bootup - 系統啟動過程
描述
系統引導涉及許多不同的組件。開機后，系統BIOS將完成最小的硬件初始化，並將控制交給存儲在持久存儲設備上的引導加載程序。然后，這個引導加載程序將從磁盤(或網絡)調用OS內核。在Linux環境下，這個內核(可選)提取並執行一個初始RAM磁盤映像(Initrd)，例如由Dracut(8)生成的，它查找根文件系統(可能為此使用systemd)。在找到並掛載根文件系統之后，initrd將控制交給存儲在OS映像上的主機的系統管理器(例如systemd)，然后該管理器負責檢測所有剩余的硬件，掛載所有必要的文件系統並生成所有配置的服務。
在關閉時，系統管理器停止所有服務，卸載所有文件系統(卸載支持它們的存儲技術)，然后(可選)跳回到initrd卸載根文件系統及其駐留的存儲。最后，關閉系統。
系統管理器啟動
在啟動時，OS映像上的系統管理器負責初始化系統運行所需的文件系統、服務和驅動程序。對於systemd系統來說，這個過程被划分為幾個階段性步驟。 在每個階段性步驟內部，任務是高度並行的，所以無法准確預測其中的單元的順序，但是不同階段之間的先后順序總是固定的。
當systemd啟動系統時，它將激活default.Target的所有依賴項(以及遞歸地激活這些依賴項的所有依賴項)。通常，default.Target只是Graphical.Target或Multi-user.Target的別名，這取決於系統是配置為圖形用戶界面，還是配置為文本控制台。為了強制啟動流程的規范性以及提高單元的並行性，預先定義了一些具有特定含義的 target 單元。
下面的圖表解釋了這些具有特定含義的 target 單元之間的依賴關系以及各自在啟動流程中的位置。 圖中的箭頭表示了單元之間的依賴關系與先后順序，整個圖表按照自上而下的時間順序執行。
重點標識的目標單元經常被用作啟動目標。有兩種方法可以指定啟動目標： (1)使用 systemd.unit= 內核命令行參數(參見systemd手冊)；(2)使用 default.target 軟連接。
因為 timers.target 以異步方式包含在 basic.target 中，所以 timer 單元可以依賴於在 basic.target 之后才啟動的服務。
initrd 啟動過程
初始的RAM磁盤實現(Initrd)也可以使用systemd來設置。在本例中，initrd內部的引導遵循以下結構。
在initrd內部，也可以將 systemd 用作init程序(由"rdinit="內核引導參數指定)，此時 initrd.target 將是默認目標。initrd內部啟動流程的上半部分與前一小節 basic.target 之前的部分完全相同，隨后的啟動流程將如下圖所示。 如果成功的將根文件系統掛載到 /sysroot 目錄，那么 sysroot.mount 單元將被激活，然后進一步激活 initrd-root-fs.target 目標。 initrd-parse-etc.service 將會分析 /sysroot/etc/fstab 文件以掛載 /usr (若需要)與帶有 x-initrd.mount 標記的掛載點。 這些掛載點都將被掛載到 /sysroot 之下，然后流程到達 initrd-fs.target 目標。 再接下來 initrd-cleanup.service 將會使用 /usr/bin/systemctl --no-block isolate initrd-switch-root.target 命令啟動 initrd-switch-root.target 目標。因為 isolate 表示立即停止所有在新的目標單元中不需要的進程， 所以此動作實際上是為接下來切換根目錄做預先的准備(也就是清理環境)。 最后，激活 initrd-switch-root.service 服務，將系統的根目錄切換至 /sysroot 目錄。 (之前的流程與上一小節完全相同) :
切換到主機上的操作系統
關機過程
systemd 系統在關機時同樣遵循固定的流程，具體如下圖所示：
要查看具體的啟動順序可以通過如下命令輸入到文件，然后通過瀏覽器打開查看。
systemd-analyze plot > boot.html
列出所有正在運行的單元，按從初始化開始到啟動所花的時間排序。
systemd-analyze blame
下面通過查看日志分析啟動過程：
##調用kernel：
Oct 14 16:59:56 localhost kernel: Linux version 4.18.11-1.el7.elrepo.x86_64 (mockbuild@Build64R7) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-28) (GCC)) #1 SMP Sat Sep 29 09:42:38 EDT 2018
Oct 14 16:59:56 localhost kernel: Command line: BOOT_IMAGE=/vmlinuz-4.18.11-1.el7.elrepo.x86_64 root=/dev/mapper/rootvg-root ro rd.lvm.lv=rootvg/root rd.lvm.lv=rootvg/swap rhgb quiet
… …
Oct 14 16:59:56 localhost kernel: SMBIOS 2.5 present.
