2. Introduction

This chapter provides an overview of basic software operations on the SR Linux.

2.1. File system layout

The file system is distributed and laid out as a closed-source third-party application on the OS. The directory structure is listed in Table 3.

Table 3: File system layout

Path	Purpose
/opt/srlinux/bin/*	Application binaries
/opt/srlinux/lib/*	Shared libraries
/etc/opt/srlinux/config.json	System configuration
/etc/opt/srlinux/banner	The system banner, pre-login
/etc/opt/srlinux/srlinux.rc	Global environment file
/etc/opt/srlinux/tls/*	User-configured certificates
/etc/opt/srlinux/appmgr/*	YAML configuration
/etc/opt/srlinux/yang/*	YANG models
/etc/opt/srlinux/checkpoint/*	Configuration checkpoints
/etc/opt/srlinux/devices/*	Discovered devices
/etc/opt/srlinux/cli/plugins/*	Operator-provided plug-ins
/var/opt/srlinux/run/*	Application PIDs
/var/log/srlinux/buffer/*	Tmpfs logging
/var/log/srlinux/buffer.persist/*	Persistent buffered logging
/var/log/srlinux/file/*	Persistent logging
/var/log/srlinux/debug/*	Debug logging, tmpfs
/var/log/srlinux/debug.persist/*	Persistent debug logging
/var/log/srlinux/monitor/*	Log_mgr tmpfs storage
/var/log/srlinux/monitor.persist/*	Log_mgr persistent storage
/var/log/srlinux/archive/*	Archive directory for previous startups
$HOME/.srlinuxrc	Per-user environment
$HOME/srlinux/cli/plugins/*	Per-user CLI plug-ins

The Solid State Drive (SSD) is used for an overlay file system, allowing the user to add persistent modifications to the system.

2.2. Boot process

The SR Linux boots using a normal Linux boot mechanism. The BIOS is set up to boot from the internal storage device. The following is the general boot sequence:

The system powers on. Assuming a fully populated system, all components initialize to the point where they bring up their link on the back door bus. Fans are under hardware control and run at 100% speed.
Both control modules start their boot sequence. During this sequence, the following occurs:
1. The BIOS tries to boot off the internal storage device.
2. Grub2 loads the kernel and initramfs into memory. The initramfs contains a squashfs of the root file system used to run SR Linux, including base CentOS and SR Linux applications.The squashfs is unpacked and loaded.
3. When the initramfs has loaded, the application manager (app_mgr) starts and loads the applications based on their start order.
4. The system is operational.
On control redundant platforms, both control modules attempt to boot at the same time. The control module in slot B will wait up to 300 seconds before becoming active, after detecting slot 1 on the back door bus.
The chassis_mgr and device_mgr initialize, push images, and boot each line card and Switch Fabric Module (SFM). This includes making any decisions based on power availability, and taking control of fans.