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Tales From the Cryp^H^H^H^H Cloud
Herein lies a selection of tales from OpenStack cloud operators. Read, and learn from their wisdom.
I was on-site in Kelowna, British Columbia, Canada setting up a new OpenStack cloud. The deployment was fully automated: Cobbler deployed the OS on the bare metal, bootstrapped it, and Puppet took over from there. I had run the deployment scenario so many times in practice and took for granted that everything was working.
On my last day in Kelowna, I was in a conference call from my hotel. In the background, I was fooling around on the new cloud. I launched an instance and logged in. Everything looked fine. Out of boredom, I ran ps aux and all of the sudden the instance locked up.
Thinking it was just a one-off issue, I terminated the instance and launched a new one. By then, the conference call ended and I was off to the data center.
At the data center, I was finishing up some tasks and remembered the lock-up. I logged into the new instance and ran ps aux again. It worked. Phew. I decided to run it one more time. It locked up.
After reproducing the problem several times, I came to the unfortunate conclusion that this cloud did indeed have a problem. Even worse, my time was up in Kelowna and I had to return back to Calgary.
Where do you even begin troubleshooting something like this? An instance that just randomly locks up when a command is issued. Is it the image? Nope—it happens on all images. Is it the compute node? Nope—all nodes. Is the instance locked up? No! New SSH connections work just fine!
We reached out for help. A networking engineer suggested it was an MTU issue. Great! MTU! Something to go on! What’s MTU and why would it cause a problem?
MTU is maximum transmission unit. It specifies the maximum number of bytes that the interface accepts for each packet. If two interfaces have two different MTUs, bytes might get chopped off and weird things happen—such as random session lockups.
OK, so where is the MTU issue coming from? Why haven’t we seen this in any other deployment? What’s new in this situation? Well, new data center, new uplink, new switches, new model of switches, new servers, first time using this model of servers… so, basically everything was new. Wonderful. We toyed around with raising the MTU at various areas: the switches, the NICs on the compute nodes, the virtual NICs in the instances, we even had the data center raise the MTU for our uplink interface. Some changes worked, some didn’t. This line of troubleshooting didn’t feel right, though. We shouldn’t have to be changing the MTU in these areas.
As a last resort, our network admin (Alvaro) and myself sat down with four terminal windows, a pencil, and a piece of paper. In one window, we ran ping. In the second window, we ran
tcpdump on the cloud controller. In the third,
tcpdump on the compute node. And the forth had
tcpdump on the instance. For background, this cloud was a multi-node, non-multi-host setup.
One cloud controller acted as a gateway to all compute nodes. VlanManager was used for the network config. This means that the cloud controller and all compute nodes had a different VLAN for each OpenStack project. We used the
-s option of
ping to change the packet size. We watched as sometimes packets would fully return, sometimes they’d only make it out and never back in, and sometimes the packets would stop at a random point. We changed
tcpdump to start displaying the hex dump of the packet. We pinged between every combination of outside, controller, compute, and instance.
Finally, Alvaro noticed something. When a packet from the outside hits the cloud controller, it should not be configured with a VLAN. We verified this as true. When the packet went from the cloud controller to the compute node, it should only have a VLAN if it was destined for an instance. This was still true. When the ping reply was sent from the instance, it should be in a VLAN. True. When it came back to the cloud controller and on its way out to the Internet, it should no longer have a VLAN. False. Uh oh. It looked as though the VLAN part of the packet was not being removed.
That made no sense.
While bouncing this idea around in our heads, I was randomly typing commands on the compute node:
vlan_interface specifies what interface OpenStack should attach all VLANs to. The correct setting should have been:
As this would be the server’s bonded NIC.
vlan20 is the VLAN that the data center gave us for outgoing Internet access. It’s a correct VLAN and is also attached to bond0.
By mistake, I configured OpenStack to attach all tenant VLANs to vlan20 instead of bond0 thereby stacking one VLAN on top of another. This added an extra 4 bytes to each packet and caused a packet of 1504 bytes to be sent out which would cause problems when it arrived at an interface that only accepted 1500.