Oct 14 16:59:56 localhost kernel: tsc: Detected 2491.908 MHz processor
Oct 14 16:59:56 localhost kernel: acpiphp: ACPI Hot Plug PCI Controller Driver version: 0.5
Oct 14 16:59:56 localhost kernel: SCSI subsystem initialized
##提取initramfs：
Oct 14 16:59:56 localhost kernel: Unpacking initramfs...
Oct 14 16:59:56 localhost kernel: Freeing initrd memory: 29564K
Oct 14 16:59:56 localhost kernel: Block layer SCSI generic (bsg) driver version 0.4 loaded (major 247)
Oct 14 16:59:56 localhost kernel: ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
Oct 14 16:59:56 localhost kernel: ehci-pci: EHCI PCI platform driver
Oct 14 16:59:56 localhost kernel: ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver
Oct 14 16:59:56 localhost kernel: ohci-pci: OHCI PCI platform driver
##使用initramfs啟動systemd:
Oct 14 16:59:56 localhost systemd[1]: systemd 219 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 -SECCOMP +BLKID +ELFUTILS +KMOD +IDN)
Oct 14 16:59:56 localhost systemd[1]: Detected virtualization kvm.
Oct 14 16:59:56 localhost systemd[1]: Detected architecture x86-64.
Oct 14 16:59:56 localhost systemd[1]: Running in initial RAM disk.
Oct 14 16:59:56 localhost systemd[1]: Set hostname to <localhost.localdomain>.
##到達swap.target：
Oct 14 16:59:56 localhost systemd[1]: Reached target Swap.
Oct 14 16:59:56 localhost systemd[1]: Starting Swap.
Oct 14 16:59:56 localhost systemd[1]: Reached target Local File Systems.
Oct 14 16:59:56 localhost systemd[1]: Starting Local File Systems.
Oct 14 16:59:56 localhost systemd[1]: Created slice -.slice.
Oct 14 16:59:57 localhost systemd: Started Setup Virtual Console.
Oct 14 16:59:57 localhost systemd: Started dracut cmdline hook.
Oct 14 16:59:57 localhost systemd: Starting dracut pre-udev hook...
Oct 14 16:59:58 localhost kernel: device-mapper: uevent: version 1.0.3
Oct 14 16:59:58 localhost systemd: Started dracut pre-udev hook.
Oct 14 16:59:58 localhost systemd: Starting udev Kernel Device Manager...
Oct 14 16:59:58 localhost systemd-udevd: starting version 219
Oct 14 16:59:58 localhost systemd: Started udev Kernel Device Manager.
Oct 14 16:59:58 localhost systemd: Started dracut pre-trigger hook.
Oct 14 16:59:58 localhost systemd: Starting udev Coldplug all Devices...
Oct 14 16:59:58 localhost systemd: Started udev Coldplug all Devices.
##到達sysinit.target:
Oct 14 16:59:58 localhost systemd: Reached target System Initialization.
Oct 14 16:59:58 localhost systemd: Starting System Initialization.
Oct 14 16:59:58 localhost systemd: Starting dracut initqueue hook...
Oct 14 16:59:58 localhost systemd: Starting Show Plymouth Boot Screen...
Oct 14 16:59:58 localhost systemd: Mounting Configuration File System...
Oct 14 16:59:58 localhost systemd: Mounted Configuration File System.
Oct 14 16:59:58 localhost systemd: Started Show Plymouth Boot Screen.
Oct 14 16:59:58 localhost systemd: Reached target Paths.
Oct 14 16:59:58 localhost systemd: Starting Paths.
##到達basic.target:
Oct 14 16:59:58 localhost systemd: Reached target Basic System.
Oct 14 16:59:58 localhost systemd: Starting Basic System.
