Keith Tate (10/1/2013)
Actually RCSI is the "big hammer" approach to snapshot isolation. Once turned on it will change all queries that are currently running in read committed isolation level (which is the default) and change them to use the version stores. With Snapshot Isolation, after you turn it on, you must specify SET TRANSACTION ISOLATION LEVEL SNAPSHOT to use it. So Snapshot Isolation can be used only where you want the change unlike RCSI.
True, however once Allow Snapshot Isolation has been turned on, all data modifications write old versions into the row version store, regardless of whether the isolation level is ever requested. It's very possible to get all the downsides and none of the benefits from snapshot isolation by enabling it but never using the isolation level. At least once RCSI is turned on you immediately get the benefits and downsides.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability