Re: nfs question - ftruncate vs pwrite

[Peter Staubach]

Kenny Simpson wrote:

> Sorry about the previous partial message...
>
> If a file is extended via ftruncate, the new empty pages are read in before the the ftruncate
> returns (taking 64mS on my machine), but if the file is extended via pwrite, nothing is read in
> and the system call is very quick (34uS).
>
> Why is there such a difference?  Is there another cheap way to grow a file and map in its new
> pages?  Am I missing some other semantic difference between ftruncate and a pwrite past the end of
> the file?

You might use tcpdump or etherreal to see what the different traffic looks
like.  I suspect that ftruncate() leads a SETATTR operation while pwrite()
leads to a WRITE operation.

      ps
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