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linux boot process

Posted on 2014-02-14
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Last Modified: 2014-03-16
in rhel4 / rhel5 / rhel5 is the linux boot process same?

can someone explain how linux boot process works?

what is upstart? is it some thing new?
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Question by:ittechlab
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Daniel Helgenberger earned 600 total points
ID: 39860082
Hello,

I assume you mean RHEL  4,5,6?

The Linux boot process can very basically broken down into these steps:
- Boot loader (grub) loads the kernel in an initial ram disk
- There kernel takes care of enumerating devices (udev) and loading appropriate drivers (kernel modules). In the end, the root filesystem is mounted read/write.
- Then, the INIT process is started. From there, the init process runs the scripts in /etc/init.d in alphabetical order until every script for the target runlevel is executed.

This works the same for RHEL4 and RHEL5. As for RHEL6, Red Hat switched to upstart. While some things are different , Upstart can run things asynchronously and handles (hotplug-) devices much better, things are basically all the same and you will not see much difference. Upstart was designed make the transition smooth.
As a general rule, always use the 'service' command and try to avoid directly invoking /etc/init.d/ scripts.

However, a note to your question: In RHEL 7 this will change drastically, as it will incorporate the new systemd witch fully replaces sysVinit / Upstart.

I hope I could shed some light into the matter? Do you want me to be more specific?

Helge
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by:ittechlab
ID: 39862696
please explain more specific also pass me some information about RHEL7 boot process as well. much appreciated.
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by:Daniel Helgenberger
Daniel Helgenberger earned 600 total points
ID: 39862717
Hello,
IMHO it is beyond this thread to rewrite all the documentation on this topic.

Please read http://en.m.wikipedia.org/wiki/Linux_startup_process
And esp. the links there.

The same is true for systemd.

One note on systemd for further reading; a good start is again the Wikipedia article, esp the links to Lennart Pöttering's blog - he wrote several posts on systemd for administrators:
http://0pointer.de/blog/projects/systemd-for-admins-1.html

At the moment there is an ongoing debate if systemd is a 'good' thing for servers. Suffice to say that after Debian adopted systemd recently (the guys who maintain upstart), these discussions are rather pointless.

So kindly read the provided docs and I am happy to assist with specific questions on the matter.
Cheers
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by:gheist
gheist earned 600 total points
ID: 39868917
RHEL4 - lilo + mkinitrd + sysvinit
RHEL5 - grub + mkinitrd + sysvinit
RHEL6 - grub + dracut + upstart
RHEL7 beta (as seen in fedora 19) - grub2 + dracut + systemd

1st loads kernel from disk
theen kernel loads initial ramdisk with some programs (like init, and others to load some extra drivers)
then it changes root to real root and runs real init (see 3) as PID1
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by:madunix
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ID: 39873298
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by:gheist
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Some are inacurate (even coming from horses mouth) "mounts partition read only"
It actually mounts small system image from initrd file which contains drivers and filesystem checkers to get system to mounting / read-write.
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by:Daniel Helgenberger
ID: 39932375
Hello Ittechlab,

I wonder what you expected us to write here to get the A grading. It is impossible to explain this topic comprehensively in a forum post.
I used layman's terms in my first posting to get you started, but unless you do not come up with specific questions I can only copy/paste the official documentation; witch is - and I hope you agree -  not achieving the objective. The same is true for your other question about multipath.

Please consider reading:
http://support.experts-exchange.com/customer/portal/articles/481419
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