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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: 3, Interesting) by boltronics on Monday May 11 2020, @02:33AM
I run Debian Stretch to host a bunch of Xen VMs, and on each system I've done this (that's running systemd), the system always crashes on shutdown - unless I manually shutdown all Dom-U (guest) hosts first. I haven't spent a significant amount of time investigating, but it seems like systemd is doing something to the logical volumes (or the associated volume groups) prior to the guests finishing shutdown - maybe deactivating the volume group because it doesn't detect mounted volumes on the Dom-0 or something silly. I'm not sure. I just know it is not an issue under SysVinit.
Anyway, imagine an environment with a bunch of NAS boxes that host VM OSs, and somebody wanting to fire up a bunch of VMs on a single host, maybe to simulate a production environment for troubleshooting something, and the number of NFS mounts would quickly add up. It's certainly a valid use case, common or not.
It's GNU/Linux dammit!