Re: network performance between host and guest...

[Erik Brakkee <erik@brakkee.org>]

Brian Jackson wrote:
> On 12/17/2010 4:29 PM, Erik Brakkee wrote:
>> Hi,
>>
>>
>> For a backup of data from a VM to a USB mounted disk I want to 
>> circumvent the USB 1.1 limitations on the guest and instead copy the 
>> data over to the host using scp/ssh. I have setup a network using 
>> virtio and NAT like this:
>>
>> <interface type='network'>
>> <mac address='52:54:00:6b:0d:36'/>
>> <source network='default'/>
>> <target dev='vnet1'/>
>> <model type='virtio'/>
>> <alias name='net1'/>
>> <address type='pci' domain='0x0000' bus='0x00' slot='0x06' 
>> function='0x0'/>
>> </interface>
>
>
> What does that equate to in command line options? Check libvirt logs 
> maybe. What version of qemu-kvm? Guest details? Host details?
>
>
>>
>> When I now create a 1GB file using dd and copy it over from the guest 
>> to the host, I am seeing a performance between 25-30 MB/s.
>
>
> Is it to and from the same disk? If so, maybe you could try a tmpfs in 
> the guest or host so you aren't constantly seeking back and forth on 
> the same disk.
>
> Also have you tried something like rsyncd instead of scp? Maybe you 
> are hitting some sort of encryption limitation.
>
>
>>
>> My question is if this is normal because I have seen others on the 
>> internet achieve far greater speeds.
>
>
> Depends on a lot of factors. Certainly raw bandwidth wise, virtio-net 
> is capable of a lot more than that. With vhost-net here, I can get 
> over 5gbps guest to host. And that's on crappy old first gen cpus (no 
> ept/etc.).
>
>
>>
>> In any case the speeds are comparable to current USB 2.0 speeds but I 
>> intend on using USB 3.0 so would like to get a little bit more out of 
>> it.
>>
>> What would I use to speed this up a bit futher?
>>
>> Cheers
>>   Erik
>>
>> -- 
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>

You are right. I was indeed not so smart of me. I was assuming that the 
overhead of SSH would be negligible. However, I am seeing similar 
transfer speeds when I copy a file on the localhost to the localhost 
using SSH (on the host, not even on a guest). Netperf tests of the 
network show much higher speeds (approx 900Mbps) so the network is not 
the problem. The bottleneck is definitely SSH.