You can use resource reservation to configure a Load Balancer Capacity Unit (LCU) baseline for a Network Load Balancer (NLB) instance. NLB ensures that the processing capacity never falls below this value. In high-load scenarios, NLB can immediately use the reserved capacity to handle traffic peaks without waiting for auto scaling to take effect. When the load exceeds the reserved capacity, NLB can still auto scale. You are charged on an hourly basis for the number of reserved LCUs.
Use cases for resource reservation:
You are preparing to launch a series of promotional events that will cause sudden traffic spikes, and you want to ensure that your NLB instance can handle the peak traffic during the events.
Your business experiences bursty traffic patterns, making it difficult to predict traffic peaks.
A service that is newly launched or migrated requires high performance immediately, rather than waiting for auto scaling.
You need to maintain a deterministic capacity to meet your business requirements.
You are migrating between load balancers and want the target load balancer's performance to match the source.
Usage notes
The resource reservation feature is not enabled by default. To enable this feature, contact your business manager.
Resource reservation and TCPSSL listeners are mutually exclusive. You cannot enable resource reservation on an NLB instance with a TCPSSL listener, nor can you add a TCPSSL listener to an instance that has resource reservation.
Configure resource reservation
By default, a public-facing NLB uses an Elastic IP address (EIP) for public access. Resource reservation does not include bandwidth reservation for the EIP. If you need more bandwidth, purchase and add the EIP to a Bandwidth Plan. If you are using an Anycast EIP, you must increase the peak bandwidth of the Anycast EIP.
The reserved LCU capacity is distributed evenly across the availability zones where the NLB instance is deployed. For optimal performance, we recommend that you deploy the NLB instance in at least two availability zones.
In the NLB console, click the ID of the target instance to go to the Instance Details page. On the LCU Reservation tab, click Edit LCU Reservation.
Choose a method to Capacity Estimation:
Historic-Reference Based Estimate: If you have historical traffic data, select the target NLB instance from the Reference ALB Instance drop-down list. The chart below displays the historical Peak LCU. Peak LCU represents the maximum LCU consumption recorded during traffic peaks.
Manual Estimate: If you do not have historical traffic data, you can estimate the traffic for your NLB instance. Enter a value for Maximum Bandwidth and refer to the system-calculated Estimated LCUs.
Enter a value for Load Balancer Capacity Units (LCU) based on the reference value and click OK. The resource reservation must be in the range of
4,500 to 22,500 LCUs, and the maximum capacity is limited by the quota.
View resource reservation
In the NLB console, click the ID of the target instance to go to the Instance Details page, and then click the LCU Reservation tab.
In the LCU Reservation section, view information such as the Reservation Status and Reserved LCU.
In the ALB LCU Usage section, compare the Reserved LCUs with the Peak LCU to evaluate the effectiveness of your Reserved LCUs.
The following table describes the possible statuses of a resource reservation.
Status | Description | Allowed actions |
Pending | The resource reservation is being configured. | No |
Available | The reserved capacity is available for use. | Yes |
Failed | The resource reservation request could not be completed. | Yes |
Rebalancing | The load balancer is rebalancing its capacity because an availability zone was added or removed. Any change in the number of availability zones for the NLB instance triggers an automatic rebalancing process to evenly redistribute the total capacity across the zones. | No |
Modify or cancel resource reservation
You can increase the reserved capacity an unlimited number of times. However, you can only decrease the capacity or cancel the reservation a combined total of two times per day.
If you reserve a capacity that exceeds the instance's maximum auto scaling performance, you cannot decrease or cancel the reservation for seven days.
In the NLB console, click the ID of the target instance to go to the Instance Details page, and then click the LCU Reservation tab.
Modify the reserved LCU capacity: Click Edit LCU Reservation, set a new value for Load Balancer Capacity Units (LCU), and then click OK.
Cancel the resource reservation: Click Cancel Capacity and then click OK.
