Message-ID: <68057416@news.ausics.net>
From: not@telling.you.invalid (Computer Nerd Kev)
Subject: Re: Shutdown - 25 Years Later
Newsgroups: comp.os.linux.misc
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Pancho <Pancho.Jones@protonmail.com> wrote:
> If they can do it for shutdown, presumably this is some method,
> some acknowledgement?

On shutdown the filesystems are remounted read-only, shown here
in dmesg (captured from serial port):

 EXT4-fs (sda3): re-mounted f109ff12-da03-4dfd-ad2a-cbd4eb713c43 ro. Quota mode: disabled.
 sd 0:0:0:0: [sda] Synchronizing SCSI cache
 reboot: Power off not available: System halted instead

"mount -o ro /deb/sda3" should do the first part during normal
operation too, but you'd need to have "/" on a ramdisk or tmpfs if
every physical filesystem is being mounted read-only.

I'm not so sure about "Synchronizing SCSI cache", is that like
running sync or something else (it didn't appear in dmesg when I
ran "sync" now)? A web search points to the "sg_sync" command
from sg3_utils as a way of triggering it manually:
https://manpages.debian.org/bookworm/sg3-utils/sg_sync.8.en.html

It sounds like they're talking about the cache in the drive itself,
making sure data is physically written out before power-off. Sounds
like a good idea. I wonder if there's a risk to unplugging
unmounted USB HDDs without doing that? Or does it only have a real
effect on actual SCSI drives (none present on the system where that
kernel message was captured)?

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