As soon as this setting was fixed, everything worked.
At the end of August 2012, a post-secondary school in Alberta, Canada migrated its infrastructure to an OpenStack cloud. As luck would have it, within the first day or two of it running, one of their servers just disappeared from the network. Blip. Gone.
After restarting the instance, everything was back up and running. We reviewed the logs and saw that at some point, network communication stopped and then everything went idle. We chalked this up to a random occurrence.
A few nights later, it happened again.
We reviewed both sets of logs. The one thing that stood out the most was DHCP. At the time, OpenStack, by default, set DHCP leases for one minute (it’s now two minutes). This means that every instance contacts the cloud controller (DHCP server) to renew its fixed IP. For some reason, this instance could not renew its IP. We correlated the instance’s logs with the logs on the cloud controller and put together a conversation:
- Instance tries to renew IP.
- Cloud controller receives the renewal request and sends a response.
- Instance “ignores” the response and re-sends the renewal request.
- Cloud controller receives the second request and sends a new response.
- Instance begins sending a renewal request to
255.255.255.255since it hasn’t heard back from the cloud controller.
- The cloud controller receives the
255.255.255.255request and sends a third response.
- The instance finally gives up.
With this information in hand, we were sure that the problem had to do with DHCP. We thought that for some reason, the instance wasn’t getting a new IP address and with no IP, it shut itself off from the network.
A quick Google search turned up this: DHCP lease errors in VLAN mode which further supported our DHCP theory.
An initial idea was to just increase the lease time. If the instance only renewed once every week, the chances of this problem happening would be tremendously smaller than every minute. This didn’t solve the problem, though. It was just covering the problem up.
We decided to have
tcpdump run on this instance and see if we could catch it in action again. Sure enough, we did.
tcpdump looked very, very weird. In short, it looked as though network communication stopped before the instance tried to renew its IP. Since there is so much DHCP chatter from a one minute lease, it’s very hard to confirm it, but even with only milliseconds difference between packets, if one packet arrives first, it arrived first, and if that packet reported network issues, then it had to have happened before DHCP.
Additionally, this instance in question was responsible for a very, very large backup job each night. While “The Issue” (as we were now calling it) didn’t happen exactly when the backup happened, it was close enough (a few hours) that we couldn’t ignore it.
Further days go by and we catch The Issue in action more and more. We find that dhclient is not running after The Issue happens. Now we’re back to thinking it’s a DHCP issue. Running
/etc/init.d/networking restart brings everything back up and running.
Ever have one of those days where all of the sudden you get the Google results you were looking for? Well, that’s what happened here. I was looking for information on dhclient and why it dies when it can’t renew its lease and all of the sudden I found a bunch of OpenStack and dnsmasq discussions that were identical to the problem we were seeing!
This bug report was the key to everything: KVM images lose connectivity with bridged network.
It was funny to read the report. It was full of people who had some strange network problem but didn’t quite explain it in the same way.
So it was a qemu/kvm bug.
At the same time of finding the bug report, a co-worker was able to successfully reproduce The Issue! How? He used
iperf to spew a ton of bandwidth at an instance. Within 30 minutes, the instance just disappeared from the network.
Armed with a patched qemu and a way to reproduce, we set out to see if we’ve finally solved The Issue. After 48 hours straight of hammering the instance with bandwidth, we were confident. The rest is history. You can search the bug report for “joe” to find my comments and actual tests.
At the end of 2012, Cybera (a nonprofit with a mandate to oversee the development of cyberinfrastructure in Alberta, Canada) deployed an updated OpenStack cloud for their DAIR project. A few days into production, a compute node locks up. Upon rebooting the node, I checked to see what instances were hosted on that node so I could boot them on behalf of the customer. Luckily, only one instance.
The nova reboot command wasn’t working, so I used virsh, but it immediately came back with an error saying it was unable to find the backing disk. In this case, the backing disk is the Glance image that is copied to
/var/lib/nova/instances/_base when the image is used for the first time. Why couldn’t it find it? I checked the directory and sure enough it was gone.
