Re: [PATCH ha-manager 3/7] manager: auto rebalance: require a minimum absolute imbalance improvement

["Daniel Kral" <d.kral@proxmox.com>]

On Sun Sep 20, 2026 at 6:22 PM CEST, Michael Ryom wrote:
> The automatic load balancer only requires a relative improvement of the
> node imbalance (default 10%). When a single resource dominates the
> cluster load, the relative margin provides almost no hysteresis: with
> two nodes, a resource load v and a node base load difference d, the
> relative improvement of moving the resource is 2d / (v + d), so it
> exceeds a margin m as soon as d > v * m / (2 - m). For a mid-size VM
> that is a base load difference of about half a percentage point, well
> within the normal fluctuation of e.g. the ZFS ARC or Ceph daemons, and
> each fluctuation then qualifies a motion (and, after it was carried
> out, its reverse).
>
> Additionally require the imbalance improvement to exceed a minimum
> absolute value of 5 percentage points. This creates a dead band for
> motions that shuffle load without meaningfully improving the cluster
> balance. No expected motion of an existing regression test is affected;
> the smallest occurring improvement there is 9.6 percentage points.
>
> Like the backoff and cooldown constants, this could be exposed as a
> ha-auto-rebalance-* option later.

I think the absolute minimum imbalance improvement is a good idea.

The relative 'margin' was only introduced to compensate for the
initially unbound imbalance value for different cluster sizes, which was
changed to be in [0,1] for all cluster sizes only in a later revision.

Though I'd prefer if this can be configured by users from the get-go.
For simplicity, I'd merge it with the ha-auto-rebalance-margin option,
and add a flag which can switch the value's semantic meaning between
'relative' (the default, because of backwards compatibility) and
'absolute'.

Except if there really are use cases which require both a relative and
an absolute minimum imbalance improvement?

Of course, 'absolute' should still mean decreasing it and not increasing
it, the same way it was for 'relative' ;).

>
> Signed-off-by: Michael Ryom <Michael@RyomHerold.dk>