Thanks for the explanation, Simon. The man page wasn't entirely clear on that
point.
A jumbogram is a
large-size packet composed of 2 to 4 normal Rx data packets that
share the same header. The fileserver does not use jumbograms by
default, as some routers are not capable of properly breaking the
jumbogram into smaller packets and reassembling them.
can we add this from your message to the man page? I'll send a patch to the
bugs address if
you think it's a good idea.
> jumbograms deliberately exceed 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.
Jumbograms
On Feb 28, 2011, at 11:07 AM, Simon Wilkinson wrote:
>
> 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.
danno
--
Dan Pritts, Sr. Systems Engineer
Internet2
office: +1-734-352-4953 | mobile: +1-734-834-7224
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