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Comment by andrewstuart | Hacker News Reader

For those who haven't dug that deep.

initramfs is essentially a tiny Linux operating root image that Linux boots to RAM, before it goes on to boot from hard disk.

You can actually configure Linux to never go onto the next step of booting from hard disk, and run the operating system purely from RAM. Indeed there are distros such as Tinycore Linux that are designed exactly this way which can make things very fast compact and secure and diskless.

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If there isn't enough memory to fully unpack the initramfs, Linux 4.x kernels will log a message and silently proceed with a partial filesystem, instead of panic/halt. The resulting failures can take time to root-cause. Haven't tested on 5.x kernels, but similar code is still present:

https://github.com/torvalds/linux/blob/master/init/initramfs...

It's essential on some embedded boards as a first step because a well crafted initramfs can fit in a few MB of flash, and it's sometimes the case that the onboard bootloader can read the flash but has no other capabilites (well, aside from usually tftp).

So you read the kernel and initramfs from there, then load the real root FS from whatever storage you need to.

What happens when out of memory on Tinycore.^1,2

1. On NetBSD, I could make RAM-only, no swap distributions that might thrash but would gracefully recover once some RAM was freed.

2. On Ubuntu, when running from a tmpfs root, I have found that running out of memory causes unrepairable filesystem errors.

You can also go in reverse, and escape the running distribution's mounted / by pivoting to a tmpfs /, kill all processes and unmount the old root. Figuring out how to do this was quite fun. Systemd makes it actually fairly easy, because you can easily hijack the PID 1 if it's systemd, which is necessary to be able to umount the old /.

systemd makes it the most easy. all you need is systemctl switch-root ROOT. on other inits, you usually need to replace /sbin/init, then use some non-portable (sysvinit is not the only competitor) method of telling init to re-exec itself. sometimes that method doesn't even stop the running services, so you need to either manually stop them, or kill them.

In what situations would you expect a re-exec to stop the services? The normal case is that the reexec preserves state.

IIRC, some init implementations have this capability. I'm not saying that it's good in general, but just that it facilitates this particular case.

You can do it non-destructively by bind mounting a binary from ramfs over /bin/init

When it reexecs, the new binary can kill and umount everything and switch roots. If it fails somehow, you just reboot and everything is back to normal.

It is also useful for some other interesting use cases. For example I have done what I call "initram shims" to give me a way to enter FDE passwords on remote systems during boot.

IIRC Chrome OS was designed to do exactly this (in its early days, anyway).

I’m thoroughly convinced google will never be able to produce light weight consumer software. That was their second try at a “light weight operating system” and they so wildly missed the mark both times.

Well remember what happens. Engineers get it working. It’s a success. Now let’s make some money. Happens with every job. I always seem to mosey on step 3 to another job :)

I think they just failed to maintain what they accomplished (I remember when they really did have sub-one-second boot). Google's culture has been dying slowly and painfully over the decade; the less people care, the sadder the products will get.

> I remember when they really did have sub-one-second boot

Wow, as someone who never owned a Chromebook, >1s boot times seem enviable, even if booting into Chrome is rather restrictive. What are boot times like nowadays?

They still advertise sub-8 second boot, but it seems like that's possible with any decently light weight Linux system nowadays.

Yea but most people are comparing Chromebook boot their Windows laptop or equivalent.

No idea what Windows boot time is these days but I'd be shocked of it's under 20 seconds.

Everyone "cheated" that by making going to sleep or hibernating the new norm.

My chromebook wakes back to playing the youtube video I was watching quicker than opening the lid.

My 9-year old desktop on Windows 10 boots in <15 seconds.

SSDs do make a huge difference.

But I'm unsure which class of hardware were you talking about.

On my HP Spectre dual-boot Windows 10 / Fedora Workstation 31, it's roughly 6s and 12s respectively.

For my windows systems, a majority of boot time seems to be in pre-boot environment (bios/uefi/whatever), once it hits the disk it's pretty fast. They're all ssds though, because Windows 10 on spinning disks seems intollerable.

I'm pretty sure Microsoft cheats the boot process, and it's a special hibernation restore, but either way, it's pretty fast. Chrome devices benefit from also having a much faster firmware than most, so even if Windows booted as fast as Chrome OS, the whole device would still be much slower to boot.

And then you have your corporate-mandated image where Windows is bogged down by a metric crapton of garbage enterprise software. My work notebook takes about a minute to come to a state where I can actually enter my username and password on the login screen. Once on the desktop, it takes about 8-10 minutes to get settled (i.e. CPU load goes to baseline).

The way to win in the corporate world is to push for whatever OS they least manage. At Yahoo, getting a mac meant less corpware on your desktop; at Facebook, you would be better off with Windows (or maybe Linux), cause IT is proficient at running random garbage on macos.

A lot of its key people ended up at a startup which was acquired by another of the FAANGs.

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