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Notes, systemd unit

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Vitaliy Filippov 8 months ago
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  1. 47
      README.md
  2. 14
      dm-era.service

47
README.md

@ -3,6 +3,9 @@
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
@ -15,34 +18,40 @@ With dm-era and these two small utilities you can perform incremental backups of
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.
# How to try dm-era for incremental backups (Debian, GRUB)
It's not a one-click solution, but it works :-)
# How to try dm-era for incremental backups
1. Setup a small partition for dm-era metadata. We'll use 1024 block (512 KB) granularity
1. Install dm-era tools (`era_invalidate`) with `apt-get install thin-provisioning-tools` on your target host.
2. Install `era_copy` and `era_apply` (`make install` from this repository) on both hosts (target host and backup host).
3. 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.
2. Zero out the new metadata partition: `dd if=/dev/zero of=<META_PARTITION> bs=1048576`
3. Copy `zz_dm-era.sh` to `/etc/initramfs-tools/scripts/local-block`
4. Zero out the new metadata partition: `dd if=/dev/zero of=<META_PARTITION> bs=1048576`
5. 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"`.
6. Enable the unit: `systemctl enable dm-era`.
7. Change the desired /etc/fstab entry and reboot (or unmount the partition, stop services, mount it back and start services).
8. 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`.
9. 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)
1. 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.
4. Add `dm_era` to `/etc/initramfs-tools/modules`.
5. Edit `/etc/fstab` and change your actual device to `/dev/mapper/<ERA_DEVICE_NAME>`,
2. Add `dm_era` to `/etc/initramfs-tools/modules`.
3. Edit `/etc/fstab` and change your actual device to `/dev/mapper/<ERA_DEVICE_NAME>`,
for example `/dev/mapper/root_era`.
6. Repeat it for more partitions if you want.
7. Run `update-initramfs -u -k all`.
8. If you do it for the root partition also change `/etc/default/grub`:
4. Run `update-initramfs -u -k all`.
5. Change `/etc/default/grub`: set
`GRUB_CMDLINE_LINUX="root=/dev/mapper/root_era"` and refresh grub config with `update-grub`.
9. Reboot.
10. Install dm-era tools (`era_invalidate`) with `apt-get install thin-provisioning-tools` on your target host.
11. Install `era_copy` and `era_apply` (`make install` from this repository) on both hosts (target host and backup host).
12. 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`.
13. 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.
It's not a one-click solution, but it works :-)
6. Reboot.
# Author and license

14
dm-era.service

@ -0,0 +1,14 @@
[Unit]
Description=dm-era for /var
DefaultDependencies=no
StartLimitInterval=0
Before=local-fs-pre.target
DefaultDependencies=no
Conflicts=shutdown.target
[Service]
Type=oneshot
ExecStart=/sbin/modprobe dm_era ; /sbin/dmsetup create md2p4_era --table "0 3625547743 era /dev/md2p3 /dev/md2p4 1024"
[Install]
WantedBy=sysinit.target
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