tar backup ignore files

[Gary Aitken]

I'm backing up a gcloud vm running ubuntu 16.04.01 using tar, and am wondering
which pieces can be safely ignored.  The system is running redmine on apache.
I'm currently ignoring the following:

/cache
/dev/*
/lost+found/*
/media/*
/proc/*
/sys/*
/tmp/*
/mnt/*
/var/cache/apt/*
*/cache/*
[.]tmp$
[.]core$
[.]snap$
/[.]cache/*
/var/lib/lxcfs

I'm wondering in particular if the /sys/* files and /var/lib/lxcfs are really
safe to ignore.  A du of /var/lib/lxcfs comes up zero, but then no-one was doing
much when I did that.  du of /sys also comes up zero.  Are these all just lock
files?

/var/cache is not empty, but I'm assuming since it's a "cache" directory it need
not be backed up.  True?

Thanks,

Gary