Thursday, August 03, 2017

Creating a decent Tribblix AMI

Previously, I've described how I created my first Tribblix AMI, then how to do it properly in hvm mode so you can run on modern instances in all regions.

That creates something that will work, but is it actually in a state that's useful?

The first thing is to add an EC2 credential service. That's the thing that will query for metadata and install the keys on the system so you can log after the instance is created. I tried the ec2-credential service from OmniOS, but for some reason it didn't work right on Tribblix. I've tweaked mine a little, forcing it to run after the network comes up, adding retries in case there's a problem, and also disabling it in non-global zones.

Of course, there's more instance metadata that I could query and use, but I haven't yet had a need for anything other than the initial key.

The other thing I've been wondering about is the configuration of Tribblix itself - specifically what the storage should look like and what the default software installation should look like.

My image is built on an 8G "disk" or EBS volume. That might seem a little small, but remember that Tribblix is pretty lean and mean. For a typical server configuration you'll probably be looking at about 1G or so, and that's without any special work. The most annoying thing here is that by default you lose 2G to each of dump and swap, so that's effectively half the disk gone. There's opportunity to modify those, especially as I'm typically using t2.micro instances on the free tier that only have 1G of memory. You might not even want dump at all. As for swap, you do want some (so that anonymous reservations don't eat into actual RAM) but you could cut that down a bit.

As I'm writing this I do wonder whether I could pull some of the instance metadata and shrink the dump and swap volumes appropriately.

The assumption I'm making here, though, is that if you're storing any reasonable amount of data that you're going to attach a separate EBS volume, and you can then size that appropriately to the need at hand. (And you can then move that data around independent of your running instance.) So I think that keeping the root volume fairly small is reasonable. It also keeps my AWS bill down, an important consideration as any charges here come out of my own pocket.

Then, what should the baseline software install look like? Tribblix uses overlays, and there's an assumption that you always start from the base overlay. I'm currently using a dedicated overlay that pulls in cli-tools - essentially you get basic shells, compression tools, basic utilities, but not much else. Many of the normal server utilities don't apply to running in the cloud, as they're aimed at monitoring or managing hardware.

The base set of packages is that installed on the ISO. That includes most storage and network drivers, which are irrelevant - on EC2 you know exactly which drivers you need, so almost all the drivers that are installed are unnecessary. What I need here is a better way of handling installation variants, so it knows the drivers aren't supposed to be there - at the moment I could remove them, but updates and upgrades would simply put them back. In the same vein, I could only ship a 64-bit kernel, as we know there are no 32-bit instance types available.

At the moment I have an LX variant, which is a bit of a hack in terms of the way I've packaged it together, but as the number of interesting variants grows I'm going to have to come up with a better way of handling it, especially as you might want multiple variants together - for instance a 64-bit LX-enabled cloud-optimised image.

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