Lesson 6: Boot and the Filesystem Hierarchy

Note

  • grub, filesystem stuff based roughly on Frostsnow’s talk
  • basics of kernel and differences between virtualization/physical (the picture that kevin draws)

The Linux Filesystem Hierarchy

Note

Based on Wade’s talk https://github.com/clinew/presentation_filesystems/blob/master/presentation.tex

What’s a filesystem?

In computing, a file system is used to control how information is stored and retrieved. Without a file system, information placed in a storage area would be one large body of information with no way to tell where one piece of information stops and the next begins.

Filesystem can mean:

  • How the system’s files are arranged on the disk
  • How the disk actually holds the files
    • FAT and NTFS are old but Windows-compatible
    • ext3 is standard, ext4 is newer, xfs has fancier journaling
      • journaling tracks changes before write
    • sysadmins will encounter NFS and its competitors like Gluster

Note

Moving from Windows?

  • Binaries, not executables.
  • Directories, not folders.
  • Read, not load.
  • Symbolic links, not shortcuts.
  • Write, not save.

The File System


_images/you_are_here.jpg 
$ ls
bin   etc         initrd.img.old  lost+found  opt   run      srv  usr
boot  home        lib             media       proc  sbin     sys  var
dev   initrd.img  lib64           mnt         root  selinux  tmp  vmlinuz

Installed programs and utilities

/bin                /usr/sbin
/sbin               /usr/local/bin
/usr/bin            /usr/local/sbin
  • PATH environment variable
$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
  • which command
$ which bash
/bin/bash

User-Specific Data & Configuration

  • Data stored at /home/<username>
    • Desktop environment creates folders Documents, Pictures, Videos, etc.

  • Configurations in dotfiles within home (/.)

  • Lost+Found is not your desktop trash can
    • Lost blocks of the filesystem.
    • Usually not an issue.
    • If your desktop provides backups of deleted files, they’ll be somewhere in /home/<username>/

Where are drives mounted?

  • Raw device appears under /dev.
$ dmesg | tail
[260930.208715]  sdb: sdb1
[260930.320756] sd 6:0:0:0: >[sdb] Asking for cache data failed
[260930.320765] sd 6:0:0:0: >[sdb] Assuming drive cache: write through
[260930.320771] sd 6:0:0:0: >[sdb] Attached SCSI removable disk

  • USB filesystem under /media, main disk /

  • You can manually mount devices with mount
    • “Everything’s a file”
    • umount to unmount

  • /etc/fstab tells things where to mount

  • /etc/mtab shows where things are currently mounted

Space on drives

  • Use df to see disk free space.
$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda8        73G   29G   41G  42% /
  • Use du to see disk usage.
$ du -sh /home/
21G /home/
  • Default output is in bytes, -h for human-readable output.

Three Tiers of Filesystem Hierarchy

  • /, essential for system booting and mounting /usr.

  • /usr, read-only system data for normal system operation.

  • /usr/local, locally-installed software.
    • Package managers usually install under / and /usr.
_images/hierarchy.jpg

Common Directories

Directory Contents
/bin Binary files
/include Header files for C/C++ programs
/lib Libraries
/sbin Binary files for root (superuser)
/boot Files essential for booting kernel, initramfs
/dev Virtual filesystem, exports hardware devices
/etc System-wide configurations
/home Individual users’ data
/media Removable storage devices
/mnt Like media – place to mount disks and things

Common Directories

Directory Contents
/opt “Add-on application software packages”
/proc Virtual filesystem exporting system data
/root homedir for root
/run Volatile information accumulated since boot
/sys Virtual filesystem exporting kernel objects
/tmp Temporary files
/var Data which varies – logs, mail, etc.
/usr/share Architecture-independent, read-only data
/usr/src Kernel source code

/proc has lots of useful system information

Which Linux kernel version are you running?

