Hi all, Marko Hoyer recently brought up the concept of a staged startup [1] on this list.
I have a specific use case for some form of staging, though I don't know if it meets Marko's definition or requirements! Perhaps systemd can handle this already, but let's see... So, I've been building a systemd package for OpenWrt [2] to test on my little VoCore coin-sized MIPS machine. (Stay with me, the weird part is over.) The root filesystem is a read-only squashfs blob stored on the VoCore's generous (!) 8MB of flash memory. During initial testing, I was happy to boot up into a read-only environment, bring up a few tmpfs mount points, and then keep mucking around with systemd. But it's time to get serious. And everyone knows that "serious" means having a writeable root filesystem. OpenWrt uses overlayfs with JFFS2 as the top layer. but I'm just using tmpfs for now. (For some values of "serious".) I wanted to make best use of systemd's built-in primitives, so here's what I've done: - default.target is symlinked to initrd.target in the read-only filesystem image - I've added some custom services to prepare all the mounts for the root switch (including one which changes the default.target symlink on the new, writeable root) Yes, I'm abusing systemd's idea of an initrd. Here's where it breaks down: - systemd dutifully starts all the services it knows about during the initrd.target run, because they're all right there on the read-only filesystem (and they fail a lot) - then systemd dutifully stops them all again to switch the root - and dutifully starts them all again once we're headed towards multi-user.target That's a *lot* of noise in the startup process! I did get the impression from the documentation that initrd.target was somehow special, but it makes complete sense that it's not. If I were using an initramfs, there wouldn't be any superfluous service files in the initramfs filesystem, and I'd be happy to know that systemd would behave *exactly* the same way it would elsewhere. One hacky idea I had to fix this: - Pull all of the systemd service symlinks out of the squashfs filesystem and store them in a tarball - Add specific symlinks to make the initrd stage works properly - In the pre-switch prepare service, unpack the tarball into the rw-mounted /sysroot Before anyone says it: No, using a real initramfs would be highly inappropriate. I do not want to store two copies of systemd and friends in 8MB of flash. It's hard enough with just the one! Is there an existing systemd solution to this problem? Is there a better way to go about it? Thanks, Jeff [1] http://lists.freedesktop.org/archives/systemd-devel/2015-January/027688.html [2] https://github.com/jdub/openwrt-systemd
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