geom_rowr kernel module

The geom_rowr kernel module for FreeBSD makes a read-only device writable by combining it with a read-write device.


This kernel module is currently under development and is being tested for adoption in helloSystem.

Use case

When creating a Live ISO, we want to use a read-only filesystem (that can reside on a medium that is physically read-only, such as a DVD) but make it appear to the system as if it was read-write (since some parts of the system may not work on a read-only system).

Theory of operation

geom_rowr is a FreeBSD kernel module using the GEOM Modular Disk Transformation Framework. It allows you to take a read-only device (e.g. a DVD) and combine it with a read-write device (e.g., a swap-backed md memory disk) to get a new device that contains the data from the read-only device but is writable.

It is designed to work with any filesystem, including ZFS. It is compatible with geom_uzip so the read-only filesystem can be compressed with uzip.

The principle of operation is as follows:

  • Reading a sector from the combined device reads it from the read-only device. Writing a sector writes it to the read-write device

  • After a sector is written, future reads of that sector are satisfied from the read-write device as it contains the latest version of the sector’s contents

As a result, immediately after creating the combined device, it appears to contain all the data from the read-only device. This means zero wait time for the user.


Execute the following commands as root:

// Prepare filesystem image
# dd if=/dev/zero of=test.img bs=1M count=512
# mdconfig -a -t vnode -f test.img -u 9
# newfs /dev/md9
# mount /dev/md9 /mnt
// Populate the filesystem
# cp -R /boot/ /mnt
# umount /mnt
# mdconfig -d -u 9

// Load kernel module
# kldload ./geom_rowr.ko
// prepare the read-only device md0 
# mdconfig -a -t vnode -o readonly -f test.img -u 0
// prepare the read-write device md1
# mdconfig -a -t swap -s 512M -u 1
// Make sure the geom(8) tool will find the .so
# cp /lib/geom
// Create the new device that combines md0 and md1
# geom rowr create rowr0 md0 md1
# mount /dev/rowr0 /mnt

// See that the files are already there
# ls /mnt
// make changes to the filesystem to see that it is read-write
# mv /mnt/*.4th /mnt/lua; rm /mnt/pxeboot

Use in the Live ISO boot process

This can be used for a Live ISO like this:

  • On the ISO, there is a small ramdisk.ufs that contains the logic to mount the main filesystem using geom_rowr

  • On the ISO, there is also the main filesystem system.ufs which is a UFS image compressed with geom_uzip

  • At boot time, the code in the ramdisk creates a r/w swap-backed md device with the same size as the main r/o filesystem image, mounts it together with the main filesystem using geom_rowr, and chroots into the combined r/w filesystem. From there, the boot process continues as usual


Device size

  • The r/o image must have as much free space as you would like to have on the combined device. This can be achieved, e.g., like this: makefs -b 20% -f 20% /usr/local/furybsd/cdroot/data/system.ufs /usr/local/furybsd/uzip which will add an extra 20% of free space to the main filesystem image

  • The free space is compressed very efficiently when the image is compressed with geom_uzip. This can be achieved, e.g., like this: mkuzip -o /usr/local/furybsd/cdroot/data/system.uzip /usr/local/furybsd/cdroot/data/system.ufs

  • The uncompressed r/o image and the r/w device need to be the exact same size in bytes. Assuming variable ${x} holds the size of the uncompressed r/o image in bytes, a matching r/w device with the same size in bytes can be created using mdconfig -a -t swap -s ${x}b -u 2 (note the b suffix to the -s option). If this is not done correctly, we get the geom: mediasize mismatch error

You can have an md(4) device be of size 3GB without it consuming 3GB of RAM. This is valid for vnode-backed md devices, swap-backed md devices and malloc-backed when the reserve option to mdconfig(8) isn’t used.

A swap-backed md device consumes close to zero memory when created and grows its memory use the more you write to it.


Because reroot cannot be made to work with geom_rowr at this time, we need to use chroot.

When using chroot, some kernel modules that load firmware (e.g., for Intel WLAN drivers) fail to load the firmware if the places where kernel modules are located - /boot/kernel and /boot/modules - will look differently inside vs. outside the chroot.

This can be fixed.

Recommended way (to be tested):

Provided that ramdisk.ufs does not contain /boot, create a symlink on ramdisk.ufs from /boot to /sysroot/boot. The symlink can be made at image creation time or by a script that runs at runtime. This combined with the default kern.module_path should be enough to create a common view between the kernel and chrooted userland utilities. The places where kernel modules are located - /boot/kernel and /boot/modules - will look the same inside and outside of the chroot.

Alternative way (known to work):

# mkdir -p /sysroot/sysroot/boot # Needed?
# mount -t nullfs /sysroot/boot /sysroot/sysroot/boot # Needed?

# echo "==> Set kernel module path for chroot"
# sysctl kern.module_path=/sysroot/boot/kernel


With ZFS there is a problem unrelated to the geom_rowr code. When ZFS performs discovery of devices which are part of a zpool, the read-only device could be found before the combined rowr device which leads to inability to import the pool read-write.

(TODO: Insert tested workaround here)


geom_rowr.ko, Copyright (c) 2021, Jordan Gordeev

The author states that once the code reaches a certain level of maturity, it will be published under the license used by the FreeBSD project.