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I need to change the default configuration of journald on Ubuntu Core 16 so that I can see docker daemon logs from previous boots.

However, /etc/systemd/journald.conf is on a read-only file system, and all information I have found regarding making changes to such a file fail.

The standard advice is to sudo mount -o remount,rw /, but this gives me the error message mount: cannot remount /dev/root read-write, is write-protected

The ubuntu core documentation indicates that the udisks2 snap should be used for disk management, but from reading the documentation on that software, I can't really ascertain that this functionality is available.

Most other use-cases for modifying /etc files seem to be resolved by manipulating substitutes.

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For multiple boot logs (How to find previous boot log after Ubuntu 16.04 restarts?) you don't need read/write access to /etc. You need read/write access to /var/log which you should have already.

To summarize the link:

$ sudo mkdir -p /var/log/journal
$ sudo systemd-tmpfiles --create --prefix /var/log/journal

Display list of previous boots

After boot history has accumulated, use:

$ journalctl --list-boots
-15 58a9e56135564cd8a52d547b19e76bf5 Fri 2018-02-02 18:34:35 MST—Fri 2018-02-02 23:07:14 M
-14 3514e056440341b1b6e5f03d109681bc Sat 2018-02-03 06:05:12 MST—Sat 2018-02-03 08:07:44 M
-13 0d1a32dc275348589f5ecdc72180c018 Sat 2018-02-03 08:08:05 MST—Sat 2018-02-03 08:08:34 M
-12 74159b593f3a401589ee6bd78e31684b Sat 2018-02-03 08:08:51 MST—Sun 2018-02-04 08:32:09 M
-11 4b394a9aad584ab2bfabe3b77eeed78f Sun 2018-02-04 08:32:26 MST—Mon 2018-02-05 16:54:02 M
-10 8e461ed2593c4fd896ca3b71eb3c0fba Mon 2018-02-05 16:54:34 MST—Tue 2018-02-06 03:54:30 M
 -9 ec7ba0e4dfe241c0b9c978d278fcca6d Tue 2018-02-06 03:54:47 MST—Tue 2018-02-06 16:25:02 M
 -8 b5c110267c214c38b63d0a367197d118 Tue 2018-02-06 16:25:19 MST—Thu 2018-02-08 16:49:03 M
 -7 75c3b117ac6a4de984dc3ced15edb7f8 Thu 2018-02-08 16:49:22 MST—Fri 2018-02-09 03:51:09 M
 -6 7338bd1007bc42dda5c8667eeefe1a59 Fri 2018-02-09 03:51:26 MST—Fri 2018-02-09 16:55:52 M
 -5 4b6cd0121327454ca3db035c7ed42df6 Fri 2018-02-09 16:56:09 MST—Sat 2018-02-10 07:55:14 M
 -4 0d56207f9ec0405ca3a3fd638334de2f Sat 2018-02-10 07:55:32 MST—Mon 2018-02-12 22:16:05 M
 -3 0f230cc546fd4aec8f5233e0074ab3e1 Tue 2018-02-13 03:57:20 MST—Wed 2018-02-14 22:58:56 M
 -2 c0d2c0141dd840cbab75d3c2254f8781 Wed 2018-02-14 22:59:13 MST—Sat 2018-02-17 22:46:14 M
 -1 aafb2573a6374e019a7165cb8eee74a0 Sun 2018-02-18 06:02:03 MST—Mon 2018-02-19 04:16:36 M
  0 8462f1969c6f4d61973e7e245014b846 Mon 2018-02-19 04:16:53 MST—Tue 2018-02-20 18:51:42 M

Display last boot log

$ journalctl -b-1
-- Logs begin at Fri 2018-02-02 18:34:35 MST, end at Thu 2018-03-01 16:43:25 MST. --
Feb 28 20:03:15 alien systemd-journald[290]: Runtime journal (/run/log/journal/) is 8.0M, 
Feb 28 20:03:15 alien kernel: Linux version 4.14.23-041423-generic (kernel@kathleen) (gcc 
Feb 28 20:03:15 alien kernel: Command line: BOOT_IMAGE=/boot/vmlinuz-4.14.23-041423-generi
Feb 28 20:03:15 alien kernel: KERNEL supported cpus:
Feb 28 20:03:15 alien kernel:   Intel GenuineIntel
Feb 28 20:03:15 alien kernel:   AMD AuthenticAMD
Feb 28 20:03:15 alien kernel:   Centaur CentaurHauls
Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point
Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registe
Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'
Feb 28 20:03:15 alien kernel: x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
Feb 28 20:03:15 alien kernel: x86/fpu: xstate_offset[3]:  832, xstate_sizes[3]:   64
Feb 28 20:03:15 alien kernel: x86/fpu: xstate_offset[4]:  896, xstate_sizes[4]:   64
Feb 28 20:03:15 alien kernel: x86/fpu: Enabled xstate features 0x1f, context size is 960 b
Feb 28 20:03:15 alien kernel: e820: BIOS-provided physical RAM map:
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000000000-0x0000000000057fff] usabl
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000058000-0x0000000000058fff] reser
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000059000-0x000000000009dfff] usabl
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x000000000009e000-0x000000000009ffff] reser
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000100000-0x0000000030a5ffff] usabl
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a60000-0x0000000030a71fff] reser
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a72000-0x0000000030a89fff] usabl
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a8a000-0x0000000030a8afff] ACPI 
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a8b000-0x0000000030ad4fff] reser
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030ad5000-0x0000000030b2dfff] usabl
Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030b2e000-0x0000000031099fff] reser
lines 1-29

Pay close attention to the parameter -b-1 it is different than other references you may see.

Keep log file size down with vacuum

I have a monthly cron job setup to keep log files down to 200MB in size: Systemd logs (`journalctl`) are too large and slow. This not only decreases storage requirements but speeds up search across all boot logs.

For example to reduce 312 MB consumption down to 200 MB (or less) use:

$ journalctl --vacuum-size=200M
Deleted archived journal /var/log/journal/d7b25a27fe064cadb75a2f2f6ca7764e/system@00056515dbdd9a4e-a6fe2ec77e516045.journal~ (56.0M).
Deleted archived journal /var/log/journal/d7b25a27fe064cadb75a2f2f6ca7764e/user-65534@00056515dbfe731d-b7bab56cb4efcbf6.journal~ (8.0M).
Deleted archived journal /var/log/journal/d7b25a27fe064cadb75a2f2f6ca7764e/user-1000@1bbb77599cf14c65a18af51646751696-000000000000064f-00056444d58433e1.journal (112.0M).
Vacuuming done, freed 176.0M of archived journals on disk.
  • Thanks. That helps me make progress even though this would fall in to the "manipulating substitutes" category. I am still not sure how I would do things like control the max retention history and so-on. – Steven Velez May 23 '18 at 14:01
  • I have another answer posted on vacuuming out history. I'll link it after work if you don't find it first. As a new user you may not know you can click the check mark next to an answer if it works for you. Plus you can up vote any answer or question around the site you find helpful. – WinEunuuchs2Unix May 23 '18 at 15:04
  • @StevenVelez I've updated the answer with instructions to keep log file size down to about 60 day history (in my case=200 MB). – WinEunuuchs2Unix May 23 '18 at 22:46
  • K. I hadn't accepted this as an answer earlier because it did not address the general concern of how to configure journald. I have updated the question to be more specific and fit the answer since it does help my situation. Thanks! – Steven Velez May 24 '18 at 4:01

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