cloud – Made by Mikal

Setting up VXLAN between nested virt VMs on Google Compute Engine

I wanted to play with a VXLAN mesh between VMs on more than one hypervisor node, but the setup for VXLAN ended up being a separate post because it was a bit long. Read that post first if you want to follow the instructions here.

Now that we have a working VXLAN mesh between our two nodes we can move on to installing libvirt (which is called libvirt-daemon-system on Debian, not libvirt-bin as on Ubuntu):

sudo apt-get install -y qemu-kvm libvirt-daemon-system
sudo virsh net-start default
sudo virsh net-autostart --network default

I’m going to use a little python helper to launch my VMs, so I need some other dependancies as well:

sudo apt-get install -y python3-pip pkg-config libvirt-dev git

git clone https://github.com/mikalstill/shakenfist
cd shakenfist
git checkout 6bfac153d249752b27d224ad9d079095b640498e

sudo mkdir /srv/shakenfist
sudo cp template.debian.xml /srv/shakenfist/template.xml
sudo pip3 install -r requirements.txt

Let’s launch a quick test VM to make sure the helper works:

sudo python3 daemon.py
sudo virsh list

You can destroy that VM for now, it was just testing the install.

sudo virsh destroy ...name...

Next we need to tweak the template that shakenfist is using to start instances so that it uses the bridge for networking (that template is the one you copied to /srv/shakenfist/template.xml earlier). Replace the interface section in the template with this on both nodes:

<interface type='bridge'>
  <mac address={{eth0_mac}}/>
  <source bridge='br-vxlan0'/>
  <model type='virtio'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

I know the bridge mentioned here doesn’t exist yet, but we’ll deal with that in a second. Before we start VMs though, we need a way of getting IP addresses to them. shakenfist can configure interfaces using config drive, but I’d prefer to use DHCP because who doesn’t love some additional complexity?

On one of the nodes install docker:


sudo apt-get install apt-transport-https ca-certificates curl gnupg2 software-properties-common
curl -fsSL https://download.docker.com/linux/debian/gpg | sudo apt-key add -
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/debian $(lsb_release -cs) stable"
sudo apt-get update
sudo apt-get install docker-ce docker-ce-cli containerd.io

Now we can setup DHCP. Create a place for the configuration file:

sudo mkdir /srv/shakenfist/dhcp

And then create the configuration file at /srv/shakenfist/dhcp/dhcpd.conf with contents like this:

default-lease-time 3600;
max-lease-time 7200;
option domain-name-servers 8.8.8.8;
authoritative;

subnet 192.168.200.0 netmask 255.255.255.0 {
  option routers 192.168.1.1;
  option broadcast-address 192.168.1.255;

  pool {
    range 192.168.200.10 192.168.200.254;
  }
}

Before we can start dhcpd, we need to move the VXLAN device into a bridge so we can add a device for the DHCP server to it. First off remove the vxlan0 device from the last post:

sudo ip link set down dev vxlan0
sudo ip link del vxlan0

And now recreate it with a bridge:

sudo ip link add vxlan0 type vxlan id 42 dev eth0 dstport 0
sudo bridge fdb append to 00:00:00:00:00:00 dst 34.70.161.180 dev vxlan0
sudo ip link add br-vxlan0 type bridge
sudo ip link set vxlan0 master br-vxlan0
sudo ip link set vxlan0 up
sudo ip link set br-vxlan0 up
sudo ip link add dhcp-vxlan0 type veth peer name dhcp-vxlan0p
sudo ip link set dhcp-vxlan0p master br-vxlan0
sudo ip link set dhcp-vxlan0 up
sudo ip link set dhcp-vxlan0p up
sudo ip addr add 192.168.200.1/24 dev dhcp-vxlan0

This block of commands:

recreated the vxlan0 interface
added it to the mesh with the other node again
created a bridge named br-vxlan0
moved the vxlan0 interface into it
created a veth pair called dhcp-vxlan0 and dhcp-vlan0p
moved the peer part of that veth pair into the bridge
and then configured an IP on the external half of the veth pair

To make the bridge survive reboots you would need to add it to either /etc/network/interfaces or /etc/netplan/01-netcfg.yml depending on your distribution, but that’s outside the scope of this post.

