Re: Magic Sysrq key option ... What is the option to record the boot logs to my hard disk before i issue a reboot command ?

From: Pavel Machek <pavel@ucw.cz>
To: "Theodore Y. Ts'o" <tytso@mit.edu>,
	AIAMUZZ <muzafsh.113@gmail.com>,
	linux-kernel@vger.kernel.org
Subject: Re: Magic Sysrq key option ... What is the option to record the boot logs to my hard disk before i issue a reboot command ?
Date: Thu, 10 Jan 2019 11:36:42 +0100	[thread overview]
Message-ID: <20190110103642.GA8228@amd> (raw)
In-Reply-To: <20181119184803.GB19377@thunk.org>

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On Mon 2018-11-19 13:48:03, Theodore Y. Ts'o wrote:
> On Mon, Nov 19, 2018 at 08:51:17PM +0530, AIAMUZZ wrote:
> > Hi,
> > 
> > I have this nagging and frustrating boot freeze i often face on my
> > Deepin OS boot ... Deepin OS i think uses 'journalctl' to record logs
> > on its system.
> > 
> > 'journalctl' however seems to record boot logs ONLY for successful
> > boots ... boot logs for instances of boot freezes/hangs that are
> > issued with a ... 'Ctrl+Alt+Del' ... key combination or the ... Magic
> > Sysrq 'b' ... key combination to get out of the frozen/hung machine
> > state are just not aved in the 'journalctl' log ...
> > 
> > Is there any option using Magic Sysrq that will enable me to record
> > the failed/frozen/hung boot information to a file for troubleshooting
> > before i reboot the system using the option 'b' ?
> > 
> > If not ... Isn't it a good idea to have such an option added to Magic
> > Sysrq options, that can save/record the boot/system logs upto that
> > point, until just before we restart the machines ?
> 
> This is a hard problem to solve, because there's no place to store the
> information, at least not in the general case.  The problem is on an
> unsuccessful boot, the root file system may not have been mounted yet.
> Heck, the storage devices might not have been probed at all!
> 
> If your hardware has a place to store dmesg output across reboots (via
> one of the CONFIG_PSTORE_* kernel configuration options) then this
> would be an easy problem --- in fact, it would be the default even
> with out needing a magic sysrq to request it.  The problem is that
> most x86 devices do not have hardware capable of supporting
> CONFIG_PSTORE.  If you have a custom BIOS which doesn't clear memory
> across a warm reset, that would make things easy.  Unless, unless you
> are a big cloud company using custom hardware and/or a custom BIOS,
> life is much more difficult.  :-(
> 
> One alternative solution you can use is to simply use a serial
> console, and have a another computer monitoring the output from the
> serial console.  This will allow you to see all of the kernel

Serial ports are not very common these days :-(.

Ethernets are, and we have netconsole, maybe that can be used?

(Actually, I wish we had sysrq key "configure netconsole up and
re-send the dmesg buffer". Could be enabled all the time, and would
eliminate frustration of looking at blinking capslock LED and thinking
"I wish I knew what is in the buffer at the moment"....) 

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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