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overlayfs.rst

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OverlayFS

This page documents overlayfs support for vagga. This is currently a work in progress.

Since unprivileged overlayfs is unsupported in mainline kernel, you may need some setup. Anyway, ubuntu's stock kernel has the patch applied.

The Plan

  1. Make of use of overlayfs in :volume:`Snapshot` volume. This will be enabled by a volume-level setting initially. In perspective the setting will be default on systems that support it.
  2. Use overlayfs for _run --writable and transient copies
  3. Use overlayfs for :step:`Container` step. This will be enabled by a container-level setting. Which, presumably, will always be disabled by default.
  4. Add vagga _build container --cache-each-step to ease debugging of container builds (actually to be able to continue failing build from any failed step)

Smaller things:

  • vagga _check_overlayfs_support

We need a little bit more explanation about why we would keep overlayfs disabled by default. The first thing to know, is that while we will mount overlays for filesystems inside the container, we can't mount overlays outside of the container.

So we want to have first class IDE support by default (so you can point to one folder for project dependencies, not variable list of layered folders)

For --cache-each-step the main reason is performance. From experience with Docker we know that snapshotting each step is not zero-cost.

Setup

This section describes quirks on various systems that are needed to enable this feature.

To check this run:

$ vagga _check_overayfs_support
supported
$ uname -r -v
4.5.0 #1-NixOS SMP Mon Mar 14 04:28:54 UTC 2016

If first command reports supported please report your value of uname -rv so we can add it to the lists below.

The original patch made by Canonical's employee is just one line, and has pretty extensive documentation about why it's safe enough.

Ubuntu

It works by default on Ubuntu trusty 14.04. It's reported successfully on the following systems:

3.19.0-42-generic #48~14.04.1-Ubuntu SMP Fri Dec 18 10:24:49 UTC 2015

Arch Linux

Since you already use custom kernel, you just need another patch. If you use the package recommended in installation page<archlinux_> your kernel already supports overlayfs too.

The AUR package has he feature enabled too, this is were you can find the PKGBUILD to build the kernel yourself.

NixOS

On NixOS you need to add a patch and rebuild the kernel. Since the patch is already in the nixos source tree, you need just the following in your /etc/nixos/configuration.nix:

nixpkgs.config.packageOverrides = pkgs: {
  linux_4_5 = pkgs.linux_4_5.override { kernelPatches = [
    pkgs.kernelPatches.ubuntu_unprivileged_overlayfs
  ]; };
};

Adjust kernel version as needed.