question about "create file" in filesystem

https://www.usenix.org/legacy/events/wiov11/tech/final_files/Hildebrand.pdf

"In this section, we show the impact of guest file
system metadata requests as they are transformed into
read and write accesses to a disk image file. The
optimizations made by many file systems for metadata
operations such as creates become irrelevant in 
virtualized NAS environments since metadata is
always transformed to data I/O"

這是什麼意思呢~~