dm-era is a Device Mapper target that acts as a proxy to an existing block device, like dm-linear, but also keeps track of which blocks were written to. It is included in mainline Linux kernel since 3.15.

Update: dm-era seems to be unsafe in kernel versions before 5.12 because of [https://www.spinics.net/lists/dm-devel/msg45023.html](two bugs) which were only fixed in 5.12.

era_copy and era_apply

era_copy parses dm-era metadata from era_invalidate output and saves changed blocks into a stream that you can save to a file or copy over network.

era_apply takes era_copy output and applies it to a file (or to a block device) to create a mirror of the original device.

With dm-era and these two small utilities you can perform incremental backups of raw block devices without the need for a COW FS, LVM or a storage hypervisor. dm-era almost doesn't hurt performance and seems to handle fsyncs correctly.

It's not a one-click solution, but it works :-)

How to try dm-era for incremental backups

Install dm-era tools (era_invalidate) with apt-get install thin-provisioning-tools on your target host.
Install era_copy and era_apply (make install from this repository) on both hosts (target host and backup host).
Setup a small partition for dm-era metadata. We'll use 1024 block (512 KB) granularity so a bitmap for 1 TB device will only take 8 MB on disk. dm-era keeps slightly more than one bitmap on disk at a time, but anyway, a 512 MB or 1 GB partition will be more than enough. For example you can shrink the main partition a bit and add the dm-era partition after it.
Zero out the new metadata partition: dd if=/dev/zero of=<META_PARTITION> bs=1048576
Copy dm-era.service to /etc/systemd/system and edit the ExecStart command. The syntax is: /sbin/dmsetup create ERA_DEVICE_NAME --table "0 SIZE_IN_SECTORS era METADATA_DEVICE DATA_DEVICE 1024".
Enable the unit: systemctl enable dm-era.
Change the desired /etc/fstab entry and reboot (or unmount the partition, stop services, mount it back and start services).
Do an initial full partition backup with block-level copy. For example, to copy an ext4 filesystem to another host over ssh run: ssh root@host "e2image -f -p -ra /dev/mapper/root_era - | gzip" | gzip -d | cp --sparse=always /dev/stdin rootfs.bin.
Now you can use backup.sh to perform incremental backups of the dm-era device over ssh from the backup host. Just change variables at the top of the script so it matches your device configuration.

dm-era on root partition (Debian, GRUB)

Copy zz_dm-era.sh to /etc/initramfs-tools/scripts/local-block and adjust DATA_DEVICE, META_DEVICE and ERA_DEVICE_NAME in it. Use partition IDs (/dev/disk/by-partuuid/* for GPT partitions, /dev/disk/by-id/md-uuid-* for mdadm, etc) to be safe because dm-era doesn't check if you supply correct partitions to it.
Add dm_era to /etc/initramfs-tools/modules.
Edit /etc/fstab and change your actual device to /dev/mapper/<ERA_DEVICE_NAME>, for example /dev/mapper/root_era.
Run update-initramfs -u -k all.
Change /etc/default/grub: set GRUB_CMDLINE_LINUX="root=/dev/mapper/root_era" and refresh grub config with update-grub.
Reboot.

Author and license

Vitaliy Filippov

GNU GPLv3.0 or later

Of course I don't take any responsibility if you kill your data while trying to setup dm-era :-)