first of all, thank you for developing and supporting these awesome products.
I witnessed a similar issue upon running continuous replication on a VM after growing one of the disks on the source side. I run delta backup to a NFS share and continuous replication to another XCP-ng host in one job.
The most important part of the relevant log messages remind me of this issue - this topic is the only similar reference I was able to find, so I hope this is the right place to post. If not, please let me know, and I'll create a new topic
Mar 21 01:00:29 xcp-ng-2 xapi: [ info||8947241 INET :::80|Importing raw VDI R:3967654dad6b|vhd_tool_wrapper] Executing /bin/vhd-tool serve --source-format vhd --source-protocol none --source-fd 1f47b1fe-a9ea-4bda-ab8e-2101e5677059 --tar-filename-prefix --destination file:///dev/sm/backend/f7aca870-12e0-8352-9f86-f52d575eb313/6cd16448-6839-476d-93b1-2f8b816c827f --destination-format raw --progress --machine --direct --destination-size 0 --prezeroed Mar 21 01:00:29 xcp-ng-2 xapi: [error||8947241 INET :::80|Importing raw VDI R:3967654dad6b|vhd_tool_wrapper] vhd-tool failed, returning VDI_IO_ERROR Mar 21 01:00:29 xcp-ng-2 xapi: [error||8947241 INET :::80|Importing raw VDI R:3967654dad6b|vhd_tool_wrapper] vhd-tool output: vhd-tool: Non-zero size required (either a pre-existing destination file or specified via --destination-size on the command line)\x0A
I'm currently running Xen Orchestra compiled from sources, on commit 5bc44363f9ac9fa534f3247e697f1b3c6991e916. XCP-ng is on version 8.2.0.
I know that this is not fully up-to-date, but after retrying that backup as a first measure, Xen Orchestra ran a full backup for that VM, so I can't reproduce that issue anymore, but I figured posting that here might help resolving the aforementioned issue, if the causes correlate.