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System adminsitrator Chris Siebenmann has found Modern versions of systemd can cause an unmount storm during shutdowns:
One of my discoveries about Ubuntu 20.04 is that my test machine can trigger the kernel's out of memory killing during shutdown. My test virtual machine has 4 GB of RAM and 1 GB of swap, but it also has 347 NFS[*] mounts, and after some investigation, what appears to be happening is that in the 20.04 version of systemd (systemd 245 plus whatever changes Ubuntu has made), systemd now seems to try to run umount for all of those filesystems all at once (which also starts a umount.nfs process for each one). On 20.04, this is apparently enough to OOM[**] my test machine.
[...] Unfortunately, so far I haven't found a way to control this in systemd. There appears to be no way to set limits on how many unmounts systemd will try to do at once (or in general how many units it will try to stop at once, even if that requires running programs). Nor can we readily modify the mount units, because all of our NFS mounts are done through shell scripts by directly calling
mount; they don't exist in/etc/fstabor as actual.mountunits.
[*] NFS: Network File System
[**] OOM Out of memory.
We've been here before and there is certainly more where that came from.
Previously:
(2020) Linux Home Directory Management is About to Undergo Major Change
(2019) System Down: A systemd-journald Exploit
(2017) Savaged by Systemd
(2017) Linux systemd Gives Root Privileges to Invalid Usernames
(2016) Systemd Crashing Bug
(2015) tmux Coders Asked to Add Special Code for systemd
(2016) SystemD Mounts EFI pseudo-fs RW, Facilitates Permanently Bricking Laptops, Closes Bug Invalid
(2015) A Technical Critique of Systemd
(2014) Devuan Developers Can Be Reached Via vua@debianfork.org
(2014) Systemd-resolved Subject to Cache Poisoning
(Score: 4, Funny) by qzm on Monday May 11 2020, @01:15AM (4 children)
So could you please point at the supported and widespread syerver read Linux distros that are available from cloud services?
(Seriously, in his discussions of the new homeD, Pottering actually handwaved away the inability to ssh into a newly booted machine - 'you shouldnt need to do that'.)
(Score: 2) by janrinok on Monday May 11 2020, @08:28AM (3 children)
You don't have to use homed on systemd distros - it is optional. If you want to leave home directories exactly as they are now you can do so - and you can still ssh into them. If, however, you would rather have encrypted home directories then you cannot use ssh. But that is nothing new - it is not possible to ssh into any encrypted storage until it has been unlocked. Encryption that can be defeated by ssh isn't encryption anyway.
This doesn't excuse Poetterings hand waving away of using ssh - but I would have hoped (against all odds) that he could have explained the situation better.
(Score: 2) by DannyB on Monday May 11 2020, @05:26PM (2 children)
It's optional -- for now. And it's called systemd-homed.
The thing about landline phones is that they never get lost. No air tag necessary.
(Score: 2) by janrinok on Tuesday May 12 2020, @06:41AM (1 child)
I know its systemd.homed - you know that from 'systemd' in the title. Homed is perfectly understandable. People use journald, systemctl , and loginctl without having to prefix them with 'systemd.' Why should homed be any different?
The day it is no longer optional is the day I will object to it. I don't object to black cars on the road, people wearing funny hats, or those that want to wear odd coloured socks. I will complain about them when they are made compulsory.
I have that version of systemd (245) on 4 of my systems but I do not use homed. It is OPTIONAL until as you say, it isn't (maybe, one day, eventually, ...) It works just like it always has done. I can ssh into my home directories without any hassle. There hasn't been an outcry when every copy of Ubuntu, Debian or Fedora users suddenly stopped working - because it hasn't happened. Like all software, you have to know how and when to use it.
(Score: 2) by DannyB on Tuesday May 12 2020, @03:46PM
I'll bet you don't write GNU/Linux either.
(nor do I)
The thing about landline phones is that they never get lost. No air tag necessary.