Fixed vs Dynamic VHDX Performance

Q: Is the performance of a dynamic VHDX the same as a fixed VHDX? I've heard different opinions.

A: A fixed-size VHDX file has all space allocated at the time of the VHDX file creation, whereas a dynamic VHDX file grows as data is written to it. For data written to existing allocated space within a fixed or dynamic VHDX, you'll see the exact same performance. For data written to a dynamic VHDX file that requires the VHDX file to grow, there's some additional storage overhead related to zeroing out existing disk content, extending the VHDX file on the file system, and modifying the VHDX file metadata. This means there's a slight performance difference between a dynamic and fixed VHDX while the dynamic VHDX file grows, but once the dynamic VHDX has stabilized in size the performance is the same.