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

[Dave T <davestechshop@gmail.com>]

On Sun, Aug 8, 2021 at 8:10 AM <devel@roosoft.ltd.uk> wrote:
>
> On 05/08/2021 17:46, Dave T wrote:
> > I recently switched from snapper and snapsync to btrbk, which I
> > generally prefer. However, I am running into one issue.
> >
> > Background - from https://digint.ch/btrbk/doc/faq.html
> > Btrbk is designed to operate on the subvolumes within a root
> > subvolume. The author recommend booting linux from a btrfs subvolume,
> > and using the btrfs root only as a container for subvolumes (i.e. NOT
> > booting from "subvolid=5").
> >
> > That's exactly what I'm doing.
> >
> > The key point is that btrbk requires mounting the toplevel subvolume
> > to perform its tasks.
> >
> > I'm using btrbk via a systemd timer. I have a daily and hourly timer
> > set up. Each timer starts by mounting the btrfs root, performing the
> > btrbk task, and unmounting the btrfs root.
> >
> > I create hourly snapshots and on a daily basis I use btrbk to transfer
> > those to two different USB HDD's as well as to a remote server via
> > SSH.
> >
> > Here's the problem. I now end up (after some time) with a nested list
> > of mounts for one particular mount point as shown below. I don't
> > understand why or how this is happening. It does have side effects
> > (disk access can hang). The apparent "cure" is to restart the nfs
> > server service, but I'm still trying to understand the issue fully.
> >
> > # cat /etc/fstab
> > UUID=28D099A-9D92-487C-8113-A231CAD0EEF2  /     btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvol=/@btrtop/snapshot
> > 0 0 #mounts my root filesystem
> > UUID=28D099A-9D92-487C-8113-A231CAD0EEF2  /mnt/btrtop/root  btrfs
> > noauto,nofail,rw,noatime,nodiratime,compress=lzo,space_cache    0 0
> > #mounts the top btrfs volume for btrbk access to all snapshots
> > /var/cache/pacman       /srv/nfs/var/cache/pacman       none  bind  0 0
> >
> > # findmnt -t btrfs
> > TARGET                                      SOURCE
> >                                        FSTYPE OPTIONS
> > /
> > /dev/mapper/xyz[/@btrtop/snapshot]                     btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > ├─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │ └─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │   └─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │     └─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │       └─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │         └─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │           └─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │             └─/srv/nfs/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > ├─/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │ └─/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │   └─/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │     └─/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │       └─/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │         └─/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
> > │           └─/var/cache/pacman
> > /dev/mapper/xyz[/@btrtop/snapshot/var/cache/pacman]    btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=3194,subvol=/@btrtop/snapshot
>
> 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.
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

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!)