This is a compilation of several posts I found while researching this procedure and I wanted to save the steps. I’m sure I’ll need them again.

My “MAC” is running 10.10.5 and the NFS server is Ubuntu Linux 14.04 with ZOL.

Set up the server first.

1. Create a new zfs file system

   % sudo zfs create tank/home/tom/timemachine

2. Ensure it’s NFS shared

   % sudo vi /etc/exports && exportfs -a

Now, on to the “MAC” after mounting the NFS share.

1. Create the sparse disk image.

   % hdiutil create -size 500g -type SPARSEBUNDLE -fs "HFS+J" -volname "Time Machine" \
   /net/NFS-SERVER/home/tom/timemachine/time-machine.sparsebundle

2. Mount the “disk”

   % open /net/NFS-SERVER/home/tom/timemachine/time-machine.sparsebundle

3. Tell Time Machine to use this volume

   % sudo tmutil setdestination "/Volumes/Time Machine"

The default zfs compression is lz4 and it does a decent job.

$ sudo zfs get compression,compressratio,used,logicalused tank/home/tom/timemachine
NAME                       PROPERTY       VALUE     SOURCE
tank/home/tom/timemachine  compression    lz4       inherited from tank
tank/home/tom/timemachine  compressratio  1.76x     -
tank/home/tom/timemachine  used           117G      -
tank/home/tom/timemachine  logicalused    203G      -

The “MAC” thinks it’s used a bit more.

$ df -h /Volumes/Time\ Machine
Filesystem     Size   Used  Avail Capacity  iused    ifree %iused  Mounted on
/dev/disk5s2  500Gi  212Gi  288Gi    43% 55487494 75500526   42%   /Volumes/Time Machine

One closing note… My testing has shown it’s easy to damage the disk image by rebooting the “MAC” whilst the disk is open. I’d suggest ejecting the disk prior to rebooting to prevent this.