You should be able to ping again. From the other node give it a try:

$ ping 192.168.200.1
PING 192.168.200.1 (192.168.200.1) 56(84) bytes of data.
64 bytes from 192.168.200.1: icmp_seq=1 ttl=64 time=19.3 ms
64 bytes from 192.168.200.1: icmp_seq=2 ttl=64 time=0.571 ms

We need to do something similar on the other node so it can run VMs as well. It is a tiny bit simpler because there wont be any DHCP there however, and remembering that you need to change 35.223.115.132 to the IP of your first node:

sudo ip link set down dev vxlan0
sudo ip link del vxlan0

sudo ip link add vxlan0 type vxlan id 42 dev eth0 dstport 0
sudo  bridge fdb append to 00:00:00:00:00:00 dst 35.223.115.132 dev vxlan0
sudo ip link add br-vxlan0 type bridge
sudo ip link set vxlan0 master br-vxlan0
sudo ip link set vxlan0 up
sudo ip link set br-vxlan0 up

Note that now we can’t do a ping test because the second VM no longer consumes an IP for the base OS.

Now we can start the docker container with dhcpd listening on dhcp-vxlan0:

sudo docker run -it --rm --init --net host -v /srv/shakenfist/dhcp:/data networkboot/dhcpd dhcp-vxlan0

This runs dhcpd interactively so we can see what happens. Now try starting a VM on the other node:

sudo python3 daemon.py

You can watch the VM booting using the “virsh console” command with the name of the vm from “virsh list“. The dhcpd process should show you something like this:

sudo docker run -it --rm --init --net host -v /srv/shakenfist/dhcp:/data networkboot/dhcpd dhcp-vxlan0
Internet Systems Consortium DHCP Server 4.3.5
Copyright 2004-2016 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Config file: /data/dhcpd.conf
Database file: /data/dhcpd.leases
PID file: /var/run/dhcpd.pid
Wrote 0 leases to leases file.
Listening on LPF/dhcp-vxlan0/06:ff:bc:7d:11:e3/192.168.200.0/24
Sending on   LPF/dhcp-vxlan0/06:ff:bc:7d:11:e3/192.168.200.0/24
Sending on   Socket/fallback/fallback-net
Server starting service.
DHCPDISCOVER from ee:95:4d:40:ca:a6 via dhcp-vxlan0
DHCPOFFER on 192.168.200.10 to ee:95:4d:40:ca:a6 (foo) via dhcp-vxlan0
DHCPREQUEST for 192.168.200.10 (192.168.200.1) from ee:95:4d:40:ca:a6 (foo) via dhcp-vxlan0
DHCPACK on 192.168.200.10 to ee:95:4d:40:ca:a6 (foo) via dhcp-vxlan0

You can see here that our new VM got the IP 192.168.200.10 from the DHCP server! It is moments like this when you don’t realise that this blog post took me hours to write that I feel really smart.

If we started a VM on the first node (the same command as for the second node), we’d now have two VMs on a virtual network which had working DHCP and could ping each other. I think that’s enough for one evening.

Setting up VXLAN on Google Compute Engine

So my ultimate goal here is to try out VXLAN between some VMs on instances in Google compute engine, but today I’m just going to get VXLAN working because that took a fair bit longer than I expected. First off, boot your instances — because I will need nested virt later I chose two instances on Google Cloud. Please note that you need to do a bit of a dance to turn on nested virt there. I also chose to use Debian for this experiment:

gcloud compute instances create vx-1 --zone us-central1-b --min-cpu-platform "Intel Haswell" --image nested-vm-image

Now do those standard things you do to all new instances:

sudo apt-get update
sudo apt-get dist-upgrade -y

Now let’s setup VXLAN between the two nodes, with a big nod to this web page. First create a VXLAN interface on each machine (if you care about the port your VXLAN traffic is on being to IANA standards, see the postscript at the end of this):

sudo ip link add vxlan0 type vxlan id 42 dev eth0 dstport 0

Now we need to put the two nodes into a mesh, where 34.70.161.180 is the IP of the node we are not running this command on and the IP address for the second command needs to be different on each machine.

sudo bridge fdb append to 00:00:00:00:00:00 dst 34.70.161.180 dev vxlan0
sudo ip addr add 192.168.200.1/24 dev vxlan0
sudo ip link set up dev vxlan0

I am pretty sure that this style of mesh (all nodes connected) wouldn’t scale past non-trivial sizes, but hey baby steps right? Finally, because we’re using Google Cloud we need to add firewall rules to allow our traffic into the instances:

Note that these rules are a source of confusion for me right now. I wanted (and configured) VXLAN. So why do I need to allow OTV for this to work? I suspect Linux has politely ignored my request and used OTV not VXLAN for my traffic.

