Snapper 2.0

Snapper 2.0 (or just Snapper) is a snapshot mechanism for the virtual memory space of the persistent operating system PhantomOS. It uses the a logging file system (e.g. ext4) to ensure that file operations (e.g. modification or deletion) are resistant to OS crashes mid-way through the operation.

A vital property of Snapper is that it needs to be disk space efficient in order not to bloat the file system. This is done by utilizing a mapping from the virtual pages (from now on referred only as pages) to a file containing their contents. Thus if a page remains unchanged its contents will still be mapped to a file from a previous snapshot, hence no new file needs to be created.

Requirements

• Genode + Goa
• Lwext4 Plugin (optional)

Installation

Install the requirements. This pre-built container image comes with all the dependencies needed for Genode:

git clone --depth 1 https://github.com/genodelabs/genode.git
git clone --depth 1 https://github.com/genodelabs/goa.git genode/goa
git clone --depth 1 https://github.com/rumenmitov/snapper genode/repos/snapper

podman run -d --name genode --cap-add SYS_PTRACE -v $PWD/genode:/genode -w /genode docker.io/rmitov/genode:25.05 tail -f /dev/null

❗ Make sure to setup Genode’s build directory before continuing! ❗

Add the following to <genode>/build/x86_64/etc/build.conf:

REPOSITORIES += $(GENODE_DIR)/repos/wundertuete
REPOSITORIES += $(GENODE_DIR)/repos/snapper

Optionally add the Lwext plugin:

git clone -b 25.05-2025-07-07 --depth 1 https://codeberg.org/jws/genode-wundertuete.git genode/repos/wundertuete

Run this inside the container:

/genode/tool/ports/prepare_port lwext4
DEPOT_DIR=/genode/repos/snapper/var/depot goa depot-dir
DEPOT_DIR=/genode/repos/snapper/var/depot FORCE_VERSION=2025-07-09 /genode/tool/depot/create rumen/bin/x86_64/vfs_lwext4

mkdir /genode/repos/snapper/raw
dd if=/dev/zero of=/genode/repos/snapper/raw/snapper_block.raw bs=1M seek=16 count=0
mkfs.ext4 -O^metadata_csum -F /genode/repos/snapper/raw/snapper_block.raw

Running Snapper

If you are using the container, the following should be run inside the container.

With Goa

cd /genode/repos/snapper
goa run --pkg snapper

This will run the main Snapper binary. If instead you want to run the tests:

goa run --pkg snappertests

To specify the number of snapshot objects that should be in the test simulation (TESTS defaults to 1000):

TESTS=50 goa run --pkg snappertests

With Run Script

cd /genode
make -C build/x86_64 run/snapper KERNEL=linux BOARD=linux

Or, to run the tests:

make -C build/x86_64 run/snappertests KERNEL=linux BOARD=linux

To (optionally) specify how many test objects to create (see Running With Goa for more details):

TESTS=50 make -C build/x86_64 run/snappertests KERNEL=linux BOARD=linux

NOTE: On subsequent runs, if you want to change TESTS, you have to rm -rf /genode/build/x86_64/test/snapper/ before running.

Configuration of Snapper

Snapper should be configurable through Genode’s XML. The configuration options are stored in Snapper::Config:

<l10>	<c30>	<r50>
OPTION	TYPE	DESCRIPTION
verbose	bool	Whether to print verbose output.
threshold	unsigned int	The maximum number of files in a snapshot sub-directory.
integrity	bool	If true, crash the system on failed integrity checks,
		otherwise log a warning.
redundancy	unsigned int	After reaching this reference count, a redundant file
		copy will be created for subsequent snapshot.
max_snapshots	unsigned int	The maximum number of complete snapshots inside
		<snapper-root>.
min_snapshots	unsigned int	The minimum number of generations that need to be present
		for a purge to be possible.
expiration	unsigned int	How many seconds a generation should be kept.
	(seconds)