Is there a "make hole" (truncate in middle) syscall?

[Rob Landley]

You can make a file with a hole by seeking past it and never writing to that 
bit, but is there any way to punch a hole in a file after the fact?  (I mean 
other with lseek and write.  Having a sparse file as the result....)

What are the downsides of holes?  (How big do they have to be to actually save 
space, is there a performance penalty to having a file with 1000 4k holes in 
it, etc...)

Rob
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