Impact of Changes to Min-Max Scaleout Per Virtual Service

Each NSX Advanced Load Balancer Service Engine (SE) group has settings for minimum and maximum scale-out per virtual service. These settings govern the number of SEs across which a virtual service can be scaled. This topic explains the impact of changing these two settings on virtual services.

For more information on scale out settings, see Service Engine Group.

A virtual service can be affected in the following scenarios:

1. The min-max settings of its SE group are dynamically changed. All virtual services placed on the SE group are affected.

2. It is moved to another SE group having different settings.

Impact of Changes

Changes to the min_scaleout_per_vs or max_scaleout_per_vs settings of an SE group result in the same behavior as described above with the following exceptions:

• Increase in the min_scaleout_per_vs of an SE group only increases the number of virtual service placements, if the new minimum value is greater than the current number of virtual service placements.

• If a virtual service is disabled and re-enabled, the number of placements is capped at max_scaleout_per_vs.

• When migrating a virtual service from one SE group to another, the NSX Advanced Load Balancer disregards the number of placements made within the group‌. The virtual service is placed according to the min_scaleout_per_vs value of the destination group.

Scenarios

The effect on virtual services when the minimum-maximum scale-out per virtual service settings of the SE group are changed is illustrated in the following section.

To understand the examples, it must be noted that internally, the number of SEs requested for a virtual service is the sum of the following two numbers:

1. The minimum scale-out per VS of its SE group (min_scaleout_per_vs).

2. The user scale-out factor which is an internal variable that starts at 0 for all virtual services. This number increases by 1 when a user scales out and decreases by 1 when the user scales in.

General Behavior

The rules governing all changes of minimum and maximum scale per virtual service are as follows:

• Decreasing the minimum scale per virtual service has no effect on the scale of existing virtual services in the group.

  • For this case, the user scale-out is increased by the amount that the minimum scale per virtual service is decreased.

  • For an existing virtual service to be scaled at the minimum level of the SE group, the user must explicitly scale in.

• Increasing the minimum scale per virtual service only increases the scale of existing virtual services if the new minimum is greater than the current scale of the virtual service.

• Increasing or decreasing the maximum scale per virtual service of an SE group has no effect on the scale of existing virtual services in the group.

  • For virtual services with more SEs than the new maximum scale, the user is still able to manually scale in.

• A virtual service which is deactivated and re-enabled preserves its existing scale, capped by the current maximum scale per VS of the SE group.

• A virtual service which is moved to another SE group will be placed on the minimum scale per VS of the new SE group.

Changing SE Group Settings

The effect on virtual services in an SE group when its minimum or maximum scale per virtual service is changed is illustrated in the following section:

• Increasing Minimum Scale per VS

For a virtual service without any user scale-out, increasing the minimum scale of the SE group increases the number of SEs of the virtual service to the new minimum.

Action	Num of SEs Requested	User Scale-out	Min Scale-out per VS
Initial state	1	0	1
Min scale per VS: 1 → 2	2	0	2

For a virtual service with user scale-out, increasing the minimum scale increases the number of SEs, only if the new minimum is greater than the current scale of the virtual service.

Example 1

Action	Num of SEs Requested	User scale-out	Min scale-out per VS
Initial state	1	0	1
User-scale out	2	1	1
Min scale per VS: 1 → 2	2	0	2

Example 2

Action	Number of SEs Requested	User Scale-out	Min Scale-out per VS
Initial state	2	1	1
Min scale per VS: 1 → 3	3	0	3

• Decreasing Minimum Scale per VS

Decreasing the minimum scale per VS of an SE group will have no effect on the scale of the existing virtual service. To maintain the same number of SEs, the user scale-out is increased by the amount of decrease in minimum scale.

Example

Action	Num of SEs Requested	User Scale-out	Min Scale-out per VS
Initial state	2	0	2
Min scale per VS: 2 → 1	2	1	1
User scale-in	1	0	1

The purpose of this behavior is to preserve the current state of all virtual services residing inside an SE group when min scale per virtual service is reduced. By increasing the user scale-out by the amount of decrease in min_scaleout_per_vs, we keep the number of SEs requested, the same.

If the desired outcome in the above example is to scale every virtual service in the SE group down to 1 SE, three options are available:

1. After changing the SE group settings, manually scale down every virtual service to reduce the user scale-out to 0.

2. Set the maximum scale per VS of the SE group to 1. Deactivate and enable all virtual services (maximum scale can also be reduced after the deactivation).

3. Move all virtual services in the SE group to another SE group where the min_scaleout_per_vs is 1.

• Changing Maximum Scale per VS

Changing the maximum scale per VS has no effect on the other variables.

If the maximum scale per VS of an SE group is reduced, all virtual services within the SE group retain the same number of SEs. So the number of SEs requested for a virtual service in this situation can be greater than the new maximum scale per VS. The user has the option of manually scaling in to reduce this number to the new max.

Example

Action	Num of SEs Requested	User Scale-out	Min Scale-out per VS	Max Scale-out per VS
Initial state	3	2	1	4
Max scale per VS: 4 → 2	3	2	1	2
User Scale-out	2	1	1	2

• VS Disable and Enable

When a virtual service is deactivated and then enabled, it is placed on (current min scale per VS + number of user scale-outs), capped by the current max scale per VS of the SE group.

If a virtual service is disabled and then enabled without changing the scale settings of the SE group, the virtual service remains at the same scale.

Example 1

Action	Num of SEs Requested	User Scale-out	Min Scale-out per VS	Max Scale-out per VS
Initial state	3	2	1	4
VS disabled	0	2	1	4
VS enabled	3	2	1	4

Example 2

Action	Num of SEs Requested	User Scale-out	Min Scale-out per VS	Max Scale-out per VS
Initial state	2	0	2	4
VS disabled	0	0	2	4
Min scale per VS: 2 → 1	0	1	1	4
VS enabled	2	1	1	4

Example 3

Action	Num of SEs Requested	User scaleout	Min scaleout per VS	Max scaleout per VS
Initial state	4	3	1	4
Max scale per VS: 4 → 1	4	3	1	1
VS disabled	0	3	1	1
VS enabled	1	0	1	1

• Moving a VS to Another SE Group With Different Settings

  Moving a virtual service to another SE group will always place the virtual service on the min_scaleout_per_vs of the new SE group.

Example 1

Action	Num of SEs Requested	User scaleout	min scaleout per VS	Max scaleout per VS
Initial state	2	0	2	4
User Scale-out	3	1	2	4
VS moved to new SE group with (min: 1, max: 2)	1	0	1	2

Since the virtual service has been moved to a new SE group, the NSX Advanced Load Balancer does not attempt to preserve its state and adheres to the settings of the new SE group.

Example 2

Action	Num of SEs Requested	User Scale-out	Min Scale-out per VS	Max Scale-out per VS
Initial state	1	0	1	4
VS moved to new SE group with (min: 2, max: 2)	2	0	2	2

A legacy active-standby SE group effectively has a minimum scale per VS of 2 and a maximum scale per VS of 2.

Summary

The following table summarizes expected changes in various scenarios:

Action	VS: num of SEs
↑ min_scaleout_per_vs	Increase only if current number of SEs < min_scaleout_per_vs
↓ min_scaleout_per_vs	Stays the same
↑ max_scaleout_per_vs	Stays the same
↓ max_scaleout_per_vs	Stays the same
VS disable/enable	Caps at max_scaleout_per_vs
Move VS to another SE group	min_scaleout_per_vs of new SE group