Who brings up lvm volumes and how when booting?

Question

I know the logical volumes info is saved in the lvm metadata, and the tools from lvm2 can be used to scan them out and setup the kernel DM by ioctl. I am curious that who does this work during booting so we can mount lv as rootfs, systemd?

Answer 1

I just found the answer by myself. http://www.system-rescue-cd.org/lvm-guide-en/Booting-linux-from-LVM-volumes/

Quote the content from above link.

Here are the important steps involved in booting from LVM:

1.The BIOS executes the boot loader which is very often Grub

2.The boot loader has its own code for reading partitions and filesystems. So it knows how to read files from the /boot partition which contains the linux kernel image (vmlinuz-x.y.z) and the initramfs (initrd-x.y.z.gz). It first loads these two files into memory. Then it executes the kernel image and it tells the kernel where the initramfs is located in memory. The boot command line is also passed to the kernel. This command line contains the important parameters for the kernel such as root=/dev/volgroup/lvroot.

3.The linux kernel starts and executes its initialization code. Then it reads the initramfs from the memory. The contents is uncompressed into a new location in the memory.

4.The contents of the initramfs is now available. The program/script called init is now executed. This script which is specific to each linux distribution is responsible for finding the root filesystem.

5.If the LVM Physical Volumes are stored on the top of a RAID disk, the init program will first execute dmraid/mdadm to make this raid disk available.

6.Then the init script will run programs such as pvscan/vgscan/lvscan to detect the LVM volumes located on the disks The LVM volumes are not usable yet. They have to be activated first. This is done by vgchange --available y or vgchange -ay.

7.The init script reads the virtual file called /proc/cmdline to see what is the name of the root filesystem specified on the boot command line.

8.The root filesystem is mounted in a temporary directory such as /rootfs and other things such as /rootfs/proc and /rootfs/dev may also be mounted.

9.The initscript executes a chroot to /rootfs. This means that this directory becomes the new root for the processes which will be executed. When a process reads /bin/something it will read /rootfs/bin/something in reality.

10.The secondary init program, the one which is stored on the root filesystem is now executed and it finishes the initialization with an access to the real root filesystem.