[mythtv-users] OT: SMB performance?

Gary Buhrmaster gary.buhrmaster at gmail.com
Thu Jan 19 21:39:33 UTC 2012


On Thu, Jan 19, 2012 at 21:00, Ben Kamen <bkamen at benjammin.net> wrote:
...
> (p.s. does anyone know if MTU's must all be set the same or do they negotiate automatically between hosts to the largest size compatible between those hosts?)

For simple values, all hosts on the same physical network
need to be set to the same physical frame size.  Ex:
a UDP app decides to send a 4K packet.  The system
has a physical MTU of 9K, so it sends it as one frame.
Another system on the same physical net has a physical
MTU of 1.5K.  It fails to see packet (too big).  UDP app
times out (if it does), and sends again.  And so on, and so on
until the app exceeds its retries.  For TCP, it is slightly more
complicated, since during initial session creation the
TCP MSS is also negotiated.  It is going to be the
smallest of both, so you might end up with a frame
size of 1.5K, so everything mostly works.

Routers may send back icmp packets (too big)
if a frame tries to go from a large MTU network
to a small MTU network (or it may fragment,
depending on the router), but on the same physical
wire, ethernet does not have that capability.

So, everything should be (the same) jumbo, or
nothing should be jumbo is the best advice for most
people (special cases exist, and for those that
are running multiple routed networks with
different MTUs, they probably do not need any
advice).

Gary


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