The pages aren't moved from memory to disk, (there's a page on disk and a copy of that page in memory), but checkpoint writes at a page level, not the individual change level, so if 2 bytes on a page have been changed, that entire page is written back to the data file.
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
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