Reflecting on Essex
This post is kind of long, and a little self indulgent. However, I really wanted to spend some time thinking about what I did for the Essex release cycle, and what I want to do for the Folsom release. I spent Essex mostly hacking on things in isolation, except for when Padraig Brady and I were hacking in a similar space. I'd like to collaborate more for Folsom, and I'm hoping talking about what I'm interested in doing in public might help with that.
I came relatively late to the Essex development cycle, having never even heard of OpenStack before joining Canonical. We can talk about how I'd worked in the cloud space for six years and yet wasn't aware of the open source implementations at some other time.
My initial introduction to OpenStack was being paged for compute nodes which were continually running out of disk. I googled around a bit and discovered that cached images for instances were never cleaned up (to start an instance, an image is fetched from glance, possibly has its format converted, is resized, and then an instance started with that resulting image, all those images were never being cleaned up). I filed bug 904532 as my absolute first interaction with the OpenStack community. Scott Moser kindly pointed me at the blueprint for how to actually fix the problem.
(Remind me if Phil Day comes to the OpenStack developer summit that I should sit down with him at some point and see how what close what was actually implemented got to what he wrote in that blueprint. I suspect we've still got a fair way to go, but I'll talk more about that later in this post).
This was a pivotal moment. I'd just spent the last six years writing python code to manage largish cloud clusters, and here was a bug which was hurting me in a python package intended to manage clusters very similar to those I had been running. I should just fix the bug, right?
It turns out that the OpenStack core developers are super easy to work with. I'd say that the code review process certainly feels like it was modelled on Google's but in general the code reviewers are nicer with their comments that what I'm used to. This makes it much easier to motivate yourself to go and spend some more time hacking that a deeply negative review would. I think Vish is especially worthy of a shout out as being an amazing person to work with. He's helpful, patient, and very smart.
In the end I wrote the image cache manager which ships in Essex. Its not perfect, but its a lot better than what came before, and its a good basis to build on. There is some remaining tech debt for image cache management which I intend to work on for Folsom. First off, the image cache only works for libvirt instances at the moment. I'd like to pull all the other hypervisors into line as best as possible. There are hooks in the virtualization driver for this, but no one has started this work as best as I am aware. To be completely honest I'd like to see the image cache manager become common code and have all the hypervisors deal with this in exactly the same manner -- that makes it easier to document, and means that on-call operations people don't need to determine what hypervisor a compute node is running before starting to debug. This is something I very much want to sit down with other nova developers and talk about at the summit.
The next step for image cache management is tracked in a very bare bones blueprint. The original blueprint envisaged that it would be desirable to pre-cache some images on all nodes. For example, a cloud host might want to offer slightly faster startup times for some images by ensuring they are pre-cached. I've been thinking about this a lot, and I can see other use cases here as well. For example, if you have mission critical instances and you wanted to tolerate a glance failure, then perhaps you want to pre-cache a class of images that serve those mission critical instances. The intention is to provide an interface and default implementation for the pre-caching logic, and then let users go wild working out their own requirements.
The hardest bit of the pre-caching will be reducing the interactions with glance I suspect. The current feeling is that calling glance from a periodic task is a bit scary, and has been actively avoided for Essex. This is especially true if Keystone is enabled, as the periodic task wont have an admin context unless we pull that from the config file. However, if you're trying to determine what images are mission critical, then you really need to talk to glance. I guess another option would be to have a table of such things in nova's database, but that feels wrong to me. We're going to have to talk about this bit more.
(It would be interesting as well to talk about the relative priority of instances as well. If a cluster is experiencing outages, then perhaps some customers would pay more to have their instances be the last killed off or something. Or perhaps I have instances which are less critical than others, so I want the cluster to degrade in an understood manner.)
That leads logically onto a scheduler change I would like to see. If I have a set of compute nodes I know already have the image for a given instance, shouldn't I prefer to start instances on those nodes instead of fetching the image to yet more compute nodes? In fact, if I already have a correctly resized COW base image for an instance on a given node, then it would make sense to run a new instance on that node as well. We need to be careful here, because you wouldn't want to run all of a given class of instance on a small set of compute nodes, but if the image was something like a default Ubuntu image, then it would make sense. I'd be interested in hearing what other people think of doing something like this.
Another thing I've tried to focus on for Essex is making OpenStack easier for operators to run. That started off relatively simply, by adding an option for log messages to specify what instance a message relates to. This means that when a user queries the state of their instance, the admin can now just grep for the instance UUID, and run from there. Its not perfect yet, in that not all messages use this functionality, but that's some tech debt that I will take on in Folsom. If you're a nova developer, then please pass instance= in your log messages where relevant!
This logging functionality isn't perfect, because if you only have the instance UUID in the method you're writing, it wont work. It expects full instance dicts because of the way the formatting code works. This is kind of ironic in that the default logging format only includes the UUID. In Folsom I'll also extend this code so that the right thing happens with UUIDs as well.
Another simple logging tweak I wrote is that tracebacks now have the time and instance included in them. This makes it much easier for admins to determine the context of a traceback in their logs. It should be noted that both of these changes was relatively trivial, but trivial things can often make it much easier for others.
There are two sessions at the Folsom dev summit talking about how to make OpenStack easier for operators to run. One was from me, and the other is from Duncan McGreggor. Neither has been accepted yet, but if I notice that Duncan's was accepted I'll drop mine. I'm very very interested in what operations staff feel is currently painful, because having something which is easy to scale and manage is vital to adoption. This is also the core of what I did at Google, and I feel I can make a real contribution here.
I know I've come relatively late to the OpenStack party, but there's heaps more to do here and I'm super enthused to be working on code that I can finally show people again.
Tags for this post: openstack canonical essex folsom image_cache_management sre
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