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I am using RHEL 6.4. Problem is it takes too much time to boots up? Can you say, what can I do to be out of this situation?

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Measure, measure, measure. Seriously. Before you try to optimise, measure where the time is spend and think if you can get rid of some services. The bootchart tool might be of help there. Usually optimisations come for a price, otherwise the distributors would have already optimised it for you. – Marco May 4 '13 at 19:12
"Usually optimisations come for a price" means what? – Living-with-Linux May 4 '13 at 19:14
If you, e.g. reduce the time grub waits for user input, you might have just one second to boot another kernel, instead of ten like before. Or if you disable services or slow network mounts, well, then you can't use them or at least it's less convenient to activate them when the are actually needed. Or if you disable the display manager, the login process is less pretty and you lose other functionality. – Marco May 4 '13 at 19:19
The best way to speed up the startup process is getting rid of unused services and a fast HDD. – Marco May 4 '13 at 19:22

Couple things:

1) Biggest thing is to disable any unnecessary services. mdmonitor, pcscd, isdn, iscsi, hplip, and hidd are pretty safe to disable to start at boot for the average workstation. Unless you're exporting nfs you can disable any service beginning in "rpc" and any portmap/rpcbind services. gpm is pretty stupid and can be disabled. If you configure your interfaces statically and don't do anything with vpn or wireless, you can disable NetworkManager. If there are some services that you want to start at boot but aren't dependencies for any other services, you can kick them off in /etc/rc.local as backgrounded tasks.

2) If you can afford it, booting from SSD and moving root to that is also going to help out a lot.

3) You can set HDD schedulers via udev so that the change will take effect earlier in the boot process benefiting startup. It's not going to be enormously beneficial for something like system initialization, but it will help.

4) Edit your GNOME Startup applications to just the stuff you need.

5) When RHEL7 comes out it will have systemd, so consider upgrading the desktops' OS when it gets released later this year (EDIT: Was released earlier this month).

That's all I can think of. What issues are being caused by the boot time?

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no need to disable the startup applications, just delay them, check here – Aquarius Power Jun 18 '14 at 22:50
There's plenty of need to disable if they're unnecessary. It's less work and it won't steal CPU cycles later. Delaying initialization is useful for nonessential services that you still want running, though. – Bratchley Jun 19 '14 at 11:45

First determine how much time booting does take now. For that set an arbitrary but clear point after which you consider the machine booted, and measure that. I recommend going for CPU going down to the background noise level and no more (startup) disc activity. Based on that you have something real to make comparisons instead of some vague feeling of boredom during the bootprocess that needs to be decreased.

Then set yourself a goal on what would be an acceptable boot time.

After that start measuring. Use bootchart. Start throwing things away that are in the kernel that you don't need and build what you need in the kernel (and not in seperate modules).

I stopped myself at booting in 15 seconds (that was back in 2009), an effort that got inspired by a presentation of someone who got booting shaved down to under a second. I did not want to spent more time optimising myself, but there was room for improvement.

Nowadays I spent less time in booting by only doing so every 6-8 weeks. That cuts down the time on booting compared to restarting every day down to about 2%. And if I have to reboot (e.g. for a kernel upgrade) I do so before I go to sleep.

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What was the time difference of the slim kernel? Just wondering if it's worth the hassle. On my system the kernel loading time is a very small fraction of the overall boot time. – Marco May 4 '13 at 19:52
I don't have the docs I made while doing that nor do I have access to the machine. The original boot time was over 90 seconds. IIRC quite a lot of time was spent probing for non-existing devices by the kernel, that is why I recommended building only those needed into the kernel. IMHO is almost always a waste of time. Don't reboot as it is often not necessary and if downtime for rebooting is unacceptable, make sure you have a fail-over setup with a second machine. – Anthon May 4 '13 at 20:07

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