[Borgbackup] Reducing backup times for raw VM images, with zfs send?

[Mason Schmitt]

Today I primarily use Borg to backup sparse KVM raw disk image files and
soon I plan to use Borg to backup VM disks stored directly on ZFS zvols (in
a Proxmox environment).

My experience with backing up KVM sparse disk image files is that even if
there have been very few changes to the VMs (mostly just writes to files in
/var/log) backups still take several hours to a locally attached USB 3
disk.  Presumably this is because the full file needs to be analyzed to see
what has changed?

I'm wondering whether the new feature in 1.2, "create
--content-from-command", might be able to help reduce the time to create
backups by feeding Borg with a list of changed blocks?  Specifically, I'm
wondering if zfs send could be used as an input to borg create
--content-from-command?

Or will the improvements in sparse file support and the chunker make a
significant enough difference to backup times that I would be better off
using Borg's native functionality and saving myself the complexity of the
zfs send method?

-- 

Mason
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