN.  IOPS increased twenty-fold during the period when the problem was occuring.  The LUN in question was the one that tempdb uses.  This is all according to our network/server admins (I have no network/SAN/Server expertise).

Everyone on the team was investigating the problem, until one team member found an expensive query in activity monitor that look suspicious.  He inspected the table and found and index that was a duplicate.  Actually, it was technically an overlap, since it had one additional included column over and above from the original index.

The overlapping index had been in production for about three weeks with no problems.

The index was dropped in production, and the performance problems stopped almost immediately after  We were not all in sync monitoring and trying fixes (never mind that this isn't a good practice).  So it's possible it was a coincidence, but no other changes were made in the same timeframe, and we're 99% sure this index was somehow the cause.

I realize a superfluous index can cause performance problems, but I have never heard of a case where the problem would be that pronounced.  Adding the index back also caused no problems (I know--also not a good practice in production).

Any idea of what the root cause of the problem could have been?  The duplication?  A corrupted index (if such a think exists)?  

Any online sources as references would be deeply appreciated as well.
Without the execution plans, it's impossible to say, but I've seen a similar scenario pop up.  Had a client once whose database was teetering on the edge of being able to fit in memory.  Someone added an index on a large table, queries started using it (which kept big chunks of it in memory.)  Things all used to fit in memory perfectly, but they hit a tipping point where they started to hammer the disk much more frequently.  We caught it via the sudden spike in disk access and a sudden drop in Page Life Expectancy.

Ideally, you should capture these Perfmon counters continuously on database servers, and alert when they drop dangerously.  That'd help correlate things so you'd know if the PLE drop also happened, and that'd indicate you might be running out of memory to cache things.