Scheduler predicate for capping node volume count

For certain volume types (e.g. AWS EBS or GCE PD), a limitted
number of such volumes can be attached to a given node.  This commit
introduces a predicate with allows cluster admins to cap
the maximum number of volumes matching a particular type attached to a
given node.

The volume type is configurable by passing a pair of filter functions,
and the maximum number of such volumes is configurable to allow node
admins to reserve a certain number of volumes for system use.

By default, the predicate is exposed as MaxEBSVolumeCount and
MaxGCEPDVolumeCount (for AWS ElasticBlocKStore and GCE PersistentDisk
volumes, respectively), each of which can be configured using the
`KUBE_MAX_PD_VOLS` environment variable.

Fixes #7835

1 files changed, 2 insertions, 0 deletions

diff --git a/scheduler_algorithm.md b/scheduler_algorithm.md
index 00a812a5..786666ca 100755
--- a/scheduler_algorithm.md
+++ b/scheduler_algorithm.md
@@ -47,6 +47,8 @@ The purpose of filtering the nodes is to filter out the nodes that do not meet c
 - `PodFitsHost`: Filter out all nodes except the one specified in the PodSpec's NodeName field.
 - `PodSelectorMatches`: Check if the labels of the node match the labels specified in the Pod's `nodeSelector` field ([Here](../user-guide/node-selection/) is an example of how to use `nodeSelector` field).
 - `CheckNodeLabelPresence`: Check if all the specified labels exist on a node or not, regardless of the value.
+- `MaxEBSVolumeCount`: Ensure that the number of attached ElasticBlockStore volumes does not exceed a maximum value (by default, 39, since Amazon recommends a maximum of 40 with one of those 40 reserved for the root volume -- see [Amazon's documentation](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/volume_limits.html#linux-specific-volume-limits)).  The maximum value can be controlled by setting the `KUBE_MAX_PD_VOLS` environment variable.
+- `MaxGCEPDVolumeCount`: Ensure that the number of attached GCE PersistentDisk volumes does not exceed a maximum value (by default, 16, which is the maximum GCE allows -- see [GCE's documentation](https://cloud.google.com/compute/docs/disks/persistent-disks#limits_for_predefined_machine_types)).  The maximum value can be controlled by setting the `KUBE_MAX_PD_VOLS` environment variable.
 
 The details of the above predicates can be found in [plugin/pkg/scheduler/algorithm/predicates/predicates.go](http://releases.k8s.io/HEAD/plugin/pkg/scheduler/algorithm/predicates/predicates.go). All predicates mentioned above can be used in combination to perform a sophisticated filtering policy. Kubernetes uses some, but not all, of these predicates by default. You can see which ones are used by default in [plugin/pkg/scheduler/algorithmprovider/defaults/defaults.go](http://releases.k8s.io/HEAD/plugin/pkg/scheduler/algorithmprovider/defaults/defaults.go).