Oct 14 16:59:58 localhost kernel: scsi host2: ahci
Oct 14 16:59:58 localhost kernel: scsi host3: ahci
Oct 14 16:59:59 localhost kernel: scsi 3:0:0:0: Direct-Access ATA VBOX HARDDISK 1.0 PQ: 0 ANSI: 5
Oct 14 16:59:59 localhost kernel: sd 2:0:0:0: [sda] 16777216 512-byte logical blocks: (8.59 GB/8.00 GiB)
Oct 14 16:59:59 localhost kernel: sd 2:0:0:0: [sda] Write Protect is off
Oct 14 16:59:59 localhost kernel: sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
Oct 14 16:59:59 localhost kernel: sda: sda1 sda2
Oct 14 16:59:59 localhost kernel: sd 2:0:0:0: [sda] Attached SCSI disk
Oct 14 17:00:00 localhost systemd: Found device /dev/mapper/rootvg-root.
Oct 14 17:00:00 localhost systemd: Starting File System Check on /dev/mapper/rootvg-root...
Oct 14 17:00:00 localhost kernel: random: crng init done
Oct 14 17:00:00 localhost kernel: random: 7 urandom warning(s) missed due to ratelimiting
Oct 14 17:00:00 localhost systemd-fsck: /dev/mapper/rootvg-root: clean, 144286/426544 files, 1062697/1705984 blocks
Oct 14 17:00:00 localhost systemd: Started File System Check on /dev/mapper/rootvg-root.
Oct 14 17:00:01 localhost systemd: Started dracut initqueue hook.
Oct 14 17:00:01 localhost systemd: Reached target Remote File Systems (Pre).
Oct 14 17:00:01 localhost systemd: Starting Remote File Systems (Pre).
Oct 14 17:00:01 localhost systemd: Reached target Remote File Systems.
Oct 14 17:00:01 localhost systemd: Starting Remote File Systems.
Oct 14 17:00:01 localhost systemd: Started dracut pre-mount hook.
##將真實的根掛載到/sysroot:
Oct 14 17:00:01 localhost systemd: Mounting /sysroot...
Oct 14 17:00:01 localhost kernel: EXT4-fs (dm-0): mounted filesystem with ordered data mode. Opts: (null)
Oct 14 17:00:01 localhost systemd: Mounted /sysroot.
##到達initrrd_root_fs.target:
Oct 14 17:00:01 localhost systemd: Reached target Initrd Root File System.
Oct 14 17:00:01 localhost systemd: Starting Initrd Root File System.
Oct 14 17:00:01 localhost systemd: Starting Reload Configuration from the Real Root...
Oct 14 17:00:01 localhost systemd: Reloading.
Oct 14 17:00:01 localhost systemd: Started Reload Configuration from the Real Root.
Oct 14 17:00:01 localhost systemd: Started dracut mount hook.
Oct 14 17:00:01 localhost systemd: Reached target Initrd File Systems.
Oct 14 17:00:01 localhost systemd: Starting Initrd File Systems.
##到達initrd.target，清理環境為切根做准備，按照啟動時相反的順序停止target：
Oct 14 17:00:01 localhost systemd: Reached target Initrd Default Target.
Oct 14 17:00:01 localhost systemd: Starting Initrd Default Target.
Oct 14 17:00:01 localhost systemd: Starting dracut pre-pivot and cleanup hook...
Oct 14 17:00:01 localhost systemd: Started dracut pre-pivot and cleanup hook.
Oct 14 17:00:01 localhost systemd: Starting Cleaning Up and Shutting Down Daemons...
Oct 14 17:00:01 localhost systemd: Stopped Cleaning Up and Shutting Down Daemons.
Oct 14 17:00:01 localhost systemd: Stopped dracut pre-pivot and cleanup hook.
Oct 14 17:00:01 localhost systemd: Stopping dracut pre-pivot and cleanup hook...
Oct 14 17:00:01 localhost systemd: Stopped target Remote File Systems.
Oct 14 17:00:01 localhost systemd: Stopping Remote File Systems.
Oct 14 17:00:02 localhost systemd: Stopped target Remote File Systems (Pre).
Oct 14 17:00:02 localhost systemd: Stopping Remote File Systems (Pre).
Oct 14 17:00:02 localhost systemd: Stopped dracut initqueue hook.
Oct 14 17:00:02 localhost systemd: Stopping dracut initqueue hook...
Oct 14 17:00:02 localhost systemd: Starting Plymouth switch root service...
