There is a very good guide on how to install zfs on OMV in this forum here. Now I'm going to show you how to set it up step by step.(Assume your hardware meet zfs requirement).
First, you need gpt-based disk or use -f parameter while create zfs pool. In the following step by step guide, I will use tank as zfs pool name and storage as the name of dataset. I'm going to use 3 hard drives to create raidz array.
All right, let start. login to your server with ssh.
1. Create a raidz pool.
To make sure your zfs works on OMV, you have to use disk id to create pool.(get it from /dev/disk/by-id).
zpool create tank raidz -o ashift=12 scsi-SATA_XXXXXXXXXX1 scsi-SATA_XXXXXXXXXX2 scsi-SATA_XXXXXXXXXX3
2. Create a 500G zfs dataset in the pool.
zfs create -V 500G tank/storage
3. Partition the dataset.
sfdisk -q /dev/tank/storage << EOF
4. Format the dataset partition with 4k block (!!!This is The Most Important Part, otherwise your OMV won't see your formated dataset!!!)
To do this, you have to use -F parameter.(Force format)
mkfs.ext4 -F -b 4096 /dev/tank/storage-1
5. Now you can use webgui to mount your dataset.
Logon to webgui, click on Filesystems under Storage. highlight the new device you just created and click on mount button.
6. Create your share folder.
Go back to your ssh screen. Get into /media. You will see a new uuid. Make a directory to be shared inside this new uuid. Switch back to webgui, click on Shared Folders under Access Right Management. Click on Add button to bring up an Add shared Folder screen. Fill in the shared folder's Name, choose the new Volume, select the directory in the Path field, assign a permission and even make a comment then click OK button. Now your share folder is ready. You can setup other services with this folder in webgui.
Okay, that 's all for this simple step by step guide. I hope everybody has a Happy New year and Happy NASing in 2013!!