I was also able to reproduce this, here are my notes for now:

## /etc/fstab

LABEL=LVROOT / ext4 errors=remount-ro 0 1
LABEL=LVVAR /var ext4 defaults 0 1
LABEL=TESTE /teste ext4 defaults 0 1

Right after boot:

inaddy@trustylivepatch:~$ systemctl list-units --all | grep fsck
systemd-...el-LVVAR.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-LVVAR.service
systemd-...el-TESTE.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service

This indicates that UPSTART is the one mounting the block devices, NOT
SYSTEMD using its mount units. SNAPS are mounting the SQUASH filesystems
using SYSTEMD UNITS, despite UPSTART scripts.

It is likely that this wasn't noticed, on systems mounting "/" only,
because the "-.mount" SYSTEMD UNIT doesn't depend on "systemd-
fsck@.service" unit, it depends only on "systemd-fsck-root.service", non
existent in TRUSTY's SYSTEMD version. Probably this made SYSTEMD to act
like no error existed.

For SYSTEMD mount units to work, it is needed that no fsck unit error
exists - like when having /var or any other mounting besides root
filesystem - allowing all SYSTEMD units created by snappy to work.

Comparing default setups for TRUSTY and ZESTY:

--------
## TRUSTY

$ dpkg -L systemd | grep fsck
/lib/systemd/systemd-fsck

$ systemctl list-units --all | grep fsck
systemd-...el-LVVAR.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-LVVAR.service
systemd-...el-TESTE.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service

$ systemctl list-units --all | grep mount
-.mount            loaded active   mounted /
teste.mount        loaded active   mounted /teste
var.mount          loaded active   mounted /var
umount.target      loaded inactive dead    Unmount All Filesystems

## ZESTY

$ dpkg -L systemd | grep fsck
/lib/systemd/system/systemd-fsck-root.service
/lib/systemd/system/systemd-fsck@.service
/lib/systemd/system/systemd-fsckd.service
/lib/systemd/system/systemd-fsckd.socket
/lib/systemd/systemd-fsck
/lib/systemd/systemd-fsckd

$ systemctl list-unit-files | grep fsck
systemd-fsck-root.service                       static
systemd-fsck@.service                           static
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service static
systemd-fsckd.service                           static
systemd-fsckd.socket                            static

$ systemctl list-unit-files | grep mount
-.mount                               generated
home-inaddy-work.mount                generated
mnt.mount                             static
mountall.service                      masked
umountfs.service                      masked
umountroot.service                    masked
umount.target                         static
--------

SYSTEMD in TRUSTY was treated differently for FSCK. TRUSTY's version
contains systemd-fsck but not systemd-fsckd, the daemon responsible for
consolidating all fsck information for SYSTEMD journal. It is also clear
that TRUSTY did not include any unit file for systemd-fsck@.service,
that might still be considered for the automatically generated mount
unit files.

You can reproduce this by trying to use TRUSTY SYSTEMD mount units:

--------
## TRUSTY

$ mount /teste

$ mount | grep teste
/dev/sdb2 on /teste type ext4 (rw)

$ systemctl status teste.mount
teste.mount - /teste
   Loaded: loaded (/etc/fstab)
   Active: active (mounted) since Wed 2017-09-27 17:27:21 BRT; 10s ago
    Where: /teste
     What: /dev/sdb2
  Process: 1754 ExecUnmount=/bin/umount /teste (code=exited, status=0/SUCCESS)

$ systemctl stop teste.mount

$ systemctl status teste.mount
teste.mount - /teste
   Loaded: loaded (/etc/fstab)
   Active: inactive (dead) since Wed 2017-09-27 17:27:33 BRT; 2s ago
    Where: /teste
     What: /dev/disk/by-label/TESTE
  Process: 1778 ExecUnmount=/bin/umount /teste (code=exited, status=0/SUCCESS)

$ mount | grep teste

$ systemctl start teste.mount
Failed to issue method call: Unit 
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service failed to load: No such file or 
directory. See system logs and 'systemctl status 
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service' for details.
--------

Also, in TRUSTY, SYSTEMD creates the mount units based on /etc/fstab
entries, just like the recent SYSTEMD does, but on them THERE IS NO
setting for fsck dependencies (Requires/After), and, still, it appears
to be considering those fsck dependencies when you try to mount a
".mount" unit, based on errors I showed above for "systemctl start
XXX.mount".

--------
## TRUSTY

$ cat var.mount

# Automatically generated by systemd-fstab-generator

[Unit]
SourcePath=/etc/fstab
DefaultDependencies=no
After=local-fs-pre.target
Conflicts=umount.target
Before=umount.target
Before=local-fs.target

[Mount]
What=/dev/disk/by-label/LVVAR
Where=/var
Type=ext4
FsckPassNo=1

## ZESTY

$ systemctl edit --full mnt.mount
[Unit]
SourcePath=/etc/fstab
Documentation=man:fstab(5) man:systemd-fstab-generator(8)
Before=local-fs.target
Requires=systemd-fsck@dev-disk-by\x2dlabel-TESTE.service
After=systemd-fsck@dev-disk-by\x2dlabel-TESTE.service

[Mount]
What=/dev/disk/by-label/TESTE
Where=/mnt
Type=ext4
--------

Unfortunately creating the fsck service unit didn't seem to help for
TRUSTY, and, according to this:

$ for file in `dpkg -L systemd`; do [ -f $file ] && grep systemd-fsck $file; 
done
Binary file /lib/systemd/systemd matches
Binary file /bin/systemd matches

It is extremely likely that systemd-fsck@.service was hardcoded as a
dependency for systemd mount unit files, and the fsck units systemd-
fsck@<DISK>.service are NOT BEING CREATED automatically. This is the
real problem to be fixed IMO.

This case should be targeted to both: snapd (trusty) AND systemd
(trusty)

** Changed in: snapd
     Assignee: (unassigned) => Rafael David Tinoco (inaddy)

** Changed in: snapd
       Status: New => Confirmed

** Also affects: systemd (Ubuntu)
   Importance: Undecided
       Status: New

** Changed in: systemd (Ubuntu)
     Assignee: (unassigned) => Rafael David Tinoco (inaddy)

** Changed in: systemd (Ubuntu)
   Importance: Undecided => Medium

** Changed in: systemd (Ubuntu)
       Status: New => In Progress

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https://bugs.launchpad.net/bugs/1718966

Title:
  Cannot install snaps on Ubuntu 14.04 with /var on its own partition

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