Oct 14 17:00:02 localhost systemd: Stopped target Initrd Default Target.
Oct 14 17:00:02 localhost systemd: Stopping Initrd Default Target.
Oct 14 17:00:02 localhost systemd: Stopped target Basic System.
Oct 14 17:00:02 localhost systemd: Stopping Basic System.
Oct 14 17:00:02 localhost systemd: Stopped target System Initialization.
Oct 14 17:00:02 localhost systemd: Stopping System Initialization.
Oct 14 17:00:02 localhost systemd: Stopped udev Kernel Device Manager.
Oct 14 17:00:02 localhost systemd: Stopped Create Static Device Nodes in /dev.
Oct 14 17:00:02 localhost systemd: Stopping Create Static Device Nodes in /dev...
Oct 14 17:00:02 localhost systemd: Stopped Create list of required static device nodes for the current kernel.
Oct 14 17:00:02 localhost systemd: Stopping Create list of required static device nodes for the current kernel...
Oct 14 17:00:02 localhost systemd: Stopped udev Coldplug all Devices.
Oct 14 17:00:02 localhost systemd: Stopping udev Coldplug all Devices...
Oct 14 17:00:02 localhost systemd: Stopped target Paths.
Oct 14 17:00:02 localhost systemd: Stopping Paths.
Oct 14 17:00:02 localhost systemd: Stopped target Sockets.
Oct 14 17:00:02 localhost systemd: Stopping Sockets.
Oct 14 17:00:02 localhost systemd: Closed udev Kernel Socket.
Oct 14 17:00:02 localhost systemd: Stopping udev Kernel Socket.
Oct 14 17:00:02 localhost systemd: Stopped target Slices.
Oct 14 17:00:02 localhost systemd: Stopping Slices.
Oct 14 17:00:02 localhost systemd: Stopped target Local File Systems.
Oct 14 17:00:02 localhost systemd: Stopping Local File Systems.
Oct 14 17:00:02 localhost systemd: Closed udev Control Socket.
Oct 14 17:00:02 localhost systemd: Stopping udev Control Socket.
Oct 14 17:00:02 localhost systemd: Starting Cleanup udevd DB...
Oct 14 17:00:02 localhost systemd: Stopped target Swap.
Oct 14 17:00:02 localhost systemd: Stopping Swap.
Oct 14 17:00:02 localhost systemd: Stopped target Timers.
Oct 14 17:00:02 localhost systemd: Stopping Timers.
Oct 14 17:00:02 localhost systemd: Stopped dracut pre-udev hook.
Oct 14 17:00:02 localhost systemd: Stopping dracut pre-udev hook...
Oct 14 17:00:02 localhost systemd: Stopped dracut cmdline hook.
Oct 14 17:00:02 localhost systemd: Stopping dracut cmdline hook...
Oct 14 17:00:02 localhost systemd: Stopped Apply Kernel Variables.
Oct 14 17:00:02 localhost systemd: Stopping Apply Kernel Variables...
Oct 14 17:00:02 localhost systemd: Started Cleanup udevd DB.
##環境清理完成，到達initrd-switch-root.target：
Oct 14 17:00:02 localhost systemd: Reached target Switch Root.
Oct 14 17:00:02 localhost systemd: Starting Switch Root.
Oct 14 17:00:02 localhost systemd: Started Plymouth switch root service.
Oct 14 17:00:02 localhost systemd: Starting Switch Root...
Oct 14 17:00:02 localhost systemd: Switching root.
Oct 14 17:00:02 localhost journal: Journal stopped
Oct 14 17:00:07 localhost systemd-journald[132]: Received SIGTERM from PID 1 (systemd).
Oct 14 17:00:07 localhost kernel: systemd: 27 output lines suppressed due to ratelimiting
Oct 14 17:00:07 localhost kernel: SELinux: Disabled at runtime.
Oct 14 17:00:07 localhost systemd[1]: Inserted module 'ip_tables'
Oct 14 17:00:07 localhost journal: Journal started
##切根完成，使用真實的根啟動systemd:
Oct 14 17:00:07 localhost systemd: systemd 219 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 -SECCOMP +BLKID +ELFUTILS +KMOD +IDN)
Oct 14 17:00:07 localhost systemd: Detected virtualization kvm.
