Bug 473110

Summary: RHEL 5.3: allow tun/tap support larger MTU sizes
Product: Red Hat Enterprise Linux 5 Reporter: Mark McLoughlin <markmc>
Component: kernelAssignee: Mark McLoughlin <markmc>
Status: CLOSED ERRATA QA Contact: Martin Jenner <mjenner>
Severity: high Docs Contact:
Priority: high    
Version: 5.3CC: kernel-mgr, mgahagan, mwagner, ovirt-maint, syeghiay, xen-maint
Target Milestone: rc   
Target Release: ---   
Hardware: All   
OS: Linux   
Whiteboard:
Fixed In Version: Doc Type: Bug Fix
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Clone Of: Environment:
Last Closed: 2009-01-20 19:58:52 UTC Type: ---
Regression: --- Mount Type: ---
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Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:
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Description Flags
tun-jumbo-frame-support.patch none

Description Mark McLoughlin 2008-11-26 15:40:26 UTC 
Currently, the tun/tap driver doesn't support setting it's MTU size to more than 1500 bytes:

  $> ip link set vnet0 mtu 9000
  SIOCSIFMTU: Invalid argument

In order to gain the performance improvement resulting from less packet overhead, it's quite common for customers to configure their network so that the entire path between two nodes support jumbo frames (i.e. frame size of 9000 bytes).

Since tun/tap doesn't support this, it's impossible to obtain this benefit with KVM/virtio guests.

The patch to implement this support is quite trivial and has been upstream since 2.6.24.
Comment 2 Mark McLoughlin 2008-11-26 15:58:31 UTC 
Upstream patch:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=4885a50476b95fa0f4caad179a80783508c2fe86
Comment 3 Mark McLoughlin 2008-11-26 17:45:32 UTC 
Created attachment 324770 [details]
tun-jumbo-frame-support.patch
Comment 6 Don Zickus 2008-12-09 21:05:36 UTC 
in kernel-2.6.18-126.el5
You can download this test kernel from http://people.redhat.com/dzickus/el5
Comment 11 errata-xmlrpc 2009-01-20 19:58:52 UTC 
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHSA-2009-0225.html