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