Size memory backed volumes

[derekwaynecarr]

Improve portability of pods that consume memory backed volumes by decoupling the size of the volume from the node that runs the pod.

[k8s-ci-robot]

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[derekwaynecarr]

/assign @dashpole @sjenning @dchen1107
/milestone v1.20

[derekwaynecarr]

The verify job failing is related to a sig-scheduling kep and not this kep.

[SergeyKanzhelev]

this feature flag is on top of another feature flag LocalStorageCapacityIsolation? Would it worth mentioning this?

[derekwaynecarr]

LocalStorageCapacityIsolation did not impact the size of memory backed volume, so I kept it separate.

[SergeyKanzhelev]

definitely. The only thing I was trying to say is that emptyDir.sizeLimit wouldn't work without the LocalStorageCapacityIsolation enabled.

[dims]

@derekwaynecarr does this KEP change the meaning of the definition of sizeLimit here?
https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/api/core/v1/types.go#L645-L651

	// Total amount of local storage required for this EmptyDir volume.
	// The size limit is also applicable for memory medium.
	// The maximum usage on memory medium EmptyDir would be the minimum value between
	// the SizeLimit specified here and the sum of memory limits of all containers in a pod.
	// The default is nil which means that the limit is undefined.
	// More info: http://kubernetes.io/docs/user-guide/volumes#emptydir
	// +optional
	SizeLimit *resource.Quantity `json:"sizeLimit,omitempty" protobuf:"bytes,2,opt,name=sizeLimit"`

I feel the definition above means there is at least that much memory as mentioned in SizeLimit, right?

[dashpole]

To be precise, general memory or disk eviction don't take emptyDir.sizeLimit into account. But as part of local ephemeral storage capacity isolation, we evict pods where the usage of an empty-dir exceeds its sizeLimit

[derekwaynecarr]

@dims this KEP actually implements the behavior described in types.go where in the past, it just took 50% of node memory. Usage was restricted based on memory accounting in practice. In practical terms, a pod that requested 10Gi of memory that was scheduled to a Linux host with 16Gi of memory would have its empty dir set to 8Gi. This made it that pods could not actually use what was requested!

[dims]

got it! thanks @derekwaynecarr

[dashpole]

/lgtm
/hold
for any additional comments. @derekwaynecarr feel free to remove when you are satisfied. I'll probably have to reapply lgtm after you fix verify anyways

cc @johnbelamaric, who is listed as the PRR reviewer.

[derekwaynecarr]

/hold cancel

rebased to pick up fix to verify, ptal @dashpole

[dashpole]

/lgtm

Thanks a bunch Derek

[SergeyKanzhelev]

cross link to k/k code for completeness: kubernetes/kubernetes#94444