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Does anyone know what the performance differences would be between MongoDB and SQL Server/Oracle if they all had enough RAM to hold the entire dataset in memory?

I'm only guessing but it seems to me that any database with their entire dataset in memory would be very fast, no?

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There were some slides showing PostgreSQL with fsync turned off performed about the same as MongoDB. There is no MongoDB secret sauce that makes it any faster than well established relational dbs with a few configuration tweaks to make the comparison even.

You are correct - any database with the entire dataset in memory will be incredibly fast. SQL Server bypasses the Windows file system cache and will aggressively manage memory to keep frequently accessed data pages resident in RAM. The read/write performance is what you would expect for a database with fine-graned lock management - when you have to go to disk things get slower, otherwise I/O is only limited by RAM and the overhead of lock management.

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