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I'm wanting to understand the Linux init process better in order to netboot a system over ceph rather than nfs.
In the process I've come across two forms of switching root. One called switch_root, and the other called pivot_root. These scripts being run from an in memory filesystem (initramfs) obtained via tftp using the pxe boot process.
When would you use one over the other? I've seen both used in some init script's placed in root.
I found a wonderful explanation here. However, let me try to put in a shorter format of what I understood in the answer.
Shorter Version
pivot_root is used and for initramfs switch_root is used.Longer Version
Now, to the detailed explanation of what I had put above.
While both an initramfs and an initrd serve the same purpose, there are 2 differences. The most obvious difference is that an initrd is loaded into a ramdisk. It consists of an actual filesystem (typically ext2) which is mounted in a ramdisk. An initramfs, on the other hand, is not a filesystem. It is simply a (compressed) cpio archive (of type newc) which is unpacked into a tmpfs. This has a side-effect of making the initramfs a bit more optimized and capable of loading a little earlier in the kernel boot process than an initrd. Also, the size of the initramfs in memory is smaller, since the kernel can adapt the size of the tmpfs to what is actually loaded, rather than relying on predefined ramdisk sizes, and it can also clean up the ram that was used whereas ramdisks tend to remain in use (due to details of the pivot_root implementation).
There is also another side-effect difference: how the root device (and switching to it) is handled. Since an initrd is an actual filesystem unpacked into ram, the root device must actually be the ramdisk. For an initramfs, there is a kernel "rootfs" which becomes the tmpfs that the initramfs is unpacked into (if the kernel loads an initramfs; if not, then the rootfs is simply the filesystem specified via the root= kernel boot parameter), but this interim rootfs should not be specified as the root= boot parameter (and there wouldn't be a way to do so, since there's no device attached to it). This means that you can still pass your real root device to the kernel when using an initramfs. With an initrd, you have to process what the real root device is yourself. Also, since the "real" root device with an initrd is the ramdisk, the kernel has to really swith root devices from one real device (the ramdisk) to the other (your real root). In the case of an initramfs, the initramfs space (the tmpfs) is not a real device, so the kernel doesn't switch real devices. Thus, while the command pivot_root is used with an initrd, a different command has to be used for an initramfs. Busybox provides switch_root to accomplish this, while klibc offers new_root.
pivot_root in the past for initramfs, switch_root didn't exist at that time. switch_root seems to be a convenience method of pivot_root which does some more cleanup and also moves /proc /sys and /dev etc and not just the root itself
Oct 5, 2015 at 14:45
/. So from my understanding it could be any path and has nothing to do with actual "disks".
May 11, 2017 at 14:43
Use pivot_root when you want to preserve the original root for some purpose. pivot_root swaps two mount points.
Use switch_root when you want to discard the original root. switch_root is an embellished chroot that will move common mount points (/dev, /proc, /sys, etc) into the new root and also attempt to delete everything in the old root.
Conceptually, using pivot_root and then doing the mount moves and deletion yourself is functionally equivalent to using switch_root.
Also important: pivot_root may not be used with an initramfs.
