Just following up to see if anyone has any thoughts on a possible resolution?

@florent see below:

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            },
            {
              "id": "1759652614248",
              "message": "clean-vm",
              "start": 1759652614248,
              "status": "success",
              "warnings": [
                {
                  "data": {
                    "path": "/xo-vm-backups/fab1e47e-6b23-ac6e-2f6f-cadee708a13b/20251005T081219Z.json",
                    "actual": 20923285504,
                    "expected": 20928436736
                  },
                  "message": "cleanVm: incorrect backup size in metadata"
                }
              ],
              "end": 1759652616019,
              "result": {
                "merge": false
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          ],
          "end": 1759652616061
        }
      ],
      "warnings": [
        {
          "message": "Backup fell back to a full"
        }
      ],
      "infos": [
        {
          "message": "will delete snapshot data"
        },
        {
          "data": {
            "vdiRef": "OpaqueRef:ed0c85b7-0bcc-e3bd-6a35-a401c6f091d2"
          },
          "message": "Snapshot data has been deleted"
        }
      ],
      "end": 1759652616061
    },
    {
      "data": {
        "type": "VM",
        "id": "3e351bfb-d758-75c6-1a17-564e6560c537",
        "name_label": "Win11Pro1"
      },
      "id": "1759651206786",
      "message": "backup VM",
      "start": 1759651206786,
      "status": "success",
      "tasks": [
        {
          "id": "1759651206831",
          "message": "clean-vm",
          "start": 1759651206831,
          "status": "success",
          "end": 1759651207013,
          "result": {
            "merge": false
          }
        },
        {
          "id": "1759651207473",
          "message": "snapshot",
          "start": 1759651207473,
          "status": "success",
          "end": 1759651209165,
          "result": "0048397c-9151-c83e-04d5-878955f8bc62"
        },
        {
          "data": {
            "id": "fde89f48-d384-4208-8243-de5755ea7434",
            "isFull": false,
            "type": "remote"
          },
          "id": "1759651209166",
          "message": "export",
          "start": 1759651209166,
          "status": "success",
          "tasks": [
            {
              "id": "1759651214533",
              "message": "transfer",
              "start": 1759651214533,
              "status": "success",
              "end": 1759653069800,
              "result": {
                "size": 92809461760
              }
            },
            {
              "id": "1759653071177:0",
              "message": "health check",
              "start": 1759653071177,
              "status": "success",
              "tasks": [
                {
                  "id": "1759653071187",
                  "message": "transfer",
                  "start": 1759653071187,
                  "status": "success",
                  "end": 1759653899955,
                  "result": {
                    "size": 0,
                    "id": "566c0b93-d3a0-b69c-351e-0d371f79c098"
                  }
                },
                {
                  "id": "1759653899956",
                  "message": "vmstart",
                  "start": 1759653899956,
                  "status": "success",
                  "end": 1759653903489
                }
              ],
              "end": 1759653906216
            },
            {
              "id": "1759653906330",
              "message": "clean-vm",
              "start": 1759653906330,
              "status": "success",
              "warnings": [
                {
                  "data": {
                    "path": "/xo-vm-backups/3e351bfb-d758-75c6-1a17-564e6560c537/20251005T080014Z.json",
                    "actual": 92809461760,
                    "expected": 92832646656
                  },
                  "message": "cleanVm: incorrect backup size in metadata"
                }
              ],
              "end": 1759653906510,
              "result": {
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          "end": 1759653906511
        }
      ],
      "warnings": [
        {
          "message": "Backup fell back to a full"
        }
      ],
      "infos": [
        {
          "message": "will delete snapshot data"
        },
        {
          "data": {
            "vdiRef": "OpaqueRef:b974e97e-1b63-cb25-1ba1-01c5ef9d9977"
          },
          "message": "Snapshot data has been deleted"
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    }
  ],
  "end": 1759653906511
}

@olivierlambert It is an NFS share on a Synology NAS

The error is no longer received when we disable "Purge snapshot data when using CBT".

We recently updated Xen Orchestra to d77d6d0d9. When performing delta backups we started receiving the following "Backup fell back to a full" error:

We are using the NBD backup option, and have selected to have CBT snapshot data purged. This affects every VM. We have done testing and have tried to removing all backups, along with the task, and start over from fresh. With the second backup, and every backup there after, this error is received. And we have confirmed a full backup is being taken. We upgraded XO to the latest d7fe4f172, and this has not resolved the issue either.

Any thoughts on steps to resolve this issue? We are seeing it on two separate systems.