[ovirt-users] Users Digest, Vol 71, Issue 37

[Moacir Ferreira]

Sorry... I guess our discussion here is in line with the "good practices" discussion. For a long time I see a lot of mentions on having a front-end and a back-end network when dealing with distributed file systems like Gluster and Ceph. What I would like to here from those who already implemented oVirt on the field is "what is the real life" approach for good performance as large file/memory transfers will strongly benefit from having a big MTU. However, big MTU must be driven correctly otherwise you may end-up having the problem we are discussing.

Moacir

________________________________
From: Moacir Ferreira <moacirferreira at hotmail.com>
Sent: Tuesday, August 8, 2017 3:16 PM
To: Fabrice Bacchella
Cc: users at ovirt.org
Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37


Exactly Fabrice! In this case the router will fragment the "bigger" MTU to fit the "smaller" MTU but only when the DF is not set. However, fragmentation on routers are made by the control plane, meaning you will overload the router CPU doing too much fragmentation. On a good NIC the announced MTU to the IP stack is very big (like 64Kb) because the off-load engine will fragment this very large MTU and send it. But on this kind of NIC the fragmentation is done by dedicated AISCs that does not require any CPU intervention to do it. Just give it a try... Assemble a lab using Linux and you will see what I am trying to explain.

Moacir

________________________________
From: Fabrice Bacchella <fabrice.bacchella at orange.fr>
Sent: Tuesday, August 8, 2017 2:50 PM
To: Moacir Ferreira
Cc: users at ovirt.org
Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37


Le 8 août 2017 à 14:53, Moacir Ferreira <moacirferreira at hotmail.com<mailto:moacirferreira at hotmail.com>> a écrit :

But if you receive a 9000 MTU frame on an "input" interface that results sending it out on an interface of a 1500 MTU, then if you set DF bit the frame will just be dropped by the router.

The frame will be dropped and the router will send an ICMP message "packet to big" to the sender, it's network stack will received that, learn that the PMTU is lower and try with smaller fragment, see https://en.wikipedia.org/wiki/Path_MTU_Discovery.

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