Filesystem inside Container?

Incus
1

My question may be stupid

When using ZFS as storage, what file system is used inside a container?
Does it inherit from the host, meaning also from ZFS?
Is it not EXT4 or another one?

2

Are you talking containers, or VMs?

If it’s a container, then it’s a zfs dataset on the host. The container has this mounted inside its chroot environment. It sees no block device.

3

It’s rather the opposite, what is the file system if on the host it’s ZFS? Is it also ZFS in the container?

4

I’m not sure how I can make it clearer than this:

  • If you create a zfs storage pool in incus, it is using ZFS on the host
  • When a container is created, a new dataset is created for it on the host
  • The filesystem for the container is really just a view of (a subset of) the host’s filesystem - in this case, a zfs dataset on the host
  • In other words, the filesystem that the container sees is simply part of the filesystem on the host.

For example:

# incus storage list
+---------+--------+---------+-------------+---------+---------+
|  NAME   | DRIVER | SOURCE  | DESCRIPTION | USED BY |  STATE  |
+---------+--------+---------+-------------+---------+---------+
| default | zfs    | zfs/lxd |             | 10      | CREATED |
+---------+--------+---------+-------------+---------+---------+

I have a container called “nfsen”. From the host side:

# zfs list zfs/lxd/containers/nfsen
NAME                       USED  AVAIL     REFER  MOUNTPOINT
zfs/lxd/containers/nfsen  34.7G   193G     34.7G  legacy

# ls -l /var/lib/incus/containers/nfsen
lrwxrwxrwx 1 root root 53 Mar 20 22:50 /var/lib/incus/containers/nfsen -> /var/lib/incus/storage-pools/default/containers/nfsen

# ls -l /var/lib/incus/storage-pools/default/containers/nfsen
total 26
-r--------  1 root root 7021 Mar 21 10:56 backup.yaml
-rw-r--r--  1 root root 1050 Jun 10  2020 metadata.yaml
drwxr-xr-x 22 root root   22 Mar 21 10:55 rootfs
drwxr-xr-x  2 root root    7 Jun 10  2020 templates

# ls -li /var/lib/incus/storage-pools/default/containers/nfsen/rootfs/etc/hosts
706 -rw-r--r-- 1 root root 221 Jun  5  2020 /var/lib/incus/storage-pools/default/containers/nfsen/rootfs/etc/hosts

Inside the container, a file such as /etc/hosts is actually /var/lib/incus/storage-pools/default/containers/nfsen/rootfs/etc/hosts on the host system, which is the file rootfs/etc/hosts within the zfs dataset zfs/lxd/containers/nfsen

The container’s view of the same file:

# incus shell nfsen
root@nfsen:~# mount | grep 'on / '
zfs/lxd/containers/nfsen on / type zfs (rw,relatime,idmapped,xattr,posixacl,casesensitive)
root@nfsen:~# ls -li /etc/hosts
706 -rw-r--r-- 1 root root 221 Jun  5  2020 /etc/hosts

What I’m trying to say is: it makes no sense to talk about the file system in the container being something other than the file system in the host, because they are the same file system.

5

Perfect, that’s exactly the answer I was expecting, sorry for the confusion. I have a MariaDB server in a container, and I’ve seen that it needs to be fine-tuned to get better performance when we’re under ZFS