This one is a bit surprising.
Assume you have a cold SQL Server which you have just re-booted. The buffer cache is empty and lots of I/O requests are going to be physical. Boo! Hiss! Down with physical I/O. Well help is at hand (if you are using SQL Server Enterprise Edition).
Ramp-up is the process of converting your I/O's from page size 8KB reads to extent level I/O's i.e. 64KB. This isn't the extent the page belongs to per se but just an equivalent size. SQL Server does this to accelerate the warming up process of the server. Essentially rather than reading pages to satisfy your query request it pulls back the surrounding 7 pages to flood the cache with data just in case with the hope that you the user will request one of those pages next. There is over-head in doing this but I don't believe it's much and the benefit of subsequent requests being potentially logical rather than physical I/O's offsets this cost. However, you may see 64KB I/O's in perfmon until this period of ramp up has finished.
Thrilling stuff eh.
However, the question I know that's on your lips is; How do I know when it's finished ramping up? Well as with most things the answer lies in a DMV. sys.dm_os_sys_info is the puppy. This DMV tells you what the target size for the buffer cache is (bpool_commit_target) and also what the current committed value is (bpool_committed) i.e. how much data is already in cache. Once these two numbers are the same then ramp up ends...till the next time.
The scantily clad documentation for this feature can be found in the I/O Basics chapter 2 paper here.
Cheers, James
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