Re: [atomic-devel] Direct LVM by default

[Colin Walters <walters verbum org>]

On Thu, Sep 25, 2014, at 11:45 AM, Karanbir Singh wrote:
 
> why have a different filesystem for docker, why not leave it in the
> tree, and expand the entire root filesystem ?

This is a complex topic.  I recommend reading:
https://github.com/docker/docker/blob/master/daemon/graphdriver/devmapper/README.md
if you haven't first.

The docker default (without AUFS or btrfs or overlayfs) is loopback
inside an existing filesystem, and this has a large performance cost;
having a filesystem inside a filesystem is like TCP inside TCP, or virt
in virt.  For example, with loopback every block gets cached twice.

By pointing docker at a directly allocated LVM block device, it's both
faster and has more powerful management opportunities.

For example, say you want to expand your Docker container storage on
Atomic in the new model.  It looks something like this:

# lvm pvcreate /dev/sdb
# lvm vgextend atomicos /dev/sdb
# lvm lvextend -l '+100%FREE' atomicos/docker--data 
# systemctl restart docker

Whereas in the old model, while you could extend the rootfs, you had
100G of thinly provisioned storage, and we didn't offer really
convenient methods to dynamically extend it.