Oct 14 17:00:07 localhost systemd: Detected architecture x86-64.
Oct 14 17:00:07 localhost systemd: Set hostname to <localhost.localdomain>.
Oct 14 17:00:07 localhost systemd: Mounted Debug File System.
Oct 14 17:00:07 localhost systemd: Starting Create Static Device Nodes in /dev...
Oct 14 17:00:07 localhost kernel: EXT4-fs (dm-0): re-mounted. Opts: (null)
Oct 14 17:00:07 localhost systemd: Started Remount Root and Kernel File Systems.
Oct 14 17:00:07 localhost systemd: Started Rebuild Dynamic Linker Cache.
Oct 14 17:00:07 localhost systemd: Starting Load/Save Random Seed...
Oct 14 17:00:07 localhost systemd: Started Rebuild Hardware Database.
Oct 14 17:00:07 localhost systemd: Starting udev Coldplug all Devices...
Oct 14 17:00:07 localhost systemd: Starting Flush Journal to Persistent Storage...
Oct 14 17:00:07 localhost systemd: Starting Configure read-only root support...
Oct 14 17:00:07 localhost systemd: Started First Boot Wizard.
Oct 14 17:00:07 localhost systemd: Started Apply Kernel Variables.
Oct 14 17:00:07 localhost systemd: Started Load/Save Random Seed.
Oct 14 17:00:07 localhost systemd: Started LVM2 metadata daemon.
Oct 14 17:00:07 localhost systemd: Starting LVM2 metadata daemon...
Oct 14 17:00:08 localhost systemd: Started udev Coldplug all Devices.
Oct 14 17:00:08 localhost systemd: Starting udev Wait for Complete Device Initialization...
Oct 14 17:00:08 localhost systemd: Started Flush Journal to Persistent Storage.
Oct 14 17:00:08 localhost kernel: RPC: Registered named UNIX socket transport module.
Oct 14 17:00:08 localhost kernel: RPC: Registered udp transport module.
Oct 14 17:00:08 localhost kernel: RPC: Registered tcp transport module.
Oct 14 17:00:08 localhost kernel: RPC: Registered tcp NFSv4.1 backchannel transport module.
Oct 14 17:00:08 localhost systemd: Mounted RPC Pipe File System.
Oct 14 17:00:08 localhost systemd: Started Configure read-only root support.
Oct 14 17:00:08 localhost systemd: Started Load legacy module configuration.
Oct 14 17:00:08 localhost kernel: Installing knfsd (copyright (C) 1996 okir@monad.swb.de).
Oct 14 17:00:08 localhost systemd: Mounted NFSD configuration filesystem.
Oct 14 17:00:08 localhost lvm: 2 logical volume(s) in volume group "rootvg" monitored
Oct 14 17:00:09 localhost systemd: Started Create Static Device Nodes in /dev.
Oct 14 17:00:09 localhost systemd: Starting udev Kernel Device Manager...
Oct 14 17:00:09 localhost systemd: Reached target Local File Systems (Pre).
Oct 14 17:00:09 localhost systemd: Starting Local File Systems (Pre).
Oct 14 17:00:09 localhost systemd-udevd: starting version 219
Oct 14 17:00:09 localhost systemd: Started udev Kernel Device Manager.
Oct 14 17:00:11 localhost systemd: Created slice system-lvm2\x2dpvscan.slice.
Oct 14 17:00:11 localhost systemd: Starting system-lvm2\x2dpvscan.slice.
Oct 14 17:00:11 localhost systemd: Starting LVM2 PV scan on device 8:16...
Oct 14 17:00:11 localhost systemd: Found device /dev/mapper/rootvg-swap.
Oct 14 17:00:11 localhost systemd: Found device VBOX_HARDDISK 1.
Oct 14 17:00:11 localhost systemd: Starting File System Check on /dev/disk/by-uuid/c701b704-6af7-4cc8-ba8f-5050b589faf2...
Oct 14 17:00:11 localhost systemd: Starting LVM2 PV scan on device 8:2...
Oct 14 17:00:11 localhost systemd: Activating swap /dev/mapper/rootvg-swap...
Oct 14 17:00:11 localhost lvm: 2 logical volume(s) in volume group "rootvg" now active
Oct 14 17:00:11 localhost systemd: Started LVM2 PV scan on device 8:2.
