Re: why is the same mount point repeatedly mounted in nested manner?

From: Goffredo Baroncelli <kreijack@libero.it>
To: Dave T <davestechshop@gmail.com>,
	devel@roosoft.ltd.uk,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Cc: Phillip Susi <phill@thesusis.net>, kilobyte@angband.pl
Subject: Re: why is the same mount point repeatedly mounted in nested manner?
Date: Mon, 9 Aug 2021 21:29:56 +0200	[thread overview]
Message-ID: <6f133a41-dbd6-ce42-b6aa-ae4e621ce816@libero.it> (raw)
In-Reply-To: <CAGdWbB5Z=ARmsU66k7O3Hp=RcMTr-wV5Z880FvMdqN=m3c8Epw@mail.gmail.com>

On 8/8/21 9:48 PM, Dave T wrote:
> On Sun, Aug 8, 2021 at 8:10 AM <devel@roosoft.ltd.uk> wrote:
>>
>> On 05/08/2021 17:46, Dave T wrote:
[...]
>>
>> Try mounting the subvolume with it's subvolume ID. System only generates
>> unit files from the fstab it does not follow them , so if you are vague
>> in your fstab the systemd unit files will also be vague.
> 
> Thank you for the tip. I appreciate your interest in my issue.
> However, I don't fully understand what to change.

I think that Alexander is suggesting to add something like ',subvolid=5' to the line of fstab where /mnt/btrtop/root is mounted.

I add that if it is a systemd bug, it would help to look at the .mount files generated by systemd:

$ sudo ls /run/systemd/generator/*.mount
....

Can you share the content of the  systemd units where you ask to mount '/mnt/btrtop/root' ? It could help the diagnose.

And finally, what happens if you mount/unmount from command line (e.g. manually) /mnt/btrtop/root ?

BR

> Here are the relevant lines from my fstab. I added line numbers
> because the lines will get wrapped in email.  I don't see what part of
> this is vague:
> 
> 1. # cat /etc/fstab
> 2. UUID=28D099A-9D92-487C-8113-A231CAD0EEF2  /     btrfs
> rw,noatime,nodiratime,compress=lzo,space_cache,subvol=/@btrtop/snapshot
> 0 0
> 3. UUID=28D099A-9D92-487C-8113-A231CAD0EEF2  /mnt/btrtop/root  btrfs
> noauto,nofail,rw,noatime,nodiratime,compress=lzo,space_cache    0 0
> 4. /var/cache/pacman       /srv/nfs/var/cache/pacman       none  bind  0 0

I don't know if it matters, but why you set as 'none' the filesystem type ? However according to askubuntu/stackoverflow it seem the right thing to do...

> 
> The path /var/cache/pacman is not a subvolume, but it resides on btrfs
> subvolume @btrtop/snapshot. @btrtop/snapshot is normally mounted at
> "/" but for btrfs tasks, it is also mounted at /mnt/btrtop/root. This
> additional mount operation seems to be causing these nested mounts of
> my bind mount for  /srv/nfs/var/cache/pacman .
> 
> P.S. I cannot test without using systemd. (I'm not even sure I
> remember how to use a non-systemd distro anymore!)
> 

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