Summary --------- The high-level boot process for ONL is fairly straight forward, but there is a lot detail. At high-level, there are three phases 1. uBoot phase 2. the ONL Loader phase 3. The final ONL operating system Detailed Boot Process -------------------------- 1. uBoot is the first level boot loader: http://www.denx.de/wiki/U-Boot 2. uBoot reads the 'nos_bootcmd' environmental variable from flash and runs the contents ('nos' is Network Operating System) 4. If $nos_boot_cmd returns, uBoot loads and runs ONIE (see below) to download the ONL installer and install the ONL loader a) The factory default $nos_boot_cmd is to a trival command that returned immediately, e.g., 'echo' 5. In normal operation, i.e., after ONIE has been run, $nos_boot_cmd is set to load and run the ONL Loader 6. The ONL loader boots the Linux kernel (later, the "boot kernel") 7. The ONL loader decides which SWI to run based on the URL in the file /etc/SWI URL=`cat /etc/SWI` 8. The ONL loader runs `/bin/boot $URL` 9. The ONL loader retrieves the SWI file a) if the URL is remote (e.g., http://, ftp://, etc.), verify that there is a locally cached copy of the SWI in /mnt/flash2 or if not, download it b) if the URL is local, verify that the device is accessible c) if the URL is a Zero Touch Networking (ZTN) URL, the execute the ZTN protocol to get the SWI (see below) 10. The ONL loader reads the 'rootfs' file out of the SWI and mounts it using overlayfs[1] (SWI contents described below) Partition Layout ------------------ Switches typically have two flash storage device: a smaller flash (e.g., 64MB flash) for booting and a larger, mass storage device (e.g., compact flash, 2+GB). Smaller Boot Flash: Partition 1: uBoot Partition 2: environmental variables (e.g., $nos_boot_cmd) Partition 3: ONIE Partition 4+: Free space (unused) Mass Storage Device: Partition 1: ONL loader kernel -- the format of this partition varies depending on what formats uBoot supports on the specific platform Partition 2: ONL Loader configuration files (mounts as "/mnt/flash" both during the loader and the main ONL phases) Partition 3: ONL SWitch Images (SWIs) partition (mounts as "/mnt/flash2" both during the loader and the main ONL phases) ONL file system layout ----------------------- root@onl-powerpc:/bin# df Filesystem 1K-blocks Used Available Use% Mounted on rootfs 72040 176 71864 1% / devtmpfs 1024 0 1024 0% /dev none 72040 176 71864 1% / tmpfs 48028 148 47880 1% /run tmpfs 5120 0 5120 0% /run/lock /dev/sda2 71177 7 71170 1% /mnt/flash /dev/sda3 3791960 98172 3693788 3% /mnt/flash2 tmpfs 96040 0 96040 0% /run/shm SWI -------- Zip file contains: $ unzip -l ONL-2.0.0_ONL-OS_2015-12-12.0252-ffce159_PPC.swi Archive: ONL-2.0.0_ONL-OS_2015-12-12.0252-ffce159_PPC.swi Length Date Time Name --------- ---------- ----- ---- 97968128 2015-12-15 20:20 rootfs-powerpc.sqsh 1063 2015-12-15 20:20 manifest.json --------- ------- 97969191 2 files 1. 'rootfs-$ARCH' : the root file system for the running ONL 2. 'manifest.json' : a list of supported platforms, version information about ONL and the architecture Footnotes ----------- [1] : https://kernel.googlesource.com/pub/scm/linux/kernel/git/mszeredi/vfs/+/overlayfs.current/Documentation/filesystems/overlayfs.txt