[Bug 1450588] Re: /var/log/dmesg No Longer Being Updated

[Dimitri John Ledkov]

./debian/rsyslog.dmesg.upstart was shipped by rsyslog package in Trusty
and/or other releases.

It was:
start on runlevel [2345]
task
script
    savelog -q -p -c 5 /var/log/dmesg
    dmesg -s 524288 > /var/log/dmesg
    chgrp adm /var/log/dmesg
end script

Thus if such functionality is desired to be preserved, imho rsyslog
should be shipping a similar configuration or unit. Note that journald
pushes all entries to rsyslog, thus i would have thought it could have
generated /var/log/dmesg using normal rsyslog logging configuration
facilities.

I'm not sure how to configure rsyslog correctly to retain these. But I
can write an equivalent systemd unit:

$ systemctl cat dmesg
# /etc/systemd/system/dmesg.service
[Unit]
Description=Save kernel messages

[Service]
Type=idle
ExecStartPre=/usr/bin/savelog -q -p -c 5 /var/log/dmesg
ExecStart=/bin/sh -c '/bin/dmesg -s 524288 > /var/log/dmesg'
ExecStartPost=/bin/chgrp adm /var/log/dmesg

[Install]
WantedBy=multi-user.target


The dmesg command can be approximated with the following journalctl command, which may have more data than dmesg ring-buffer:

$ journalctl --boot 0 --dmesg --output short-monotonic --quiet --no-
pager --no-hostname

however, that is slightly incompatible format due to the extra `kernel:`
stanza after the timestamp

[1195009.811669] kernel: aufs au_plink_put:436:umount[15188]: pseudo-
link is not flushed

--boot 0 -> means to show output from current boot only
--dmesg -> means to show dmesg logs only
--output short-monotonic -> means to prefix things with regular [monotonic] time in dmesg like format
--quiet -> disables the headers with when the logs/boot started finished
--no-pager -> dumps to stdout, instead of less
--no-hostname -> supresses the hostname  entry which is injected between time and kernel: normally

E.g. on my system journalctl command above recovers things from 0.000000
time microcode updated, instead of from very late messages.

Imho journalctl should suppress the `kernel:` as that should be implied
by --dmesg flag.

Depending on how much things happen during boot, i'm not sure if dmesg
should be used, or journalctl to retrieve the boot log.

** Changed in: systemd (Ubuntu)
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1450588

Title:
  /var/log/dmesg No Longer Being Updated

Status in rsyslog package in Ubuntu:
  New
Status in systemd package in Ubuntu:
  Invalid

Bug description:
  After upgrading to Ubuntu 15.04 Vivid, /var/log/dmesg is no longer
  updated after boot.

  It appears that this was previously done via /etc/init/dmesg.conf

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