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Host volumes were considered regular feasibility checks. This had two
unintended consequences.
The first happened when scheduling an allocation with a host volume on a
set of nodes with the same computed class but where only some of them
had the desired host volume.
If the first node evaluated did not have the host volume, the entire
node class was considered ineligible for the task group.
```go
// Run the job feasibility checks.
for _, check := range w.jobCheckers {
feasible := check.Feasible(option)
if !feasible {
// If the job hasn't escaped, set it to be ineligible since it
// failed a job check.
if !jobEscaped {
evalElig.SetJobEligibility(false, option.ComputedClass)
}
continue OUTER
}
}
```
This results in all nodes with the same computed class to be skipped,
even if they do have the desired host volume.
```go
switch evalElig.JobStatus(option.ComputedClass) {
case EvalComputedClassIneligible:
// Fast path the ineligible case
metrics.FilterNode(option, "computed class ineligible")
continue
```
The second consequence is somewhat the opposite. When an allocation has
a host volume with `per_alloc = true` the node must have a host volume
that matches the allocation index, so each allocation is likely to be
placed in different nodes.
But when the first allocation found a node match, it registered the node
class as eligible for the task group.
```go
// Set the task group eligibility if the constraints weren't escaped and
// it hasn't been set before.
if !tgEscaped && tgUnknown {
evalElig.SetTaskGroupEligibility(true, w.tg, option.ComputedClass)
}
```
This could cause other allocations to be placed on nodes without the
expected host volume because of the computed node class fast path. The
node feasibility for the volume was never checked.
```go
case EvalComputedClassEligible:
// Fast path the eligible case
if w.available(option) {
return option
}
// We match the class but are temporarily unavailable
continue OUTER
```
These problems did not happen with CSI volumes kind of accidentally.
Since the `CSIVolumeChecker` was not placed in the `tgCheckers` list it
did not cause the node class to be considered ineligible on failure
(avoiding the first problem).
And, as illustrated in the code snippet above, the eligible node class
fast path checks `tgAvailable` (where `CSIVolumeChecker` is placed)
before returning the option (avoiding the second problem).
By also placing `HostVolumeChecker` in the `tgAvailable` list instead of
`tgCheckers` we also avoid these problems on host volume feasibility.