VM Storage¶

Overview¶

ZFS provides two approaches for VM disk storage:

Approach	Use Case	Pros	Cons
Directory pool	Simple setups, qcow2 images	Easy management, flexible	Extra filesystem layer
Zvols	Performance, Windows VMs	Direct block device, better I/O	Less flexible

Directory-Based Storage¶

The simpler approach uses a ZFS dataset as a libvirt storage pool. VM images (qcow2) are stored as files.

This is already configured in KVM Setup:

virsh pool-define-as vm-pool dir - - - - /mnt/tank/vm
virsh pool-start vm-pool
virsh pool-autostart vm-pool

Zvol-Based Storage¶

Zvols are block devices backed by ZFS. VMs access them directly without a filesystem layer.

Create a Zvol¶

sudo zfs create -V 100G tank/vm/win11

The -V flag creates a volume (zvol) instead of a filesystem dataset.

Zvol Properties¶

Set properties at creation time:

sudo zfs create -V 100G \
    -o volblocksize=64K \
    -o compression=lz4 \
    tank/vm/win11

Property	Recommended	Notes
volblocksize	64K	Balance between performance and space efficiency
compression	lz4	Good for most workloads
sync	standard	Use `disabled` only for non-critical VMs

volblocksize is immutable

Once set, volblocksize cannot be changed. Choose carefully based on workload.

Access Zvol¶

Zvols appear as block devices:

ls -la /dev/zvol/tank/vm/

Output:

win11 -> /dev/zd0

libvirt Integration¶

Define Zvol Storage Pool¶

Create a pool for zvols:

<!-- zvol-pool.xml -->
<pool type='zfs'>
  <name>zvol-pool</name>
  <source>
    <name>tank/vm</name>
  </source>
</pool>

virsh pool-define zvol-pool.xml
virsh pool-start zvol-pool
virsh pool-autostart zvol-pool

Create VM with Zvol¶

Using virt-install:

virt-install \
    --name win11 \
    --ram 16384 \
    --vcpus 8 \
    --os-variant win11 \
    --disk path=/dev/zvol/tank/vm/win11,bus=virtio \
    --cdrom /path/to/win11.iso \
    --network network=default \
    --graphics spice \
    --boot uefi

Attach Zvol to Existing VM¶

virsh attach-disk win11 /dev/zvol/tank/vm/win11-data vdb --persistent

Snapshots¶

Snapshot Before Changes¶

Before Windows updates or risky changes:

sudo zfs snapshot tank/vm/win11@before-update

List Snapshots¶

zfs list -t snapshot -r tank/vm

Rollback¶

VM must be stopped

Always shut down the VM before rolling back.

virsh shutdown win11
sudo zfs rollback tank/vm/win11@before-update
virsh start win11

Clone VM¶

Create a copy of a VM disk:

sudo zfs snapshot tank/vm/win11@clone-base
sudo zfs clone tank/vm/win11@clone-base tank/vm/win11-test

Performance Considerations¶

When to Use Zvols¶

Windows VMs (better compatibility with NTFS)
Database VMs requiring consistent I/O
Performance-critical workloads

When to Use Directory Pool¶

Linux VMs with qcow2
VMs needing thin provisioning features
Simpler management requirements

Tuning¶

For database VMs:

sudo zfs create -V 50G \
    -o volblocksize=16K \
    -o compression=lz4 \
    -o primarycache=metadata \
    tank/vm/postgres

For general VMs:

sudo zfs create -V 100G \
    -o volblocksize=64K \
    -o compression=lz4 \
    tank/vm/ubuntu-server

Management Commands¶

# List zvols
zfs list -t volume

# Check zvol usage
zfs list -o name,volsize,used,refer -t volume

# Resize zvol (grow only recommended)
sudo zfs set volsize=150G tank/vm/win11

# Destroy zvol (VM must be removed first)
sudo zfs destroy tank/vm/win11