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Will enabling Backup Fleecing help reduce the load on guest VMs?

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Yes, enabling Backup Fleecing can help reduce the load on guest VMs. Backup Fleecing offloads the data backup process from the guest VMs, which can free up resources within the VM. By minimizing the processing and storage demands on guest VMs, Backup Fleecing can improve performance, especially during backup operations.
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When a VM backup starts, QEMU installs a "copy-before-write" filter in its block layer. This filter ensures that before new guest writes occur, any old data still needed for the backup is first sent to the backup target. The guest write operation is paused until this transfer completes, meaning that guest IO for sectors not yet backed up will be limited by the backup target's speed.

With Backup Fleecing, however, this old data is cached in a fleecing image instead of being sent immediately to the backup target. This approach can improve guest IO performance and even prevent potential hangs, though it does require additional storage space.
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