vi : must it always copy the entire file to be edited?

[WATERMAN at USC-ECL.ARPA]

 apparently vi ALWAYS copies the entire input file, even if invoked
 with readonly switch.  Is there a way to have it not do that? When
 editing a large file, this causes a problem for us in the partition where
 the temp directory is.  ln 'd temp to a directory under /usr where we have
 more space, but it is still annoying to have to wait for vi to cp the entirety
 of a large file before editing.

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