Billing
LCU fees for NLB are charged hourly. A billing cycle is one hour. Usage for less than a full hour in a billing cycle is billed as one hour. Within a billing cycle, the system compares the initial number of reserved LCUs with the number of LCUs that are ready after a modification, and uses the maximum of these values to calculate the reserved LCU fee.
If the number of actual LCUs consumed is less than or equal to the number of reserved LCUs, the bill includes only the reserved LCU fee:
Reserved LCU Fee = Reserved LCU Unit Price × Number of Reserved LCUs
If the actual LCUs consumed exceed the reserved LCUs, the bill includes both the LCU fee and the reserved LCU fee:
LCU fee = LCU unit price × (Consumed LCUs - Reserved LCUs)Reserved LCU Fee = Reserved LCU Unit Price × Number of Reserved LCUs
The unit price for a reserved LCU is the same as the LCU unit price in the NLB billing rules. The actual price is subject to the price on the purchase page.
Reserved LCU capacity cannot be offset by a resource plan. Only the portion of LCU consumption that exceeds the reserved capacity can be offset by a resource plan.
The following table shows billing examples for an NLB instance with different usage and resource reservation configurations in various billing cycles.
Billing cycle | Operations and events | Consumed LCUs | Reserved LCUs | Fees |
10:00:00–10:59:59 |
| 800 | - | LCU fee for the hour = CNY 0.037/LCU × 800 = CNY 29.6 |
11:00:00–11:59:59 |
| 1,000 | 5,000 | Reserved LCU fee for the hour = CNY 0.037/LCU × 5,000 = CNY 185 |
12:00:00–12:59:59 |
| 6,000 | 5,000 | LCU fee for the hour = CNY 0.037/LCU × (6,000 - 5,000) = CNY 37 Reserved LCU fee for the hour = CNY 0.037/LCU × 5,000 = CNY 185 |
13:00:00–13:59:59 |
| 5,000 | 6,000 | Reserved LCU fee for the hour = CNY 0.037/LCU × 6,000 = CNY 222 |
14:00:00–14:59:59 |
| 800 | 6,000 | Reserved LCU fee for the hour = CNY 0.037/LCU × 6,000 = CNY 222 |
Quotas
To request a quota increase, contact your business manager.
Parameter | Description | Default |
nlb_quota_reserved_capacity_units_per_loadbalancer | The maximum LCU capacity that can be reserved for each NLB instance. | 22500 |
nlb_quota_reserved_capacity_units_per_region | The maximum total LCU capacity that can be reserved for all NLB instances in a region. | 45000 |
FAQ
How to determine a reasonable LCU capacity?
We recommend that you estimate the resources as described in the Configure resource reservation section and use a stress test to determine an initial value. You can then dynamically adjust the reservation based on the actual LCU usage trends in CloudMonitor.
Impacts of incorrect reservation size
Reserving too much: This results in unnecessary costs because you are charged for the reserved capacity even if your actual consumption is lower.
Reserving too little: This forces NLB to rely on auto scaling during traffic peaks. This process can introduce delays, potentially causing increased latency or request failures.
When do billing changes take effect?
After a decrease or cancellation operation is successful, the change takes effect at the start of the next billing cycle. For example, if you successfully cancel a resource reservation at 14:30, you are still charged for the maximum number of reserved LCUs that were active during the 14:00:00–14:59:59 billing cycle. The reservation fees will stop accruing from 15:00 onwards.
Why can't I configure resource reservation?
Check the following conditions:
Your NLB instance has a TCPSSL listener. Resource reservation is not supported for these instances.
The resource reservation feature is not enabled for your account. Contact your business manager to enable it.
Another resource reservation request is in progress. You must wait for the current request to complete before initiating a new one.
Can a resource plan offset reserved LCUs?
No. Only the portion of LCU consumption that exceeds the reserved capacity can be offset by a resource plan.