ZFS support in libvirt

An upcoming release of libvirt, 1.2.8 that should be released early September, will include an initial support of managing ZFS volumes.

That means that it's possible to boot VMs and use ZFS volumes as disks. Additionally, it allows to control volumes using the libvirt API. Currently, supported operations are:

• list volumes in a pool
• create and delete volumes
• upload and download volumes

It's not possible to create and delete pools yet, hope to implement that in the next release.

Defining a pool

Assume we have some pools and want to use one of them in libvirt:

# zpool list
NAME       SIZE  ALLOC   FREE   FRAG  EXPANDSZ    CAP  DEDUP  HEALTH  ALTROOT
filepool  1,98G  56,5K  1,98G     0%         -     0%  1.00x  ONLINE  -
test       186G  7,81G   178G     0%         -     4%  1.00x  ONLINE  -

Let's take filepool and define it with libvirt. This could be done using this virsh command:

virsh # pool-define-as --name zfsfilepool --source-name filepool --type zfs
Pool zfsfilepool defined

virsh #  pool-start zfsfilepool
Pool zfsfilepool started

virsh # pool-info zfsfilepool
Name:           zfsfilepool
UUID:           5d1a33a9-d8b5-43d8-bebe-c585e9450176
State:          running
Persistent:     yes
Autostart:      no
Capacity:       1,98 GiB
Allocation:     56,50 KiB
Available:      1,98 GiB

virsh # 

As you can see, we specify a type of the pool, its source name, such as seen in zpool list output and a name for it in libvirt. We also need to start it using the pool-start command.

Managing volumes

Let's create a couple of volumes in our new pool.

virsh # vol-create-as --pool zfsfilepool --name vol1 --capacity 1G
Vol vol1 created

virsh # vol-create-as --pool zfsfilepool --name vol2 --capacity 700M
Vol vol2 created

virsh # vol-list zfsfilepool
 Name                 Path                                    
------------------------------------------------------------------------------
 vol1                 /dev/zvol/filepool/vol1                 
 vol2                 /dev/zvol/filepool/vol2                 

virsh #

Dropping a volume is also easy:

virsh # vol-delete --pool zfsfilepool vol2
Vol vol2 deleted

Uploading and downloading data

Let's upload an image to our new volume:

virsh # vol-upload --pool zfsfilepool --vol vol1 --file /home/novel/FreeBSD-10.0-RELEASE-amd64-memstick.img 

... and download

virsh # vol-download --pool zfsfilepool --vol vol1 --file /home/novel/zfsfilepool_vol1.img

Note: if you would check e.g. md5 sum of the downloaded files, the result would be different as downloaded file will be of the same size as a volume. However, if you trim zeros, it'll be the same.

$ md5 FreeBSD-10.0-RELEASE-amd64-memstick.img zfsfilepool_vol1.img 
MD5 (FreeBSD-10.0-RELEASE-amd64-memstick.img) = e8e7cbd41b80457957bd7981452ecf5c
MD5 (zfsfilepool_vol1.img) = a77c3b434b01a57ec091826f81ebbb97
$ truncate -r FreeBSD-10.0-RELEASE-amd64-memstick.img zfsfilepool_vol1.img
$  md5 FreeBSD-10.0-RELEASE-amd64-memstick.img zfsfilepool_vol1.img             
MD5 (FreeBSD-10.0-RELEASE-amd64-memstick.img) = e8e7cbd41b80457957bd7981452ecf5c
MD5 (zfsfilepool_vol1.img) = e8e7cbd41b80457957bd7981452ecf5c
$

Booting a VM from volume

Finally got to the most important part. In use a volume as disk device for VM 'devices' section of the domain XML should be updated with something like this:

    <disk type='volume' device='disk'>
      <source pool='zfsfilepool' volume='vol1'/>
      <target dev='vdb' bus='virtio'/>
    </disk>

Few notes

Note #1: this code is just a few weeks old, so quite likely there are some rough edges. Feel free to report problems to novel%freebsd.org if you spot any problems.

Note #2: this code is FreeBSD-only for now. However, it should not be hard to make it work on Linux with zfsonlinux.org. Its developers were kind enough to add some useful missing flags in some of the CLI tools. However, these changes are not available in any released version so far. There are some more minor differences between zfs on Linux and FreeBSD, but that should not be hard to address. I was planning to get to it as soon as a new version of zfs on linux with the necessary flags is available. However, if you are interested in that and ready to help with testing -- feel free to poke me so it could be done sooner.