netdev - RE: Question regarding protocol specific mtu for FCoE

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Message-ID: <7C88852EF6F99F4EB538472FCFEBE2223A7E70E9@orsmsx509.amr.corp.intel.com>
Date:	Wed, 3 Jun 2009 15:22:29 -0700
From:	"Zou, Yi" <yi.zou@...el.com>
To:	Ben Hutchings <bhutchings@...arflare.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.
Well, for all all NIC devices that are capable of doing converged
traffic, the additional MTU seems to me fits just what you mentioned as
the automatic MTU increase, since a different MTU is checked. Of course,
the vlan above needs to be also fixed to allow this as well. Also, FCoE
should not required to be created only on a VLAN interface.

Thanks.

yi

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