[Lxc-users] Sharing container rootfs

[Serge Hallyn]

Quoting Michael H. Warfield (mhw at WittsEnd.com):
> On Mon, 2013-06-10 at 08:48 -0500, Serge Hallyn wrote: 
> > Quoting Michael H. Warfield (mhw at WittsEnd.com):
> > > On Fri, 2013-06-07 at 08:45 +0000, Purcareata Bogdan-B43198 wrote: 
> > ...
> > > I use to do something similar a lot under the old linux-vservers project
> > > (now defunct for several years - mailing list is now dead).  They used a
> > > COW (Copy On Write) system to maintain a common READ ONLY root system
> > > and per-vserver modified layers of changes each server made while
> > > running.  It was quite a nice feature.
> > > 
> > > In theory, this is the idea of using a rootfs image with a unionfs rw
> > > layer on top of that for the running container.  That way, you only have
> > > one copy of a binary on disk and only one copy of the shared executable
> > > code in memory, yet the containers all have unique modifiable root file
> > > systems.  So it works in principle.  Implementation can be another
> > > matter.
> > > 
> > > I think I recall having done this with OpenVZ (after linux-vserver
> > > failed in ongoing IPv6 support forced me over to OpenVZ) but that also
> > > would have been a long time ago.  More recently (but still more than a
> > > year ago) I tried the same technique using unionfs with LXC which failed
> > > horribly.  Functionally, it should appear to be similar to a bind mount
> > > but bind mounts are currently problematical with some of the hacks we've
> > > had to implement to work around systemd conventions.  I haven't tried it
> > > in well over a year.  I suppose I should try that again.  Maybe it would
> > > work now...
> 
> > This is (IIUC) what lxc-start-ephemeral is meant to do - and also what
> > 'lxc-clone -B overlayfs -o containerbase -n containerA' is meant for, where
> > containerbase is a canonical, directory-backed container which all other
> > containers are based upon, and containerA becomes a usable container
> > with an overlayfs or aufs write layer mounted over containerbase's
> > readonly rootfs.
> 
> Oh you UC, all right.  Now that's perfect.  Maybe I missunderstood what
> "ephemeral" did.  I assumed that, after the container was stopped, all
> the "ephemeral" data would be lost (IOW a throw-away instantiation).  If

BY default it is, but there is a --keep-data option which stops that
from happening.

-serge