netdev - RE: Question regarding protocol specific mtu for FCoE

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Message-Id: <1244062637.3191.23.camel@achroite>
Date:	Wed, 03 Jun 2009 21:57:17 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"Zou, Yi" <yi.zou@...el.com>
Cc:	Rick Jones <rick.jones2@...com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"Leech, Christopher" <christopher.leech@...el.com>,
	"Dev, Vasu" <vasu.dev@...el.com>,
	"Love, Robert W" <robert.w.love@...el.com>,
	"Ma, Steve" <steve.ma@...el.com>,
	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
Subject: RE: Question regarding protocol specific mtu for FCoE

On Wed, 2009-06-03 at 13:43 -0700, Zou, Yi wrote:
> >On Wed, 2009-06-03 at 12:16 -0700, Zou, Yi wrote:
> >[...]
> >> FCoE is on L2 layer, no path specific MTU, everything goes out as
> >> whatever mtu known to the nic. Since the nic is expected to be used
> >for
> >> converged traffic involving multiple traffic types, e.g. LAN, FCoE, I
> >> was wondering if it makes sense to have the additional MTU.
> >Essentially,
> >> the nic driver will be able to setup via netdev for different MTUs
> >for
> >> converged traffic.
> >
> >Wouldn't you use separate VLANs for FCoE and other traffic?  So maybe
> >we
> >should allow for per-VLAN rather than per-protocol MTU.
> >
> >Ben.
> >
> From what I can tell, you will not be able to set vlan device's MTU
> that is larger than the real_dev->mtu, as in vlan_dev_change_mtu().

Yes, I know this restriction exists at the moment.  But the device MTU
(the limit for dependent virtual devices) could perhaps be decoupled
from the interface MTU (the limit for protocols) and automatically
raised when necessary.  I think something like that is necessary for
your proposal too.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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