scp more perfectly fills the pipe than NFS/TCP

[Dan Nelson]

In the last episode (Dec 19), Zaphod Beeblebrox said:
> Here's an interesting conundrum.  I don't know what's different between
> the TCP that scp uses from the TCP that NFS uses, but given the same two
> FreeBSD machines, SCP fills the pipe with packets better.
> 
> Examine the following graphic: http://www.eicat.ca/~dgilbert/example-mrtg.png
> 
> The system doing the scp and the NFS server is FreeBSD-7.2-p1.  The system
> receiving the scp and the NFS client is FreeBSD-8.0-p1
> 
> The scp transfer is the left hand side of the graph and the NFS transfer
> is on the right.
> 
> The NFS is mounted with "-3 -T -b -l -i" and no other options.  Files are
> being moved over NFS with the system "mv" command.  The files in each case
> are large (50 to 500 meg files).

If you increase the NFS blocksize (-r 32768 for example) you will get
slightly better performance, but you will likely never match the scp
results.  They're doing two different things under the hood: scp is
streaming the entire file in one operation, while NFS is performing many
"read 8k at offset 0", "read 8k at offset 8k", etc requests one after
another, so a high-latency connection will take a performance hit due to the
latency in issuing each command.  According to the mount_nfs manpage, it
looks like there is some prefetching that can be enabled with the "-a ##"
option.  It doesn't say what the default is, though.

-- 
	Dan Nelson
	dnelson at allantgroup.com