Hi Rich, On Fri, Aug 05, 2011 at 09:04:50PM -0400, Rich Freeman wrote: > On Fri, Aug 5, 2011 at 8:42 PM, William Hubbs <willi...@gentoo.org> wrote: > > On Fri, Aug 05, 2011 at 10:06:48PM +0200, Sven Vermeulen wrote: > >> How does the tool that creates an initramfs know which files to copy from > >> /usr and /var anyhow? > > > > My understanding is that nothing gets copied from /usr and /var, and it > > doesn't have to. > > > > Here is my basic understanding of how the boot sequence works: > > > > 1) rootfs is mounted on /. Rootfs contains the contents of the > > initramfs. > > Ok, so the initfs is typically in /boot, though it need not be. It > needs to be someplace the bootloader can find it, and with grub legacy > that typically means on a bare hard drive partition, or one using md > raid-1 with older metadata. The initramfs doesn't need to find itself > - the bootloader loads it into ram and passes its address to the > kernel when executing it.
Not quite. It is actually inside the kernel binary. You are thinking of an initrd. Look at these files: /usr/src/linux/Documentation/filesystems/ramfs-rootfs-initramfs.txt. /usr/src/linux/Documentation/early-userspace/README > > > 2) the init program inside the initramfs is run. > > 3) This init program will mount the real /usr and /var. > > So, this is the chicken and egg bit that led to the question about how > the tool that creates initramfs knows what to copy from /usr, etc. If > /usr can be mounted without any files from /usr, then clearly booting > without /usr mounted shouldn't be a problem (just mount it early). If > it does need files from /usr, then the initramfs has to contain those > files since /usr isn't mounted yet. All we are trying to do is get /usr and /var mounted before running udev. All you really need in the initramfs is whatever is necessary to get the file systems mounted. > Keep in mind the initramfs does not need to fully initialize the > system, and it doesn't need to do it in a way compatible with anything > that will subsequently load. It doesn't need to populate /dev the way > the rest of the stuff in the distro expects it to be, since the /dev > it is populating will get unmounted/wiped/etc before switching roots. Actually the /dev in the initramfs will not be wiped; it will become /dev in the running system. The other thing we will need is to have CONFIG_DEVTMPFS set to y in the kernel config. This will create a tmpfs containing all devices that the kernel knows about, which should be mounted on /dev. This CONFIG_DEVTMPFS=y setting is also preferred by upstream udev. > Initramfs just has to quickly operate in a scratch filesystem, mount > the real filesystems, then wipe itself out and chroot. It doesn't > need to be compatible with the latest version of > dbus/hal/X11/openrc/etc - it just needs to be internally > self-consistent. My understanding is it doesn't do anything like a chroot. Instead, it mounts the real root directly on top of itself. > For whatever reason it still isn't working for me (using dracut) - for > some bizarre reason it can't auto-detect my raid, even though running > mdadm --assemble --scan and exiting the dash shell allows it to boot. > Oh well - I'm sure I'll work it out. Then of course somebody needs to > patch it to mount /usr as well, or grab a copy of whatever Fedora is > doing if it is open source. I think it is more simple than this; I will be looking into the steps to make a trivial initramfs soon. William
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