[lxc-users] Establish a bind mount to a running container

[Jäkel, Guido]

>-----Original Message-----
>From: lxc-users [mailto:lxc-users-bounces at lists.linuxcontainers.org] On Behalf Of Stéphane Graber
>Sent: Friday, October 07, 2016 3:38 PM
>To: LXC users mailing-list
>Subject: Re: [lxc-users] Establish a bind mount to a running container
>
>On Fri, Oct 07, 2016 at 01:26:26PM +0000, Jäkel, Guido wrote:
>> >-----Original Message-----
>> >From: lxc-users [mailto:lxc-users-bounces at lists.linuxcontainers.org] On Behalf Of Stéphane Graber
>> >Sent: Friday, October 07, 2016 11:46 AM
>> >To: LXC users mailing-list
>> >Subject: Re: [lxc-users] Establish a bind mount to a running container
>> >
>> >On Fri, Oct 07, 2016 at 07:03:21AM +0000, Jäkel, Guido wrote:
>> >> Dear experts,
>> >>
>> >> I wonder if it's possible to establish a bind mount filesystem resource from the LXC host to an already running
>container in
>> >an manual way, but analogous as it is done at startup time.
>> >>
>> >> I already figured out that the releasing an existing link is no thing; just umount it from inside the container. But is
>there a
>> >way to establish one while shifting the destination of a bind mount into the right namespace?
>> >>
>> >> I ask about, because in a couple of days I have to change a (NFS) filesystem source (because of an hardware migration)
>> >that is common to a large number of running containers but not frequently used and I want to avoid to restart all the
>> >containers with it services.
>> >>
>> >> thank you for advice
>> >>
>> >> Guido
>> >
>> >It's very difficult due to a number of restrictions in place in the kernel.
>> >
>> >The only way of doing this that I'm aware of is what we do in LXD. We
>> >create a path on the host before the container starts, put that on a
>> >rshared mountpoint, then bind-mount that directory into the container
>> >under some arbitrary path.
>> >
>> >Then when you want to inject a new mount in the container, you can mount
>> >it in a sub-directory of that path you create on the host, which will
>> >then have the container inherit the mount entry thanks to the host
>> >mountpoint being rshared and the container's mountpoint being rslave.
>> >
>> >Once the mountpoint shows up in the container, you can then move it to
>> >whatever path you actually want it on.
>>
>>
>> Dear Stéphane,
>>
>> I sorry, but I don't get it yet; some of your terms and where to do it are dubious to me. Maybe an example may light it up
>to me:
>>
>> Let say, I want to inject the path  host:/mnt/some_host_mountpoint/some_directory  as a bind mount to a running
>container; it should end up on  container:/import/some_container_moutpoint  . On the host, the mountpoint
>host:/mnt/some_host_mountpoint is mouted to a NFS source, let say  nfshost:/some_export
>>
>> Now please, where to issue which commands?
>
>You'll have to do some research yourself or hope that someone can give
>you step by step instructions :)
>
>
>A guestimate (completely untested) would be:
>
>Setup steps, before you first start the container:
>
> 1) mkdir /tmp/shared-c1
> 2) mount --bind /tmp/shared-c1 /tmp/shared-c1
> 3) mount --make-rshared /tmp/shared-c1
> 4) Add to /var/lib/lxc/c1/config => lxc.mount.entry=/tmp/shared-c1 /.shared none bind,create=dir 0 0
> 5) lxc-start -n c1
>
>At which point, you could inject a new mount with:
>
> 1) mkdir /tmp/share-c1/some_directory
> 2) mount -- bind /mnt/some_host_mountpoint/some_directory /tmp/share-c1/some_directory
> 3) lxc-attach -n c1 -- mkdir -p /import/some_container_mountpoint
> 4) lxc-attach -n c1 -- mount --move /.shared/some_directory /import/some_container_mountpoint
>
>
>Again, that's a very rough approximation from what I remember the LXD
>code is doing (though we're doing it in a mix of Go and C).


Thanks again. I don't read through it but the first thing I read: "Steps to to before first start of the container". Does that mean, that there's no chance in my case, because all the containers are already running? 

Or may I (miss)use an established bind mount (or even the rootfs mount) to create the path /tmp/shared "here" at it's source path on the host , then "over-bind-mount-and-rshared" it on the host and then continue the injection?