I reviewed the
nova database and saw the instance’s entry in the
nova.instances table. The image that the instance was using matched what virsh was reporting, so no inconsistency there.
I checked Glance and noticed that this image was a snapshot that the user created. At least that was good news—this user would have been the only user affected.
Finally, I checked StackTach and reviewed the user’s events. They had created and deleted several snapshots—most likely experimenting. Although the timestamps didn’t match up, my conclusion was that they launched their instance and then deleted the snapshot and it was somehow removed from
/var/lib/nova/instances/_base. None of that made sense, but it was the best I could come up with.
It turns out the reason that this compute node locked up was a hardware issue. We removed it from the DAIR cloud and called Dell to have it serviced. Dell arrived and began working. Somehow or another (or a fat finger), a different compute node was bumped and rebooted. Great.
When this node fully booted, I ran through the same scenario of seeing what instances were running so I could turn them back on. There were a total of four. Three booted and one gave an error. It was the same error as before: unable to find the backing disk. Seriously, what?
Again, it turns out that the image was a snapshot. The three other instances that successfully started were standard cloud images. Was it a problem with snapshots? That didn’t make sense.
A note about DAIR’s architecture:
/var/lib/nova/instances is a shared NFS mount. This means that all compute nodes have access to it, which includes the
_base directory. Another centralized area is
/var/log/rsyslog on the cloud controller. This directory collects all OpenStack logs from all compute nodes. I wondered if there were any entries for the file that virsh is reporting:
Ah-hah! So OpenStack was deleting it. But why?
A feature was introduced in Essex to periodically check and see if there were any
_base files not in use. If there were, OpenStack Compute would delete them. This idea sounds innocent enough and has some good qualities to it. But how did this feature end up turned on? It was disabled by default in Essex. As it should be. It was decided to be turned on in Folsom. I cannot emphasize enough that:
Actions which delete things should not be enabled by default.
Disk space is cheap these days. Data recovery is not.
Secondly, DAIR’s shared
/var/lib/nova/instances directory contributed to the problem. Since all compute nodes have access to this directory, all compute nodes periodically review the _base directory. If there is only one instance using an image, and the node that the instance is on is down for a few minutes, it won’t be able to mark the image as still in use. Therefore, the image seems like it’s not in use and is deleted. When the compute node comes back online, the instance hosted on that node is unable to start.
The Valentine’s Day Compute Node Massacre
Although the title of this story is much more dramatic than the actual event, I don’t think, or hope, that I’ll have the opportunity to use “Valentine’s Day Massacre” again in a title.
This past Valentine’s Day, I received an alert that a compute node was no longer available in the cloud—meaning,
showed this particular node in a down state.
I logged into the cloud controller and was able to both
ping and SSH into the problematic compute node which seemed very odd. Usually if I receive this type of alert, the compute node has totally locked up and would be inaccessible.
After a few minutes of troubleshooting, I saw the following details:
- A user recently tried launching a CentOS instance on that node
- This user was the only user on the node (new node)
- The load shot up to 8 right before I received the alert
- The bonded 10gb network device (bond0) was in a DOWN state
- The 1gb NIC was still alive and active
I looked at the status of both NICs in the bonded pair and saw that neither was able to communicate with the switch port. Seeing as how each NIC in the bond is connected to a separate switch, I thought that the chance of a switch port dying on each switch at the same time was quite improbable. I concluded that the 10gb dual port NIC had died and needed replaced. I created a ticket for the hardware support department at the data center where the node was hosted. I felt lucky that this was a new node and no one else was hosted on it yet.
An hour later I received the same alert, but for another compute node. Crap. OK, now there’s definitely a problem going on. Just like the original node, I was able to log in by SSH. The bond0 NIC was DOWN but the 1gb NIC was active.
And the best part: the same user had just tried creating a CentOS instance. What?
