iSCSI target + multipath lab

This lab builds a small iSCSI fabric with two paths and verifies that Proxmox survives the loss of either path without I/O interruption.

Environment

Steps

1. Configure the iSCSI target (tgt-host)

apt install -y targetcli-fb

# Create a backing store
targetcli /backstores/block create lun0 /dev/sdb
targetcli /backstores/block create lun1 /dev/sdc

# Create an iSCSI target with two portals
targetcli /iscsi create iqn.2026-08.lab.runbook:tgt1
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1/portals create 10.0.10.1
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1/portals create 10.0.11.1

# Map LUNs and enable CHAP
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1/luns create /backstores/block/lun0
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1/luns create /backstores/block/lun1
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1 set attribute authentication=1
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1/acls create iqn.2026-08.lab.runbook:pve1
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1/acls/iqn.2026-08.lab.runbook:pve1 set auth userid=pveuser
targetcli /iscsi/iqn.2026-08.lab.runbook:tgt1/tpg1/acls/iqn.2026-08.lab.runbook:pve1 set auth password=CHAPpassword
targetcli saveconfig

2. Discover and login from Proxmox

# On pve-host
iscsiadm -m discovery -t sendtargets -p 10.0.10.1
iscsiadm -m node -T iqn.2026-08.lab.runbook:tgt1 -o update -n node.session.auth.authmethod -v CHAP
iscsiadm -m node -T iqn.2026-08.lab.runbook:tgt1 -o update -n node.session.auth.username -v pveuser
iscsiadm -m node -T iqn.2026-08.lab.runbook:tgt1 -o update -n node.session.auth.password -v CHAPpassword
iscsiadm -m node -T iqn.2026-08.lab.runbook:tgt1 --login

Confirm both paths are visible:

iscsiadm -m session -P 1
# Expected: 2 sessions (one per portal)

3. Configure multipath

apt install -y multipath-tools
cat > /etc/multipath.conf << 'EOF'
defaults {
  user_friendly_names yes
  find_multipaths yes
}
EOF
systemctl enable --now multipathd
multipath -ll
# Expected: one dm device per LUN with 2 active paths

4. Add iSCSI storage to Proxmox

In the GUI: Datacenter → Storage → Add → iSCSI. Use the multipath device (/dev/mapper/mpath0) as the LUN path.

5. Create a test VM on the iSCSI LUN

qm create 901 --name iscsi-test --memory 2048 --cores 2 \
  --scsi0 iscsilun:32 --net0 virtio,bridge=vmbr0 --ostype l26
qm start 901

6. Test failover

Run I/O inside the VM:

# Inside VM
fio --name=randwrite --ioengine=libaio --iodepth=32 \
  --rw=randwrite --bs=4k --direct=1 --size=1G --filename=/tmp/fiotest

Pull one NIC on the target (or ip link set eth0 down on the target):

# On tgt-host
ip link set eth0 down

The fio job should continue with slightly elevated latency but no errors. Check multipath status from PVE:

multipath -ll
# Expected: 1 active, 1 failed path

Restore the link:

# On tgt-host
ip link set eth0 up
multipath -ll
# Expected: 2 active paths again

Verification

• Multipath shows 2 paths per LUN, both active
• VM I/O continues during a single-path outage
• After path recovery, multipath re-balances automatically

Cleanup

qm stop 901 && qm destroy 901
iscsiadm -m node -T iqn.2026-08.lab.runbook:tgt1 --logout
# On tgt-host: targetcli clearconfig confirm=True