zfs send of zvol creates new zvol but no data is sent

[minsonj2016]

zfs-2.2.2-0ubuntu9.4
zfs-kmod-2.2.2-0ubuntu9.3
kubuntu 24.04

I created a zvol for a vm

zfs create -V 40gb ZData1/win10-vol1

I then dd'ed my existing raw vm image to it . This works and the VM runs with no complaints

I want to 'zfs send/recv' to another zpool so I create a snapshot of the zvol

zfs snapshot ZData1/win10-vol1@init-09012025

Then I

zfs send -R -v ZData1/win10-vol1@init-09012025 | zfs recv ZBackup7/win10-vol1

No complaints but no data is sent , only thing that happens is ZBackup7/win10-vol1 is created . The original zvol is 40GB yet the commands completes instantly .

I'm either doing some thing wrong or do not understand some aspect of this .

I have other 'send/recv' operations with datasets working with no issues

?

[filip-paczynski]

Strange indeed. Can you try .... | zfs recv -v ....? This should output some info.
Also what does zfs list -r -tall ZBackup7/win10-vol1 say?
How do You know that no data was transferred?

Just a wild shot, but I have previously encountered some problems with auto-mounters, especially ones integrated into desktop uis.
When You connect, for example, an USB flash stick, does it automatically mount?
Can You disable this functionality?

[minsonj2016]

Well I started over

zfs send -v -R ZData1/win10-vol1@init-09022025 | zfs recv -v ZBackup7/win10-vol1
full send of ZData1/win10-vol1@init-09022025 estimated size is 6.61K
total estimated size is 6.61K
TIME SENT SNAPSHOT ZData1/win10-vol1@init-09022025
receiving full stream of ZData1/win10-vol1@init-09022025 into ZBackup7/win10-vol1@init-09022025
received 3.24K stream in 0.43 seconds (7.51K/sec)

so then if I look at the new zvol

zfs list -r -tall ZBackup7/win10-vol1
NAME USED AVAIL REFER MOUNTPOINT
ZBackup7/win10-vol1 40.6G 1.57T 56K -
ZBackup7/win10-vol1@init-09022025 0B - 56K -

ls -l /ZBackup7/win10-vol1
-rw-r--r-- 1 libvirt-qemu rdma 42949672960 Sep 2 13:54 /ZBackup7/win10-vol1

That looks correct and in fact the VM works .

However it could not have sent 40GB in .43 seconds .
Maybe I dont understand .
It looks like it just sent the snapshot which is referring back to the source zvol .

The 2 zvols are on 2 physically different devices

Very confused at this point

[filip-paczynski]

Welcome to the magic of ZFS!

You can have 40GB zvol, but ZFS knows what was written to it, and it knows that only what was written needs to be transferred. This also applies to resilvering - ZFS will only checksum/synchronize things that actually hold data - not the whole device.

[minsonj2016]

Well I sent my windows 11 vm (60G) , destroyed the source zvol , change qemu to look at the new zvol and it worked .
I still cant believe an entire windows 11 vm gets transferred in .07 seconds :}

[filip-paczynski]

Well, the output of zfs list only mentioned 56k actually written. So perhaps this is not the main VM disk?
Even enterprise-level nvmes will struggle to achieve such rates.

[filip-paczynski]

Just to be clear: there is no inter-pool / between-pools kind of dependencies with ZFS. These are simply impossible by design.