Oct 14 17:00:11 localhost kernel: Adding 1048572k swap on /dev/mapper/rootvg-swap. Priority:-2 extents:1 across:1048572k FS
Oct 14 17:00:11 localhost systemd: Activated swap /dev/mapper/rootvg-swap.
Oct 14 17:00:11 localhost systemd: Reached target Swap.
Oct 14 17:00:11 localhost systemd: Starting Swap.
Oct 14 17:00:11 localhost lvm: 1 logical volume(s) in volume group "datavg" now active
Oct 14 17:00:11 localhost systemd: Started LVM2 PV scan on device 8:16.
Oct 14 17:00:11 localhost systemd: Found device /dev/mapper/datavg-lv_data.
Oct 14 17:00:11 localhost systemd: Starting File System Check on /dev/mapper/datavg-lv_data...
Oct 14 17:00:11 localhost systemd: Started udev Wait for Complete Device Initialization.
Oct 14 17:00:11 localhost systemd: Starting Activation of DM RAID sets...
Oct 14 17:00:12 localhost systemd-fsck: /dev/sda1: clean, 699/128016 files, 160136/512000 blocks
Oct 14 17:00:12 localhost systemd: Started File System Check on /dev/disk/by-uuid/c701b704-6af7-4cc8-ba8f-5050b589faf2.
Oct 14 17:00:12 localhost kernel: EXT4-fs (dm-2): mounted filesystem with ordered data mode. Opts: (null)
Oct 14 17:00:12 localhost systemd: Mounting /boot...
Oct 14 17:00:12 localhost kernel: EXT4-fs (sda1): mounting ext3 file system using the ext4 subsystem
Oct 14 17:00:12 localhost systemd: Started Activation of DM RAID sets.
Oct 14 17:00:12 localhost systemd: Reached target Encrypted Volumes.
Oct 14 17:00:12 localhost systemd: Starting Encrypted Volumes.
Oct 14 17:00:12 localhost kernel: EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)
Oct 14 17:00:12 localhost systemd: Mounted /boot.
Oct 14 17:00:12 localhost systemd: Reached target Local File Systems.
Oct 14 17:00:12 localhost systemd: Starting Local File Systems.
Oct 14 17:00:12 localhost systemd: Starting Tell Plymouth To Write Out Runtime Data...
Oct 14 17:00:12 localhost systemd: Starting Import network configuration from initramfs...
Oct 14 17:00:12 localhost systemd: Started Reconfigure the system on administrator request.
Oct 14 17:00:12 localhost systemd: Started Commit a transient machine-id on disk.
Oct 14 17:00:12 localhost systemd: Started Relabel all filesystems, if necessary.
Oct 14 17:00:12 localhost systemd: Starting Preprocess NFS configuration...
Oct 14 17:00:12 localhost systemd: Started Mark the need to relabel after reboot.
Oct 14 17:00:12 localhost systemd: Started Rebuild Journal Catalog.
Oct 14 17:00:12 localhost systemd: Started Update is Completed.
Oct 14 17:00:12 localhost systemd: Started Preprocess NFS configuration.
Oct 14 17:00:12 localhost systemd: Started Tell Plymouth To Write Out Runtime Data.
Oct 14 17:00:12 localhost systemd: Started Import network configuration from initramfs.
Oct 14 17:00:12 localhost systemd: Starting Create Volatile Files and Directories...
Oct 14 17:00:12 localhost systemd: Started Create Volatile Files and Directories.
Oct 14 17:00:12 localhost systemd: Starting Security Auditing Service...
Oct 14 17:00:13 localhost systemd: Started Security Auditing Service.
Oct 14 17:00:13 localhost systemd: Starting Update UTMP about System Boot/Shutdown...
Oct 14 17:00:13 localhost systemd: Started Update UTMP about System Boot/Shutdown.
##到達sysinit.target，激活timers.target,paths.target,sockets.target:
Oct 14 17:00:13 localhost systemd: Reached target System Initialization.
Oct 14 17:00:13 localhost systemd: Starting System Initialization.
Oct 14 17:00:13 localhost systemd: Started Daily Cleanup of Temporary Directories.
Oct 14 17:00:13 localhost systemd: Starting Daily Cleanup of Temporary Directories.
Oct 14 17:00:13 localhost systemd: Reached target Timers.
