|Vitaliy Filippov fd1685098a|
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 (
apt-get install thin-provisioning-toolson your target host.
make installfrom 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
/etc/systemd/systemand 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.shto 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)
ERA_DEVICE_NAMEin 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.
/etc/fstaband change your actual device to
/dev/mapper/<ERA_DEVICE_NAME>, for example
update-initramfs -u -k all.
GRUB_CMDLINE_LINUX="root=/dev/mapper/root_era"and refresh grub config with
Author and license
GNU GPLv3.0 or later
Of course I don't take any responsibility if you kill your data while trying to setup dm-era :-)