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Solène Rapenne 2025-01-22 23:22:06 +01:00
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developer/system/template-implementation.md
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@ -73,19 +73,9 @@ When TemplateVM is stopped, the xen script moves root-cow.img to root-cow.img.ol
#### Rollback template changes
There is possibility to rollback last template changes. Saved root-cow.img.old contains all changes made during last TemplateVM run. Rolling back changes is done by reverting this "binary patch".
There is possibility to rollback last template changes. Using the automatic snapshot that is normally saved every time the template is shutdown.
This is done using snapshot-merge device-mapper target (available from 2.6.34 kernel). It requires that no other snapshot device uses underlying block devices (root.img, root-cow.img via loop device). Because of this all AppVMs based on this template must be halted during this operation.
Steps performed by **qvm-revert-template-changes**:
1. Ensure that no other VMs uses this template.
2. Prepare snapshot device with ***root-cow.img.old*** instead of *root-cow.img* (*/etc/xen/scripts/block-snapshot prepare*).
3. Replace *snapshot* device-mapper target with *snapshot-merge*, other parameters (chunk size etc) remains untouched. Now kernel starts merging changes stored in *root-cow.img.old* into *root.img*. d-m device can be used normally (if needed).
4. Waits for merge completed: *dmsetup status* shows used snapshot blocks – it should be equal to metadata size when completed.
5. Replace *snapshot-merge* d-m target back to *snapshot*.
6. Cleanup snapshot device (if nobody uses it at the moment).
7. Move *root-cow.img.old* to *root-cow.img* (overriding existing file).
Refer to volume backup and revert [documentation](/doc/volume-backup-revert) for more information.
### Snapshot device in AppVM