In my previous post about virtualenv, I took a look at a way of making python environments a little bit more generic so that they could be moved around and redeployed at ease. I mentioned docker.io as a new tool that uses a general concept of “containers” to do similar things, but more broadly. I’ve dug a bit into docker, and these are my initial thoughts. Unfortunately, it seems relatively Fedora un-friendly right now.
The first thing to look at is to examine what, exactly, a “container” is. In essence, it’s just a file system: there’s pretty much nothing special about it. I was slightly surprised by this; given the claims on the website I assumed there was something slightly more clever going on, but the only “special sauce” is the use of aufs to layer one file system upon another. So from the point of view of storage alone, there really isn’t much difference between a container and a basic virtual machine.
From the point of view of the runtime, there isn’t an awful lot of difference between a virtual machine and a container either. docker sells itself as a lightweight alternative to virtual machines, but of course there is no standard definition of a “virtual machine”. At one end of the spectrum are the minimal hardware OSen that can be used to assign different host resources, including CPU cores, to virtual machines, and those types of VM are effectively not much different to real hardware – the configuration is set on the fly, but basically it’s real metal. On the other end of the spectrum you have solutions like Xen, which make little to no use of the hardware to provide virtualisation, and instead rely on the underlying OS to provide the resources that they dish out. docker is just slightly further along the spectrum than Xen: instead of using a special guest kernel, you use the host kernel. Instead of paravirtualisation ops, you use a combination of cgroups and lxc containers. Without the direct virtualisation of hardware devices, you don’t need the various special drivers to get performance, but there are also fewer security guarantees.
There are a couple of benefits of docker touted, and I’m not totally sold on all of them. One specific claim is that containers are “hardware independent”, which is only true in a quite weak way. There is no specific hardware independence in containers that I can see; except that docker.io only runs on x86_64 hardware. If your container relies on having access to NX bit, then it seems to me you’re relying on the underlying hardware having such a feature – docker doesn’t solve that problem.
The default container file system is set up to be copy-on-write, which makes it relatively cheap diskspace-wise. Once you have a base operating system file system, the different containers running on top of it are probably going to be pretty thin layers. This is where the general Fedora un-friendliness starts, though: in order to achieve this “layering” of file systems, docker uses aufs (“Another Union File System”), and right now this is not a part of the standard kernel. It looks unlikely to get into the kernel either, as it hooks into the VFS layer in some unseemly ways, but it’s possible some other file system with similar functionality could be used in the future. Requiring a patched kernel is a pretty big turn-off for me, though.
I’m also really unsure about the whole idea of stacking file systems. Effectively, this is creating a new class of dependency between containers, ones which the tools seem relatively powerless to sort out. Using a base Ubuntu image and then stacking a few different classes of daemon over it seems reasonable; having more than three layers begins to seem unreasonable. I had assumed that docker would “flatten out” images using some hardlinking magic or something, but that doesn’t appear to be the case. So if you update that underlying container, you potentially break the containers that use it as a base – it does seem to be possible to refer to images by a specific ID, but the dockerfile FROM directive doesn’t appear to be able to take those.
The net result of using dockerfiles appears to be to take various pieces of system configuration out of the realm of SCM and into the build system. As a result, it’s a slightly odd half-way house between a Kickstart file and (say) a puppet manifest: it’s effectively being used to build an OS image like a Kickstart, but it’s got these hierarchical properties that stratify functionality into separate filesystem layers that look an awful lot like packages. Fundamentally, if all your container does it take a base and install a package, the filesystem is literally going to be that package, unpacked, and in a different file format.
The thing that particularly worries me about this stacking is memory usage – particularly since docker is supposed to be a lightweight alternative. I will preface this with the very plain words that I haven’t spent the time to measure this and am talking entirely theoretically. It would be nice to see some specific numbers, and if I get the time in the next week I will have a go at creating them.
Most operating systems spend a fair amount of time trying to be quite aggressive about memory usage, and one of the nice things about dynamic shared libraries is that they get loaded into process executable memory as a read-only mapping: that is, each shared library will only be loaded once and the contents shared across processes that use it.
There is a fundamental difference between using a slice of an existing file system – e.g., setting up a read-only bind mount – and using a new file system, like an aufs. My understanding of the latter approach is that it’s effectively generating new inodes, which would mean that libraries that are loaded through such a file system would not benefit from that memory mapping process.
My expectation, then, is that running a variety of different containers is going to be more memory intensive than a standard system. If the base containers are relatively light, then the amount of copying will be somewhat limited – the usual libraries like libc and friends – but noticeable. If the base container is quite fat, but has many minor variations, then I expect the memory usage to be much heavier than the equivalent.
This is a similar problem to the “real” virtual machine world, and there are solutions. For virtual machines, the same-page mapping subsystem (KSM) does an admirable job in figuring out which sections of a VM’s memory are shared between instances, and evicting copies from RAM. At a cost of doing more compute work, it does a better job that the dynamic loader: shared copies of data can be shared too, not just binaries. This can make virtual machines very cheap to run (although, if suddenly the memory stops being shareable, memory requirements can blow up very quickly indeed!). I’m not sure this same machinery is applicable to docker containers, though, since KSM relies on advisory flagging of pages by applications – and there is no application in the docker system which owns all those pages in the same way (for example) qemu would do.
So, enough with the critical analysis. For all that, I’m still quite interested in the container approach that docker is taking. I think some of the choices – especially the idea about layering – are poor, and it would be really nice to see them implement systemd’s idea of containers (or at least, some of those ideas – a lot of them should be quite uncontroversial). For now, though, I think I will keep watching rather than doing much active: systemd’s approach is a better fit for me, I like the additional features like container socket activation, and I like that I don’t need a patched kernel to run it. It would be amazing to merge the two systems, or at least make them subset-compatible, and I might look into tools for doing that. Layering file systems, for example, is only really of interest if you care a lot about disk space, and disk space is pretty cheap. Converting layered containers into systemd’able containers should be straightforward, and potentially interesting.