Oct 14 17:00:13 localhost systemd: Starting Timers.
Oct 14 17:00:13 localhost systemd: Listening on D-Bus System Message Bus Socket.
Oct 14 17:00:13 localhost systemd: Starting D-Bus System Message Bus Socket.
Oct 14 17:00:13 localhost systemd: Reached target Paths.
Oct 14 17:00:13 localhost systemd: Starting Paths.
Oct 14 17:00:13 localhost systemd: Listening on RPCbind Server Activation Socket.
Oct 14 17:00:13 localhost systemd: Starting RPCbind Server Activation Socket.
Oct 14 17:00:13 localhost systemd: Reached target Sockets.
Oct 14 17:00:13 localhost systemd: Starting Sockets.
##到達basic.target，激活multi-user.target或graphical.target:
Oct 14 17:00:13 localhost systemd: Reached target Basic System.
Oct 14 17:00:13 localhost systemd: Starting Basic System.
Oct 14 17:00:13 localhost systemd: Started OpenSSH Server Key Generation.
Oct 14 17:00:13 localhost systemd: Starting firewalld - dynamic firewall daemon...
Oct 14 17:00:13 localhost systemd: Starting System Logging Service...
Oct 14 17:00:13 localhost systemd: Starting Kernel Samepage Merging...
Oct 14 17:00:17 localhost systemd: Starting D-Bus System Message Bus...
Oct 14 17:00:17 localhost systemd: Started Load CPU microcode update.
Oct 14 17:00:17 localhost systemd: Starting Login Service...
Oct 14 17:00:17 localhost systemd: Starting NTP client/server...
Oct 14 17:00:17 localhost systemd: Started Software RAID monitoring and management.
Oct 14 17:00:17 localhost systemd: Starting Dump dmesg to /var/log/dmesg...
Oct 14 17:00:17 localhost systemd: Starting Modem Manager...
Oct 14 17:00:17 localhost systemd: Started irqbalance daemon.
Oct 14 17:00:17 localhost systemd: Starting irqbalance daemon...
Oct 14 17:00:17 localhost systemd-logind: New seat seat0.
Oct 14 17:00:18 localhost systemd: Started Dump dmesg to /var/log/dmesg.
Oct 14 17:00:18 localhost systemd: Started Login Service.
Oct 14 17:00:18 localhost systemd: Starting Kernel Samepage Merging (KSM) Tuning Daemon...
Oct 14 17:00:18 localhost systemd: Started NTP client/server.
Oct 14 17:00:18 localhost systemd: Started GSSAPI Proxy Daemon.
Oct 14 17:00:18 localhost systemd: Started RPC security service for NFS server.
Oct 14 17:00:18 localhost systemd: Started RPC security service for NFS client and server.
Oct 14 17:00:18 localhost systemd: Reached target NFS client services.
Oct 14 17:00:18 localhost systemd: Starting NFS client services.
Oct 14 17:00:20 localhost systemd: Started firewalld - dynamic firewall daemon.
Oct 14 17:00:20 localhost systemd: Starting LSB: Bring up/down networking...
Oct 14 17:00:32 localhost network: Bringing up interface enp0s3: [ OK ]
Oct 14 17:00:32 localhost systemd: Started LSB: Bring up/down networking.
Oct 14 17:00:32 localhost systemd: Reached target Network.
Oct 14 17:00:32 localhost systemd: Starting Network.
Oct 14 17:00:32 localhost systemd: Starting Dynamic System Tuning Daemon...
Oct 14 17:00:32 localhost systemd: Started OpenSSH server daemon.
Oct 14 17:00:32 localhost systemd: Starting OpenSSH server daemon...
Oct 14 17:00:32 localhost systemd: Reached target Network is Online.
Oct 14 17:00:32 localhost systemd: Starting Network is Online.
Oct 14 17:00:32 localhost systemd: Starting Postfix Mail Transport Agent...
Oct 14 17:00:32 localhost systemd: Starting Vsftpd ftp daemon...
Oct 14 17:00:32 localhost systemd: Reached target Remote File Systems (Pre).
Oct 14 17:00:32 localhost systemd: Starting Remote File Systems (Pre).
Oct 14 17:00:32 localhost systemd: Reached target Remote File Systems.
