[Bug 243730] ZFS incompatibility send/receive with Ubuntu 19.10 to FreeBSD 12.x

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=243730

            Bug ID: 243730
           Summary: ZFS incompatibility send/receive with Ubuntu 19.10 to
                    FreeBSD 12.x
           Product: Base System
           Version: 12.1-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: lammert.nijhof at gmail.com

Since June I use send/receive to send incremental backups from my Ubuntu
desktop to my FreeBSD 12.x backup-server. After installing Ubuntu 19.10 with
root on zfs, the weekly incremental backup failed. After some try outs with
help of the community I detected that both systems had the feature
large-dnode=active. However Ubuntu had dnodesize=auto and FreeBSD had
dnodesize=legacy. Those values were chosen by both installers and not by me. 

I'm dealing with relative large incremental backups of ~250 GB of Virtual
Machines, so an incremental backup takes 30-60 minutes. The performance
bottleneck is the Pentium 4 (FreeBSD) running at 95% CPU load, while
transferring at 200 Mbps.

On FreeBSD I see the size of the dataset increase nicely (Conky using zfs
list). When it reaches the end of transfer the snapshot with all data just
transferred is destroyed and Ubuntu gives the error message. 

After changing all relevant dnodesizes on Ubuntu to legacy and reloading all
data in those datasets, my weekly incremental backups worked again.

I filed an Ubuntu bug-report long ago
https://bugs.launchpad.net/bugs/1854982

Except for helping with the bypass, nothing happened and the 2 organization
don't seem to talk to each other. I've tried 12 different combinations of
send/receive, but the essence of the incompatibility is described here.

Ubuntu and FreeBSD did choose different defaults for large-dnodes and
dnodesizes, but to solve bugs related to feature incompatibility you have to
communicate! The problem will not disappear completely, because you start using
the same source. There will be probably months between release dates, so
feature incompatibility probably will remain an issue.

My problems is bypassed, but the default dnodesize incompatibility between
Ubuntu 19.10 and FreeBSD 12.1 remains.

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