Xen devel - Jan 2011 - Live migration-delay in gratuitous arp

Hi,

I was trying to perform live migration with a VM on a LAN and measure 
the downtime seen by an application (e.g. ping), the time for which the 
application cannot communicate with the VM. The application is run on a 
machine different from those between which the VM is migrated. I noticed 
that the VM remains unreachable for nearly 1-1.5sec. However the 
xend.log file says that the time (the delta) for the last round of the 
migration is around 100msec. So, the VM is actually paused for just 
around 100msec but the application cannot reach it for 1-1.5sec. I 
believe this is because of the delay in the gratuitous ARP being sent. 
Is the implementation such that the ARP is not sent immediately after 
the migration is finished? Is there an easy tweak (without modifying 
source code) by which I can correct this or do I have to modify the 
source code?

I am using Xen from the repos of CentOS 5.5 on a 64bit machine.

Thanks,
Virajith



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On 1/11/2011 9:51 PM, Virajith Jalaparti wrote:
> Hi,
>
> I was trying to perform live migration with a VM on a LAN and measure
> the downtime seen by an application (e.g. ping), the time for which the
> application cannot communicate with the VM. The application is run on a
> machine different from those between which the VM is migrated. I noticed
> that the VM remains unreachable for nearly 1-1.5sec. However the
> xend.log file says that the time (the delta) for the last round of the
> migration is around 100msec. So, the VM is actually paused for just
> around 100msec but the application cannot reach it for 1-1.5sec. I
> believe this is because of the delay in the gratuitous ARP being sent.
> Is the implementation such that the ARP is not sent immediately after
> the migration is finished? Is there an easy tweak (without modifying
> source code) by which I can correct this or do I have to modify the
> source code?
>
> I am using Xen from the repos of CentOS 5.5 on a 64bit machine.
I''d be interested to hear if running arping in screen during migration 
on your domU has any effect.

Jamon

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I''d be interested to hear if running arping in screen during migration
    on your domU has any effect.

    Jamon




Hi Jamon,

I did not see any difference even if I do this. I ran "arping -I eth0 -b
-U<domU ip address>" but still was getting a downtime of around
1.5sec.
I also tried arping from one of the dom0''s but that does not help
either.

Virajith



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Hello,
Try to run the following script from *another* dom0 in the same LAN. This
script could be considered as an "hack" to update the CAM table of the
switch :

#!/bin/bash
if [ -z "$1" ]; then
  echo "Error: give the DomU IP to ping as parameter."
  exit 1
fi
HOSTNAME=$1
while /bin/true;do
        /sbin/ip neigh flush all &>/dev/null
        ping -c1 -w1 $HOSTNAME
        sleep 0.2
done

Jordan


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I had tried something similar earlier but it did not work. When even I 
run the script you sent, the atleast 10-15 of the consecutive pings 
don''t receive any replies. I guess this implies that some thing is
wrong
with the ARP sending mechanism of the VM. Apparently, this issue was 
discussed earlier at 
http://lists.xensource.com/archives/html/xen-devel/2007-03/msg00268.html 
and they say that this is a bug in Xen-3.0.3 which is the version in the 
repos of CentOS 5.5.

Virajith

Jordan Pittier wrote:
> Hello,
> Try to run the following script from *another* dom0 in the same LAN. 
> This script could be considered as an "hack" to update the CAM
table
> of the switch : 
>
> #!/bin/bash
> if [ -z "$1" ]; then
>   echo "Error: give the DomU IP to ping as parameter."
>   exit 1
> fi
> HOSTNAME=$1
> while /bin/true;do
>         /sbin/ip neigh flush all &>/dev/null
>         ping -c1 -w1 $HOSTNAME 
>         sleep 0.2
> done
>
> Jordan
>
>
> ------------------------------------------------------------------------
>
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