Re: zfs support in makefs

From: Brooks Davis <>
Date: Wed, 18 May 2022 23:04:27 UTC
On Wed, May 18, 2022 at 03:03:17PM -0400, Mark Johnston wrote:
> Hi,
> For the past little while I've been working on ZFS support in makefs(8).
> At this point I'm able to create a bootable FreeBSD VM image, using the
> standard FreeBSD ZFS layout, and run through the regression test suite
> in bhyve.  I've also been able to create and boot an EC2 AMI.

Very cool!

> === Interface ===
> Creating a pool with a single dataset is easy:
> $ makefs -t zfs -s 10g -o poolname=test ./zfs.img /path/to/input
> Upon importing such a pool, you'll get a dataset named "test" mounted at
> /test containing everything under /path/to/input.
> It's possible to set properties on the root dataset:
> $ makefs -t zfs -s 10g -o poolname=test -o fs=test:setuid=off:atime=on ./zfs.img /path/to/input
> It's also possible to create additional datasets:
> $ makefs -t zfs -s 10g -o poolname=test -o fs=test/ds1:mountpoint=/test/dir1 ./zfs.img /path/to/input
> The parameter syntax is
> "-o fs=<dataset name>[:<prop1>=<val1>[:<prop2>=<val2>[:...]]]".  Only a
> few properties are supported, at least for now.
> Dataset mountpoints behave the same as they would if created with the
> standard ZFS tools.  So by default the root dataset's mountpoint is
> /test, test/ds1's mountpoint is /test/ds1, etc..  If a dataset overrides
> its default mountpoint, its children inherit that mountpoint.
> makefs builds the output filesystem using a single input directory tree.
> Thus, makefs -t zfs requires that at least one of the dataset's
> mountpoints map to /path/to/input; that is, there is a "root" mount
> point.
> The -o rootpath parameter defines this root mount point.  By default it's
> "/<poolname>".  All datasets in the pool must have their mountpoints
> under this path, and one dataset's mountpoint must be equal to this
> path.  To build bootable images, one sets -o rootpath=/.
> Putting it all together, one can build a image using the standard layout
> with an invocation like this:
> makefs -t zfs -o poolname=zroot -s 20g -o rootpath=/ -o bootfs=zroot/ROOT/default \
>     -o fs=zroot:canmount=off:mountpoint=none \
>     -o fs=zroot/ROOT:mountpoint=none \
>     -o fs=zroot/ROOT/default:mountpoint=/ \
>     -o fs=zroot/tmp:mountpoint=/tmp:exec=on:setuid=off \
>     -o fs=zroot/usr:mountpoint=/usr:canmount=off \
>     -o fs=zroot/usr/home \
>     -o fs=zroot/usr/ports:setuid=off \
>     -o fs=zroot/usr/src \
>     -o fs=zroot/usr/obj \
>     -o fs=zroot/var:mountpoint=/var:canmount=off \
>     -o fs=zroot/var/audit:setuid=off:exec=off \
>     -o fs=zroot/var/crash:setuid=off:exec=off \
>     -o fs=zroot/var/log:setuid=off:exec=off \
>     -o fs=zroot/var/mail:atime=on \
>     -o fs=zroot/var/tmp:setuid=off \
>     ${HOME}/tmp/zfs.img ${HOME}/tmp/world
> I'll admit this is somewhat clunky, but it doesn't seem worse than what
> we have to do otherwise, see poudriere-image for example:
> What do folks think of this interface?  Is there anything missing, or
> anything that doesn't make sense?

I find it slightly confusing that -o options have a default namespace of
pool options unless they have an fs=*: prefix, but making users type
"pool:" for other options doesn't seem to make sense so this is probably
the best solution.

The density of data in the filesystem specification does suggest that
someone might want to create a UCL config file format eventually, but
what's here already seems entirely workable.

-- Brooks