We have gone through several migrations already, mixing and matching sources (SQL 2000, 2005, X86, X64, Enterprise to Standard, etc). We now got most of our databases running under SQL Server 2008 R2.
In every instance we have found performance issues related to HyperThreading, bad plans, missing registry key or outdated iSCSI drivers. We have reached a point where we are now ready to analyze and solve the challenges soon after the migration has been performed.
We recently migrated a core DB, from 2005 to 2008 R2, from X86 to X64 and from Enterprise Edition to Standard Edition. System was effective but the application was sporadically timing out. After digging through the logs, collecting info based on server side traces, and using Adam Machanic’s sp_whoisactive tool, we found the culprit. Eight services pulling data using 1 stored procedure was bringing the box to it’s knees. With updated stats, better hardware and more memory available, we never expected this to happen. But it did.
We added an index and narrowed the scope of a where clause inside the stored procedure. The procedure went from ~30 mins and ~100 million reads to 10 seconds and ~15,000 reads.
Small changes, huge differences. The key is to spot the issue, understand and work it around. Then schedule a production push and QA the process.
Things look very good now.