We should now be able to ping those newly configured IP addresses from each machine:

ping 192.168.200.2 -c 1
PING 192.168.200.2 (192.168.200.2) 56(84) bytes of data.
64 bytes from 192.168.200.2: icmp_seq=1 ttl=64 time=1.76 ms

Which produces traffic like this on the underlay network:

tcpdump -n -i eth0 host 34.70.161.180
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
09:01:58.159092 IP 10.128.0.9.59341 > 34.70.161.180.8472: OTV, flags [I] (0x08), overlay 0, instance 42
IP 192.168.200.1 > 192.168.200.2: ICMP echo request, id 20119, seq 1, length 64
09:01:58.160786 IP 34.70.161.180.48471 > 10.128.0.9.8472: OTV, flags [I] (0x08), overlay 0, instance 42
IP 192.168.200.2 > 192.168.200.1: ICMP echo reply, id 20119, seq 1, length 64
^C
2 packets captured
2 packets received by filter
0 packets dropped by kernel

Hopefully this is helpful to someone else. Thanks again to Joe Julian for a very helpful post.

Postscript: Dale Shaw pointed out on twitter that I might still be talking VXLAN, just on a weird port. This is supported by this comment I found on the internets: “when VXLAN was first implemented in linux, UDP ports were not specified. Many vendors use 8472, and Linux uses the same port. Later, IANA allocated 4789 as the port. If you need to use the IANA port, you need to specify it with dstport”.

Further thoughts on Azure instance start times

My post from the other day about slow instance starts on Azure caused some commentary (mainly on reddit) that prompted me to think more about all this. In the end, there were a few more experiments I wanted to run to see if I could squeeze more performance out of Azure.

First off, looking at the logs from my initial testing it looks like resource groups are slow. The original terraform creates a resource group as part of the test and then cleans it up at the end. What if instead we had a single permanent resource group and created instances within that?

Here is a series of instance starts and deletes using the terraform from the last post:

You’ll notice that there’s no delete value for the last instance. That’s because terraform crashed and never deleted the instance. You can also see that instance starts are somewhat consistent, except for being slower in the second half of the test than the first, and occasionally spiking out to very very slow. Oh, and deletes are almost always really slow.

What happens if we use a permanent resource group and network? This means that all the “instance start terraform” is doing is creating a network interface and then an instance which uses that network interface. It has to be faster, but does it resolve our issues?

The dashed lines are the graph from above, the solid lines are the new data without resource group creation. You can see that abstracting away the resource group work has made a significant performance improvement. Instance start times are now generally under 100 seconds (which is still three times slower than AWS, and four or five times slower than Google).

So is it just that the Australian Azure zones are slow? I re-ran the new terraform against a US datacenter (East US). Here’s a zoom in of just the instance creates with the resource group extracted to make that clearer, for both data centers:

Interestingly, the Australian data center actually performs better than the US one, which isn’t what I would expect at all. You can also see in this test run that we do still see some unexpectedly slow instance launches, although they feel less frequent and smaller when they happen. That might also just be that I’m testing over a weekend and the data center might be more idle.

Looping back, I think we’ve learnt that resource groups are expensive. The last thing I wanted to dig into was what exactly was happening in those spikes where we had resource groups included. Luckily, they were happening about the point I started logging the terraform trace output of the run.

For example, run azure_1576926569_7_0_apply took 18 minutes and 3 seconds to create the instance. For those 18 minutes, terraform logs that the instance was marked by the Azure API as in provisioningState “Creating”. This correlates with operation id c983b272-fa32-4814-b858-adab3da4d9b1 sitting in state “InProgress”, unfortunately there isn’t a reason logged for why that is. So I guess its not possible as an Azure user to work out why things are sometimes slow.

To summarise some advice for terraform users on Azure — don’t create resource groups if you can avoid it. Create global resource groups and then place new objects into them instead. That said, you’re still going to have slower and less consistent performance than other clouds.

Finally, is instance start time a valid metric for cloud performance? Probably not. That said, it is table stakes to be in the conversation. Slow instance starts affect my overall experience of the cloud, as well as the workability of horizontal scaling techniques. This is especially true for instance start times which vary wildly like Azure’s do — I simply can’t trust that I can grow a horizontal scaling set with any sort of reasonable timeframe.

Why is Azure so slow to start instances?

I’ve been playing with terraform recently, and decided to see how different the terraform for launching a simple Ubuntu instance in various clouds is. There are two big questions there for me — how big is the variation between OpenStack derived clouds; and how painful is it to move between the proprietary clouds? Part of this is because terraform doesn’t present a standardised layer of cloud functionality, it has a provider per cloud.

(Although, I suspect there’s nothing stopping someone from writing a libcloud provider or something like that. It is an interesting idea which requires some additional thought.)

