[Bug?] Load Balancer: Performance mode fails to power on hosts previously shut down by Density mode

[frb94]

[Bug?] Load Balancer: Performance mode fails to power on hosts previously shut down by Density mode

Hi everyone,

I'm experiencing a possible bug with the Xen Orchestra Load Balancer plugin when switching from Density to Performance mode.

Density successfully consolidates VMs and shuts down an unused host, but switching back to Performance does not automatically power that host on, even though Wake-on-LAN is correctly configured and works manually.

After investigating the source code, I suspect that the host filtering logic prevents Performance mode from detecting hosts that were disabled during shutdown.

Environment

• Xen Orchestra: Built from source
• Build: (November 10, 2026) Xen Orchestra, commit 03e93 | Master, commit 83c3e
• Plugin: xo-server-load-balancer
• Storage: Shared storage, live VM migrations working
• Power-on method: Wake-on-LAN

The pool contains three hosts:

Host	Role	Load Balancer configuration
XCPNG1	Additional host	Excluded from the plan
XCPNG2	Pool Master	Always running
XCPNG3	Secondary host	Managed by Load Balancer

Xen Orchestra itself runs as a VM on the Pool Master (XCPNG2).

Steps to reproduce

1. Start XCPNG2 and XCPNG3.
2. Set the Load Balancer plan to Performance.
3. Plugin distributes VMs between both hosts (approximately 50/50).
4. Switch the plan to Density.
5. Density successfully migrates the VMs from XCPNG3 to XCPNG2 and automatically shuts down XCPNG3.
6. Switch the same plan back to Performance.
7. Save the configuration and wait for multiple Load Balancer execution cycles.

Expected behavior

According to the official Xen Orchestra Load Balancing documentation, Performance mode should automatically power on halted hosts when a power-on mode is configured.

XCPNG3 should therefore power on automatically.

Actual behavior

XCPNG3 remains powered off.

No automatic power-on occurs.

However, clicking the Play / Power on button for XCPNG3 in the Xen Orchestra UI successfully starts the host.

The issue does not appear to be related to Wake-on-LAN connectivity.

Load Balancer configuration

• Mode: Performance
• Performance behavior: Conservative
• CPU critical threshold: 90%
• RAM free critical threshold: 10000 MB
• Migration cooldown: 1 minute
• Maximum concurrent migrations: 1
• Excluded hosts: XCPNG1

The same pool and plan work correctly in Density mode.

Investigation

1. XAPI power-on mode is configured

Running the following command on the Pool Master:

xe host-list params=name-label,uuid,power-on-mode

Returns:

XCPNG3
power-on-mode: wake-on-lan

XCPNG1
power-on-mode: wake-on-lan

XCPNG2
power-on-mode: wake-on-lan

2. XCPNG3 is disabled after Density shuts it down

Running:

xe host-list \
  uuid=dd2cec0b-7849-412e-a040-e2826c35c6cc \
  params=name-label,enabled,power-on-mode

Returns:

name-label    : XCPNG3
enabled       : false
power-on-mode : wake-on-lan

3. Performance mode attempts to power on halted hosts

In:

packages/xo-server-load-balancer/src/performance-plan.js

The execute() method begins with:

async execute() {
  // Try to power on a hosts set.
  try {
    await Promise.all(
      filter(
        this._getHosts({ powerState: 'Halted' }),
        host => host.powerOnMode !== ''
      ).map(host => {
        const { id } = host
        return this.xo.getXapi(id).powerOnHost(id)
      })
    )
  } catch (error) {
    console.error(error)
  }
}

This suggests that Performance should attempt to power on halted hosts independently of CPU/RAM thresholds or migration cooldown.

4. Potential issue in _getHosts()

In:

packages/xo-server-load-balancer/src/plan.js

Lines 256–265:

_getHosts({ powerState = 'Running' } = {}) {
  return filter(
    this.xo.getObjects(),
    object =>
      object.type === 'host' &&
      includes(this._poolIds, object.$poolId) &&
      object.power_state === powerState &&
      !includes(this._excludedHosts, object.id) &&
      object.enabled === true
  )
}

object.enabled === true

If a halted host has enabled: false, _getHosts({ powerState: 'Halted' }) appears to exclude it before Performance can call powerOnHost().

5. Density uses shutdownHost()

In:

packages/xo-server-load-balancer/src/density-plan.js

debug(`Shutdown Host (${fmtSrcHost}).`)

try {
  await xapiSrc.shutdownHost(srcHost.id)
} catch (error) {
  debug(`Unable to shutdown Host (${fmtSrcHost}).`, { error })
}

The underlying implementation in:

packages/xo-server/src/xapi/index.mjs

Contains:

async shutdownHost(hostId, { force = false, bypassEvacuate = false } = {}) {
  const host = this.getObject(hostId)
  if (bypassEvacuate) {
    await this.call('host.disable', host.$ref)
  } else {
    await this.clearHost(host, force)
  }
  await this.callAsync('host.shutdown', host.$ref)
}

This is consistent with the observation that XCPNG3 ends up with enabled: false after being shut down by Density.

Suspected root cause

My hypothesis is:

1. Density evacuates XCPNG3.
2. Density calls shutdownHost().
3. XCPNG3 becomes halted and disabled (enabled: false).
4. Performance calls _getHosts({ powerState: 'Halted' }).
5. _getHosts() filters out XCPNG3 because enabled !== true.
6. powerOnHost() is never called for XCPNG3.

This would explain why manual power-on through the Xen Orchestra UI works while automatic power-on through the Load Balancer does not.

Possible fix

Would it make sense to allow disabled hosts to be returned by _getHosts() only when searching for halted hosts to power on?

For example, the condition:

object.enabled === true

could potentially become:

(powerState === 'Halted' || object.enabled === true)

Questions

1. Is this a known issue in the current Load Balancer implementation?
2. Is enabled: false expected after Density shuts down a host?
3. Should Performance be able to restart a host in this state?
4. Is the enabled === true filter intentional for halted hosts?
5. Is there an existing workaround or an upstream fix?

The goal is to use Density at night to consolidate VMs and shut down unused hosts, then switch to Performance during the day to bring those hosts back online automatically.

Thanks in advance for any insights!