Reserved Instance Overview

Concept

A reserved instance (RI) is not an actual instance, but a billing discount that can be applied to the use of pay-per-use ECSs in your account. When the attributes of your pay-per-use ECSs match those of an RI, the RI billing benefit automatically applies to your ECSs. The combination of RIs and pay-per-use billing fully utilizes the flexibility of pay-per-use resources at lower costs.

A purchased RI is billed, regardless of whether it is used or not.

**Table 1** ECS billing modes
Billing Mode	What It Is	How to Use
RI	A billing discount applied to pay-per-use ECSs.	When the attributes of your pay-per-use ECSs match those of an RI, the RI billing benefit automatically applies to your ECSs.
Pay-per-use	ECS billed based on usage frequency and duration. Such an ECS can be created or deleted at any time.	A basic computing unit that consists of vCPUs, memory, OS, and EVS disks. After purchasing such an ECS, you can use it on the cloud.

For instructions about how to purchase an RI, see Enabling and Purchasing a Reserved Instance.
For instructions about how to modify an RI, see Modifying RI Attributes.

What Is Attribute Mapping Between an RI and a Pay-per-Use ECS?

A regional RI is purchased within a region and without an AZ specified.

Attribute mapping of a regional RI: indicates whether the region, OS type, ECS series, and vCPU/memory ratio of a pay-per-use ECS are the same as those specified in a regional RI.

Application Scenarios

If your ECSs are to be used in a short term, it is a good practice to use pay-per-use rates. If you plan to use ECSs for one or three years, it is a good practice to use RIs. RIs offer discounts for pay-per-use ECSs with matched attributes.

Working Rules

For example, you have a running pay-per-use ECS in your account. After you purchase an RI that matches the attributes of this ECS, the billing benefit of the RI is automatically applied to your ECS when the RI takes effect. A purchased RI takes effect at the next hour.

Table 2 lists RI attributes. You can purchase your desired RIs based on these attributes.

**Table 2** RI attributes
Parameter	Description
Region	Regional RI: indicates an RI purchased in a region, without an AZ specified. Capacity reservations are not supported for regional RIs.
Flavor	When purchasing a regional RI, ensure that the ECS series and vCPU/memory ratio specified in the RI are the same as those of the target pay-per-use ECS.
OS	The OS of the ECS to be bought, which must match the OS specified in your RI. For example, if you want to use a Linux RI, select a Linux public or private image when purchasing an ECS.
Term	The service duration of an RI. A year is defined as 31,536,000 seconds (365 days).
Offering Class	Standard: Certain attributes, such as the ECS size can be modified during the term. However, the ECS type cannot be changed.
Payment Option	No upfront