Whilst tweaking mvi recently, I went back and had another look at just how minimal an illumos install I could make.
And, given sufficiently aggressive use of the rm command, the answer appears to be that it's possible to boot illumos in 48 meg of RAM.
No, it's not April 1st. 48 meg of RAM is pretty low. It's been a long time since I've seen a system that small.
I've added some option scripts to the mvi repo. (The ones with -fix.sh as part of their names.) You don't have to run mvi to see what these do.
First, I start with mvix.sh, which is fairly minimal up front.
Then I go 32bit, which halves the size.
Then I apply the extreme option, which removes zfs (it's the single biggest driver left), along with a bunch of crypto and other unnecessary files. And I clean up lots of bits of grub that aren't needed.
I then abstracted out a nonet and nodisk option. The nonet script removes anything that looks like networking from the kernel, and the bits of userland that I add in order to be able to configure an interface. The nodisk script removes all the remaining storage device drivers (I only included the ones that you normally see when running under a hypervisor in the first place), along with the underlying scsi and related driver frameworks.
What you end up with is a 17M root file system, which compresses down to a 6.8M root archive, which gets packaged up in an 8.7M iso.
For those interested the iso is here. It should run in VirtualBox - in 32-bit mode, and you should be able to push the memory allocated by VirtualBox down to 48M. Scary.
Of course, it doesn't do much. It boots to a prompt, the only tools you have are ksh, ls, and du.
(Oh, and if you run du against the devices tree, it will panic.)
While doing this, I found that there are a lot of dependencies between modules in the illumos kernel. Not all of them are obvious, and trying to work out what's needed and what can be removed has involved large amounts of trial and error. That said, it takes less than 5 seconds to create an iso, and it takes longer for VirtualBox to start than for this iso to boot, so the cycle is pretty quick.