Oct 14 17:00:32 localhost systemd: Starting Remote File Systems.
Oct 14 17:00:32 localhost systemd: Starting Permit User Sessions...
Oct 14 17:00:32 localhost systemd: Starting Virtualization daemon...
Oct 14 17:00:32 localhost systemd: Starting The Apache HTTP Server...
Oct 14 17:00:32 localhost systemd: Starting Availability of block devices...
Oct 14 17:00:33 localhost systemd: Started Vsftpd ftp daemon.
Oct 14 17:00:33 localhost systemd: Started Permit User Sessions.
Oct 14 17:00:33 localhost systemd: Started Availability of block devices.
Oct 14 17:00:33 localhost systemd: Starting Wait for Plymouth Boot Screen to Quit...
Oct 14 17:00:33 localhost systemd: Started Command Scheduler.
Oct 14 17:00:33 localhost systemd: Starting Command Scheduler...
Oct 14 17:00:33 localhost systemd: Starting GNOME Display Manager...
Oct 14 17:00:34 localhost systemd: Started GNOME Display Manager.
Oct 14 17:00:35 localhost systemd: Started Dynamic System Tuning Daemon.
Oct 14 17:00:35 localhost systemd: Started The Apache HTTP Server.
Oct 14 17:00:38 localhost systemd: Started Postfix Mail Transport Agent.
Oct 14 17:00:40 localhost systemd: Received SIGRTMIN+21 from PID 294 (plymouthd).
Oct 14 17:00:40 localhost systemd: Started Wait for Plymouth Boot Screen to Quit.
Oct 14 17:00:40 localhost systemd: Reached target Multi-User System.
Oct 14 17:00:40 localhost systemd: Starting Multi-User System.
Oct 14 17:00:40 localhost systemd: Reached target Graphical Interface.
Oct 14 17:00:40 localhost systemd: Starting Graphical Interface.
Oct 14 17:00:40 localhost systemd: Starting Update UTMP about System Runlevel Changes...
Oct 14 17:00:40 localhost systemd: Started Stop Read-Ahead Data Collection 10s After Completed Startup.
Oct 14 17:00:40 localhost systemd: Started Update UTMP about System Runlevel Changes.
Oct 14 17:00:40 localhost systemd: Startup finished in 1.248s (kernel) + 6.518s (initrd) + 37.578s (userspace) = 45.345s.
至此，啟動完成。
查看initramfs內容：
#mkdir /tmp/initramfs
# cp /boot/initramfs-3.10.0-327.el7.x86_64.img /tmp/initramfs
# /usr/lib/dracut/skipcpio initramfs-3.10.0-327.el7.x86_64.img | zcat | cpio -id --no-absolute-filenames
解壓后的目錄內容如下:
total 18676
-rw-r--r-- 1 root root 19076118 Oct 14 21:28 initramfs-3.10.0-327.el7.x86_64.img
drwxr-xr-x 2 root root 4096 Oct 14 21:29 sys
-rwxr-xr-x 1 root root 3041 Oct 14 21:29 shutdown
lrwxrwxrwx 1 root root 7 Oct 14 21:29 lib -> usr/lib
lrwxrwxrwx 1 root root 23 Oct 14 21:29 init -> usr/lib/systemd/systemd
drwxr-xr-x 2 root root 4096 Oct 14 21:29 var
drwxr-xr-x 2 root root 4096 Oct 14 21:29 tmp
drwxr-xr-x 2 root root 4096 Oct 14 21:29 sysroot
lrwxrwxrwx 1 root root 8 Oct 14 21:29 sbin -> usr/sbin
drwxr-xr-x 2 root root 4096 Oct 14 21:29 run
drwxr-xr-x 2 root root 4096 Oct 14 21:29 root
drwxr-xr-x 2 root root 4096 Oct 14 21:29 proc
lrwxrwxrwx 1 root root 9 Oct 14 21:29 lib64 -> usr/lib64
drwxr-xr-x 12 root root 4096 Oct 14 21:29 etc
drwxr-xr-x 2 root root 4096 Oct 14 21:29 dev
lrwxrwxrwx 1 root root 7 Oct 14 21:29 bin -> usr/bin
drwxr-xr-x 7 root root 4096 Oct 14 21:29 usr
init是鏈接到systemd，systemd是Linux的第一個進程(PID=1)。