Re: nfs question - ftruncate vs pwrite

--- Peter Staubach <staubach@xxxxxxxxxx> wrote:
> 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.

Ethereal results interpreted with wild speculation:
The pwrite case:
This does a bunch of reads, but the server always returns a short read responding with EOF. It
seems that a pwrite does cause a getattr call, but that's it.
Once memory is exhausted, the pages are written out.

The ftruncate case:
This does a setattr, then does a read - this time the server responds with a large amount of
0's.

Since this is using the buffer cache (not opened with O_DIRECT), and since we know we are
extending the file... is it strictly necessary to read in pages of 0's from the server?

-Kenny



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