I was totally confused at this point, so I texted our network admin to see if he was available to help. He logged in to both switches and immediately saw the problem: the switches detected spanning tree packets coming from the two compute nodes and immediately shut the ports down to prevent spanning tree loops:
He re-enabled the switch ports and the two compute nodes immediately came back to life.
Unfortunately, this story has an open ending… we’re still looking into why the CentOS image was sending out spanning tree packets. Further, we’re researching a proper way on how to mitigate this from happening. It’s a bigger issue than one might think. While it’s extremely important for switches to prevent spanning tree loops, it’s very problematic to have an entire compute node be cut from the network when this happens. If a compute node is hosting 100 instances and one of them sends a spanning tree packet, that instance has effectively DDOS’d the other 99 instances.
This is an ongoing and hot topic in networking circles —especially with the raise of virtualization and virtual switches.
Down the Rabbit Hole
Users being able to retrieve console logs from running instances is a boon for support—many times they can figure out what’s going on inside their instance and fix what’s going on without bothering you. Unfortunately, sometimes overzealous logging of failures can cause problems of its own.
A report came in: VMs were launching slowly, or not at all. Cue the standard checks—nothing on the Nagios, but there was a spike in network towards the current master of our RabbitMQ cluster. Investigation started, but soon the other parts of the queue cluster were leaking memory like a sieve. Then the alert came in—the master Rabbit server went down and connections failed over to the slave.
At that time, our control services were hosted by another team and we didn’t have much debugging information to determine what was going on with the master, and we could not reboot it. That team noted that it failed without alert, but managed to reboot it. After an hour, the cluster had returned to its normal state and we went home for the day.
Continuing the diagnosis the next morning was kick started by another identical failure. We quickly got the message queue running again, and tried to work out why Rabbit was suffering from so much network traffic. Enabling debug logging on nova-api quickly brought understanding. A
tail -f /var/log/nova/nova-api.log was scrolling by faster than we’d ever seen before. CTRL+C on that and we could plainly see the contents of a system log spewing failures over and over again - a system log from one of our users’ instances.
After finding the instance ID we headed over to
/var/lib/nova/instances to find the
Sure enough, the user had been periodically refreshing the console log page on the dashboard and the 5G file was traversing the Rabbit cluster to get to the dashboard.
We called them and asked them to stop for a while, and they were happy to abandon the horribly broken VM. After that, we started monitoring the size of console logs.
To this day, the issue doesn’t have a permanent resolution, but we look forward to the discussion at the next summit.
Havana Haunted by the Dead
Felix Lee of Academia Sinica Grid Computing Centre in Taiwan contributed this story.
I just upgraded OpenStack from Grizzly to Havana 2013.2-2 using the RDO repository and everything was running pretty well—except the EC2 API.
I noticed that the API would suffer from a heavy load and respond slowly to particular EC2 requests such as
/var/log/nova/nova-api.log on Havana:
This request took over two minutes to process, but executed quickly on another co-existing Grizzly deployment using the same hardware and system configuration.
/var/log/nova/nova-api.log on Grizzly:
While monitoring system resources, I noticed a significant increase in memory consumption while the EC2 API processed this request. I thought it wasn’t handling memory properly—possibly not releasing memory. If the API received several of these requests, memory consumption quickly grew until the system ran out of RAM and began using swap. Each node has 48 GB of RAM and the “nova-api” process would consume all of it within minutes. Once this happened, the entire system would become unusably slow until I restarted the nova-api service.
So, I found myself wondering what changed in the EC2 API on Havana that might cause this to happen. Was it a bug or a normal behavior that I now need to work around?
After digging into the nova (OpenStack Compute) code, I noticed two areas in
api/ec2/cloud.py potentially impacting my system:
Since my database contained many records—over 1 million metadata records and over 300,000 instance records in “deleted” or “errored” states—each search took a long time. I decided to clean up the database by first archiving a copy for backup and then performing some deletions using the MySQL client. For example, I ran the following SQL command to remove rows of instances deleted for over a year:
Performance increased greatly after deleting the old records and my new deployment continues to behave well.