$ cat /proc/version
Linux version 3.5.0-17-generic (buildd@allspice) (gcc version 4.7.2
(Ubuntu/Linaro 4.7.2-2ubuntu1) ) #28-Ubuntu SMP Tue Oct 9 19:31:23 UTC 2012

Learn about system’s hardware

$ less /proc/cpuinfo
$ less /proc/meminfo

Some parts of /proc can be written as well as read...

$ echo 3 > /proc/sys/vm/drop_caches # drop caches

Commands for working with filesystems

Creating filesystems

$ mkfs

Mounting filesystems

$ mount
# -t for type
# -o for options
# requires device path and mount point

Loopback devices

$ losetup
$ /dev/loop*
# makes it look like a device instead of a file

devfs

sd*
sr*
/dev/null
/dev/random
/dev/urandom
/dev/zero

Blocks and dd

  • Block size is the size of chunks allocated for files

  • dd
    • Disk duplicator (or disk dump).
      • if=<path>, input file.
      • of=<path>, ooutput file.
      • bs=<size>, block size.
      • count=<size>, number of block to transfer.
$ dd if=/dev/random of=/dev/sda
# What will this do?

Filesystem Consistency

  • Metadata vs. data
    • Metadata is extra information the filesystem tracks about the file
    • Data is the file’s contents
  • Filesystem is consistent if all metadata is intact
    • fsck is FileSystem Consistency Check

More about Journaling

  • Filesystem consistency tool; protections against system freezes, power outages, etc.

  • Replaying the journal.

  • ext3’s three modes of journaling:

    • journal:Data and metadata to journal.
    • ordered:Data updates to filesystem, then metadata committed to journal.
    • writeback:Metadata comitted to journal, possibly before data updates.

The Boot Process

  • Bootstrapping
  • Steps in the process
  • Boot loaders
  • Startup scripts
  • Boot levels
_images/xkcd-fight.png

Bootstrapping

Note

kernel loaded into memory, initialization tasks, and available to users

Init
  • kernel spawns init which is always PID 1
  • controls the boot process
  • can be a simple script to a binary
  • Pull itself up by its own bootstraps
  • Automatic and manual booting
  • Driver Loading
  • Period of vulnerability
    • configuration errors
    • missing hardware
    • damaged filesystems
  • init – Always Process ID (PID) #1
    • First process to start
    • Either a binary or can be a simple script (even a bash shell!)

Steps in boot process

Note

Kernel
  • 1st stage – bootloader, 2nd, boot the kernel
  • boot from boot loader
  • load into memory
  • located in /boot/ on Linux
Hardware config
  • locate & initialize hardware
  • print out what it does
System processes
  • init, kswapd, pdflush, etc
  • init only real process
  • Others look like processes for scheduling (appear as [kswapd] with ps)

_images/booting.png
  1. Kernel initialization
  2. Hardware configuration
  3. System processes
  4. Operator intervention (single-user)
  5. Execution of start-up scripts
  6. Multi-user operation

Booting

Note

On hardware specific to UNIX (i.e. Sun)
  • firmware knows how to use devices
  • talk to the network
  • understand filesystems
  • all accessible via the commandline
BIOS smarter than they used to be
  • Not standardized
  • Most servers support PXE
  • PCs vs Proprietary hardware
    • BIOS, UEFI, OpenBoot PROM, etc
  • BIOS
    • Basic Input/Output System
    • Very simple compared to OpenBoot PROM / UEFI
    • Select devices to boot from
    • MBR (first 512 bytes)
  • UEFI
    • Unified Extensible Firmware Interface
    • Successor to BIOS
    • Flexible pre-OS environment including network booting

Boot Loaders (Grub)

