jumbo (was Re: [OpenAFS] Documenting the recommended fileserver options)

Simon Wilkinson sxw@inf.ed.ac.uk
Mon, 28 Feb 2011 16:07:55 +0000

On 28 Feb 2011, at 14:31, Dan Pritts wrote:

> On Feb 24, 2011, at 8:03 PM, Jason Edgecombe wrote:
>>   fileserver -L -udpsize 131071 -sendsize 131071 -rxpck 700 -p 128 - 
>> b 600 -nojumbo -cb 1500000
> I was curious about jumbogram support so i went to read the man page.
> The man page suggests that if your router does not support  
> fragmenting of packets, jumbo won't work for clients without jumbo  
> support.  Does this mean there is no path mtu discovery support in  
> the fileserver?  not that that is without issues, of course.

1.6 has path discovery for some operating systems. Derrick can say  
more about how complete it is, but I think there are issues for some  
of our cache managers.

However, path MTU discovery doesn't actually buy you that much. It can  
only tell you the maximum packet size that can be passed unfragmented  
between the RX endpoints. jumbograms deliberately exceeds the MTU of  
the path, typically by 4 times, in order to get better performance  
(fragmented packets are actually more efficient in our RX  
implementation than having to process more single packets). Path  
discovery doesn't tell you anything about whether these packets will  
be successfully passed, or dropped on the floor.