Re: [vserver] Another barrier question... Can I "mount" on the barrier?

From: Herbert Poetzl <>
Date: Fri 22 Oct 2010 - 07:48:17 BST
Message-ID: <>

On Thu, Oct 21, 2010 at 07:59:39PM +0100, Ed W wrote:
> On 20/10/2010 22:57, Ed W wrote:
> >Hi, I think this has been answered before, but I can't find the
> >question in the archive:

> >- I have my guest directories as /vserver/images/{guest1,guest2,etc}
> >- I would like to put my guests on a different filesystem

> >I see that I am absolutely fine if my guest filesystem contains
> >"images/guest1" and I mount this on /vserver, because the barrier goes
> >on "images" in the mounted filesystem

> >However, does it work to create "/vserver/images", put the barrier on
> >that, then "mount /dev/vg/myguests /vserver/images" ? I thought I saw
> >that this doesn't work, but would like to check and confirm?

> Help please..?
> Actually, I think the questions boil down to:

> - Can the barrier be on the mount point itself?
> ie given a filesystem containing vserver clients,
> can this be mounted under /vservers/ with
> the barrier on the /vservers/ mount point?

every mount point (with a mount ontop) actually
consists of two directories (usually in two
different filesystems), one being 'the mountpoint'
and the other being the mounted filesystems root

> - Where does the barrier go if we bind mount things
> into multiple locations?

> eg given a filesystem containing "/vservers/{guest1,guest2}",
> which is mounted at /mnt/vs/. I then bind
> mount "/mnt/vs/vservers" to /vservers.
> Where does the barrier now go?

the barrier still goes on /path/to/guest/..

> Presumably it must go on at least /mnt/vs/vservers/?
> Is a barrier also needed on /vservers/?

a barrier on /path/to/guest/.. is more than
sufficient, but of course, you can put any number
of barriers at higher levels, just keep in mind
that only certain filesystems support them and that
they will effectively stop a guest from workin when
placed at /path/to/guest or below

> Basically I get that the barrier has to go on
> "/vservers/guest/..",


> but how do things change if we have a bunch of mounts
> from other filesystems, which are then bind mounted
> into final position?

nothing changes, still the barrier goes on /path/to/guest/..
(just iterate over all guests and you will be fine)

> Just in case this seems too abstract, I'm evaluating
> some kind of clustered filesystem for my guests and
> for various reasons the guests are split between various
> filesystems.

keep in mind that you need filesystem support for the
barrier (special xattrs) and not all filesystems have
that (although it should be easy to add where needed)

> I'm just trying to understand how bind mounting it
> all together changes where the barrier needs to be.

if you stick to the 'barrier goes on /path/to/guest/..'
(of course, after all your filesystem and bind mounts
are in place) then you should be fine in all cases
(given that the filesystems support the barrier)


> Thanks
> Ed W
Received on Fri Oct 22 07:48:30 2010

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