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@ -3,6 +3,9 @@ |
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dm-era is a Device Mapper target that acts as a proxy to an existing block device, like dm-linear, |
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but also keeps track of which blocks were written to. It is included in mainline Linux kernel since 3.15. |
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Update: dm-era seems to be unsafe in kernel versions before 5.12 because of |
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[https://www.spinics.net/lists/dm-devel/msg45023.html](two bugs) which were only fixed in 5.12. |
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# era_copy and era_apply |
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era_copy parses dm-era metadata from `era_invalidate` output and saves changed blocks into a stream |
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@ -15,34 +18,40 @@ With dm-era and these two small utilities you can perform incremental backups of |
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without the need for a COW FS, LVM or a storage hypervisor. dm-era almost doesn't hurt performance |
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and seems to handle fsyncs correctly. |
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# How to try dm-era for incremental backups (Debian, GRUB) |
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It's not a one-click solution, but it works :-) |
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# How to try dm-era for incremental backups |
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1. Setup a small partition for dm-era metadata. We'll use 1024 block (512 KB) granularity |
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1. Install dm-era tools (`era_invalidate`) with `apt-get install thin-provisioning-tools` on your target host. |
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2. Install `era_copy` and `era_apply` (`make install` from this repository) on both hosts (target host and backup host). |
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3. Setup a small partition for dm-era metadata. We'll use 1024 block (512 KB) granularity |
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so a bitmap for 1 TB device will only take 8 MB on disk. dm-era keeps slightly more than one bitmap |
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on disk at a time, but anyway, a 512 MB or 1 GB partition will be more than enough. |
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For example you can shrink the main partition a bit and add the dm-era partition after it. |
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2. Zero out the new metadata partition: `dd if=/dev/zero of=<META_PARTITION> bs=1048576` |
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3. Copy `zz_dm-era.sh` to `/etc/initramfs-tools/scripts/local-block` |
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4. Zero out the new metadata partition: `dd if=/dev/zero of=<META_PARTITION> bs=1048576` |
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5. Copy `dm-era.service` to `/etc/systemd/system` and edit the ExecStart command. |
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The syntax is: `/sbin/dmsetup create ERA_DEVICE_NAME --table "0 SIZE_IN_SECTORS era METADATA_DEVICE DATA_DEVICE 1024"`. |
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6. Enable the unit: `systemctl enable dm-era`. |
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7. Change the desired /etc/fstab entry and reboot (or unmount the partition, stop services, mount it back and start services). |
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8. Do an initial full partition backup with block-level copy. For example, to copy an ext4 filesystem |
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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`. |
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9. Now you can use `backup.sh` to perform incremental backups of the dm-era device |
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over ssh from the backup host. Just change variables at the top of the script so it matches |
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your device configuration. |
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# dm-era on root partition (Debian, GRUB) |
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1. Copy `zz_dm-era.sh` to `/etc/initramfs-tools/scripts/local-block` |
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and adjust `DATA_DEVICE`, `META_DEVICE` and `ERA_DEVICE_NAME` in it. |
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Use partition IDs (`/dev/disk/by-partuuid/*` for GPT partitions, `/dev/disk/by-id/md-uuid-*` for mdadm, etc) |
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to be safe because dm-era doesn't check if you supply correct partitions to it. |
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4. Add `dm_era` to `/etc/initramfs-tools/modules`. |
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5. Edit `/etc/fstab` and change your actual device to `/dev/mapper/<ERA_DEVICE_NAME>`, |
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2. Add `dm_era` to `/etc/initramfs-tools/modules`. |
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3. Edit `/etc/fstab` and change your actual device to `/dev/mapper/<ERA_DEVICE_NAME>`, |
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for example `/dev/mapper/root_era`. |
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6. Repeat it for more partitions if you want. |
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7. Run `update-initramfs -u -k all`. |
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8. If you do it for the root partition also change `/etc/default/grub`: |
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4. Run `update-initramfs -u -k all`. |
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5. Change `/etc/default/grub`: set |
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`GRUB_CMDLINE_LINUX="root=/dev/mapper/root_era"` and refresh grub config with `update-grub`. |
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9. Reboot. |
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10. Install dm-era tools (`era_invalidate`) with `apt-get install thin-provisioning-tools` on your target host. |
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11. Install `era_copy` and `era_apply` (`make install` from this repository) on both hosts (target host and backup host). |
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12. Do an initial full partition backup with block-level copy. For example, to copy an ext4 filesystem |
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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`. |
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13. Now you can use `backup.sh` to perform incremental backups of the dm-era device |
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over ssh from the backup host. Just change variables at the top of the script so it matches |
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your device configuration. |
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It's not a one-click solution, but it works :-) |
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6. Reboot. |
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# Author and license |
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