[resolved] Handbook: why not unmount sys before reboot?

dcljr · Last edited by dcljr on Fri Aug 29, 2014 4:32 am; edited 1 time in total

In the Handbook (both AMD64 and x86 versions), after configuring the bootloader and before rebooting, we see:

Oniryczny · Guru Joined: 01 Dec 2011 Posts: 419 Location: Poland

DONAHUE · Posted: Tue Aug 05, 2014 2:43 pm Post subject:

Which just exits the X terminal I'm working in. Anything wrong with that? No.
Anyway, so why doesn't the Handbook tell users to unmount /mnt/gentoo/sys? A Good Question, suggest filing a bug.
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krinn · Watchman Joined: 02 May 2003 Posts: 7470

Well i don't read the handbook if anyone file a bug, ask to correct the only thing that seems stupid :
umount -l /mnt/gentoo/boot

As this might damage boot. Nobody should lazy umount boot that hold real datas (kernel, grub... some hdd or controller may have not flush datas to disk yet).

And proc sys and dev, nobody cares about them anyway. So umount them with a hammer or not umounting them change nothing.

Hu · Moderator Joined: 06 Mar 2007 Posts: 21624

I agree with krinn. If you use a lazy unmount on a filesystem backed by real storage, you need to wait for the unmount to complete before you reboot. The unmount will begin when the filesystem is unused. If you have processes with open files or directories on that filesystem, then the filesystem does not become unused until those processes release the descriptors. Lazy unmount can be used freely for NFS and virtual filesystems. For real filesystems, it should be used only if you know exactly why you need a lazy unmount instead of an eager unmount.

dcljr · Posted: Fri Aug 08, 2014 12:25 pm Post subject:

To those who are complaining about lazily unmounting /mnt/gentoo/boot: it is true, is it not, that my solution...

Hu · Moderator Joined: 06 Mar 2007 Posts: 21624

No, your solution is not technically safe. A lazy unmount immediately detaches the specified mount from the namespace, so after umount -l /mnt/gentoo/boot, you can immediately unmount /mnt/gentoo even if the filesystem formerly mounted at /mnt/gentoo/boot is still in use. That is why I said "For real filesystems, it should be used only if you know exactly why you need a lazy unmount instead of an eager unmount." Both root and boot are real filesystems, so both of them need a non-lazy unmount unless you know exactly why you need a lazy unmount.

dcljr · Posted: Sun Aug 10, 2014 8:21 am Post subject:

Right. OK, so this then:

Cyker · Veteran Joined: 15 Jun 2006 Posts: 1746

Interesting... I don't think I bothered doing any of that and just rebooted the system :lol:

Hu · Moderator Joined: 06 Mar 2007 Posts: 21624

dcljr: right, that would be safe. You usually do not need to use a lazy unmount even for virtual filesystems, but using it there is not dangerous as it can be for real filesystems.

Cyker: if you skip the umount process, then the system may automatically perform eager unmounts of the filesystems on the way down, which is safe, equivalent to what dcljr posted, and uses fewer keystrokes. If you mean that you used lazy unmounts for real filesystems and it worked, then that just means you got lucky. Getting lucky is likely in this case, since a lazy unmount will probably free the filesystem within a few seconds if you do not have any processes using it. The eager unmount has the advantage that it will fail if the filesystem is in use, rather than detaching immediately and releasing it at some unspecified later time, which may or may not occur before the kernel turns off the device.

dcljr · Posted: Mon Aug 18, 2014 9:40 am Post subject:

Bugs submitted (first by me, second by someone else) and acted upon:

krinn · Watchman Joined: 02 May 2003 Posts: 7470

I'm the one who made the 2nd report, because of bad privacy policies of gentoo bugzilla.