Note

Grub
  • next generation PC boot loader
  • no need to “re-run grub” config updates
  • Grub config
  • disks are index based from zero
  • grub install commands
  • netboot, pretty, serial
  • device.map, grub.conf
  • robust with weird disk geometry
  • Grand Unified Bootloader
  • Dynamic fixes during booting
  • Can read the filesystem
  • Index based – (hd0,0) = sda1
  • Grub “version 1” vs. “version 2”
    • Version 2 has more features, but more complicated
    • Latest Debian, Ubuntu and Fedora use v2
grub> root (hd0,0)    (Specify where your /boot partition resides)
grub> setup (hd0)     (Install GRUB in the MBR)
grub> quit            (Exit the GRUB shell)

grub-install

Single User Mode

Note

Show on VM
  • enter grub, hit ESC, pick kernel, hit “e” for edit
  • use arrows

Typically ask for root password

  • What is it used for?
_images/single-user-mode.png
  • Troubleshoot problems
  • Manual Filesystem Checks
  • Booting with bare services
  • Fix boot problems
  • Add “single” to kernel option
  • Solaris/BSD
    • boot -s

Startup Script Tasks

Note

Verbose and print out description of what its doing.

Old days were to manually adjust scripts, not anymore. Most are configurable now.


_images/fsck.jpg
  • Setting up hostname & timezone
  • Checking disks with fsck
  • Mounting system’s disks
  • Configuring network interfaces
  • Starting up daemons & network services

System-V Boot Style

Note

  • System-V Most common today
  • Show system changing between different run levels.
  • Slightly different between Distros
  • Linux derived from System-V originally
  • Alternative init systems
    • systemd - Fedora 15+, Redhat 7+ and Debian* (dependency driven)
    • upstart - Ubuntu, Redhat 6 (event driven, faster boot times)

Run levels:

level 0 sys is completely down (halt)
level 1 or S single-user mode
level 2 through 5 multi-user levels
level 6 reboot level

/etc/inittab

Note

Look at inittab

  • Tells init what to do on each level
  • Starts getty (terminals, serial console)
  • Commands to be run or kept running
  • inittab not used with systemd or upstart
# The default runlevel.
id:2:initdefault:

# What to do in single-user mode.
~~:S:wait:/sbin/sulogin

# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

# terminals
1:2345:respawn:/sbin/getty 38400 tty1
T0:23:respawn:/sbin/getty -L ttyS0 9600 vt100

init.d Scripts

Note

sshd init script
  • case statement
  • functions
  • chkconfig
  • One script for one service/daemon
  • Start up services such as sshd, httpd, etc
  • Commands
    • start, stop, reload, restart
  • sshd init script
$ service sshd status
openssh-daemon (pid  1186) is running...

$ service sshd restart
Stopping sshd:                                             [  OK  ]
Starting sshd:                                             [  OK  ]

Starting services on boot

Note

Show sshd script show list, adding, removing, enabling, disabling

  • rclevel.d (rc0.d, rc1.d)
  • S = start, K = stop/kill
  • Numbers to set sequence (S55sshd)
  • chkconfig / update-rc.d
    • Easy way to enable/disable services in RH/Debian
  • Other distributions work differently
$ chkconfig --list sshd
sshd            0:off 1:off 2:on  3:on  4:on  5:on  6:off

$ chkconfig sshd off

$ chkconfig --list sshd
sshd            0:off 1:off 2:off 3:off 4:off 5:off 6:off

Configuring init.d Scripts

Note

show sendmail & network config examples for CentOS

/etc/defaults seems to be more common between UNIX’s

  • /etc/sysconfig (RH) or /etc/defaults (Debian)
  • source Bash scripts
  • Daemon arguments
  • Networking settings
  • Other distributions are vastly different
$ cat /etc/sysconfig/ntpd
# Drop root to id 'ntp:ntp' by default.
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -g"

Shutting Down

Note

Modern systems are less touchy with hard resets, but still need to be careful. Only for emergencies.

Shutdown -h

  • Not Windows, don’t reboot to fix issue
  • Can take a long time (i.e. servers)
  • Reboot only to
    • load new kernel
    • new hardware
    • system-wide configuration changes
  • shutdown, reboot, halt, init
  • wall - send system-wide message to all users
$ wall hello world
Broadcast message from root@devops-bootcamp (pts/0) (Fri Jan 31 00:40:29 2014):

hello world

Homework