My terraform implementations for each cloud are on github if you’re interested. I don’t want to spend a lot of analysis on the actual terraform, because I think the really interesting thing I found isn’t where I expected it to be (there’s a hint in the title for this post). That said, the OpenStack clouds vary mostly by capabilities. vexxhost for example seems to only offer flavors that require boot-from-volume. The proprietary clouds are complete re-writes, but are generally relatively simple and well documented.

However, that interesting accidental thing — as best as I can tell, Microsoft Azure is really really slow to launch instances. The graph below presents five instance launches on each cloud I tested:

As you can see, Vault, Vexxhost, and AWS are basically all in the same ballpark. Google and Azure are outliers, with Google being crazy fast (but also very slow to delete instances, a metric not presented here), and Azure being more than three times slower than everyone else.

Instance launch time isn’t a great metric to be honest, but it does matter. For example if you were trying to autoscale a web tier or a kubernetes cluster, then waiting over two minutes just for the instance to boot before it can be configured and added to the cluster is probably not ok.

I wonder why Azure is so slow?

I did some further exploring after writing this post and was able to improve performance by changing how I handled resource groups in the terraform. The performance still isn’t great though. You can read more about that in a separate post if you’d like.

Further adventures with base images in OpenStack

I was bored over the New Years weekend, so I figured I’d have a go at implementing image cache management as discussed previously. I actually have an implementation of about 75% of that blueprint now, but its not ready for prime time yet. The point of this post is more to document some stuff I learnt about VM startup along the way so I don’t forget it later.

So, you want to start a VM on a compute node. Once the scheduler has selected a node to run the VM on, the next step is the compute instance on that machine starting the VM up. First the specified disk image is fetched from your image service (in my case glance), and placed in a temporary location on disk. If the image is already a raw image, it is then renamed to the correct name in the instances/_base directory. If it isn’t a raw image then it is converted to raw format, and that converted file is put in the right place. Optionally, the image can be extended to a specified size as part of this process.

Then, depending on if you have copy on write (COW) images turned on or not, either a COW version of the file is created inside the instances/$instance/ directory, or the file from _base is copied to instances/$instance.

This has a side effect that had me confused for a bunch of time yesterday — the checksums, and even file sizes, stored in glance are not reliable indicators of base image corruption. Most of my confusion was because image files in glance are immutable, so how come they differed from what’s on disk? The other problem was that the images I was using on my development machine were raw images, and checksums did work. It was only when I moved to a slightly more complicated environment that I had enough data to work out what was happening.

We therefore have a problem for that blueprint. We can’t use the checksums from glance as a reliable indicator of if something has gone wrong with the base image. I need to come up with something nicer. What this probably means for the first cut of the code is that checksums will only be verified for raw images which weren’t extended, but I haven’t written that code yet.

So, there we go.

Openstack compute node cleanup

I’ve never used openstack before, which I imagine is similar to many other people out there. Its actually pretty cool, although I encountered a problem the other day that I think is worthy of some more documentation. Openstack runs virtual machines for users, in much the same manner as Amazon’s EC2 system. These instances are started with a base image, and then copy on write is used to write differences for the instance as it changes stuff. This makes sense in a world where a given machine might be running more than one copy of the instance.

However, I encountered a compute node which was running low on disk. This is because there is currently nothing which cleans up these base images, so even if none of the instances on a machine require that image, and even if the machine is experiencing disk stress, the images still hang around. There are a few blog posts out there about this, but nothing really definitive that I could find. I’ve filed a bug asking for the Ubuntu package to include some sort of cleanup script, and interestingly that led me to learn that there are plans for a pretty comprehensive image management system. Unfortunately, it doesn’t seem that anyone is working on this at the moment. I would offer to lend a hand, but its not clear to me as an openstack n00b where I should start. If you read this and have some pointers, feel free to contact me.

Anyways, we still need to cleanup that node experiencing disk stress. It turns out that nova uses qemu for its copy on write disk images. We can therefore ask qemu which are in use. It goes something like this:

$ cd /var/lib/nova/instances
$ find -name "disk*" | xargs -n1 qemu-img info | grep backing | \
  sed -e's/.*file: //' -e 's/ .*//' | sort | uniq > /tmp/inuse

/tmp/inuse will now contain a list of the images in _base that are in use at the moment. Now you can change to the base directory, which defaults to /var/lib/nova/instances/_base and do some cleanup. What I do is I look for large image files which are several days old. I then check if they appear in that temporary file I created, and if they don’t I delete them.

I’m sure that this could be better automated by a simple python script, but I haven’t gotten around to it yet. If I do, I will be sure to mention it here.