After you purchase reserved instances, you make commitments to using pay-as-you-go instances for a certain period of time. Reserved instances are used to offset bills of pay-as-you-go instances only when they match the pay-as-you-go instances. If you do not have any pay-as-you-go instances under your account, the reserved instances are idle and continue to be charged. This topic describes the matching rules of reserved instances and provides some examples.

Matching rules

The match between reserved instances and pay-as-you-go instances cannot be managed manually. After you purchase a reserved instance, the reserved instance automatically matches one or more pay-as-you-go instances that have certain attributes within the term. After the reserved instance is matched, it checks for eligible pay-as-you-go bills on an hourly basis and deducts fees based on its computing power. You can view pay-as-you-go instances that match the reserved instance. For more information, see View matched pay-as-you-go instances.

The matching conditions of a regional reserved instance are different from those of a zonal reserved instance. The following table describes the attributes that determine the match between reserved instances and pay-as-you-go instances.
Attribute Regional reserved instance Zonal reserved instance
Region and zone A regional reserved instance can match one or more pay-as-you-go instances across zones within a specific region. A zonal reserved instance can match one or more pay-as-you-go instances only in the same zone.
Instance type The following section describes the instance size flexibility and resource reservation status:
  • Within the same instance family, regional reserved instances of small instance types can match pay-as-you-go instances of large instance types or regional reserved instances of large instance types can match pay-as-you-go instances of small instance types.
  • Resource reservation is not supported.
The following section describes the instance size flexibility and resource reservation status:
  • A zonal reserved instance can match only pay-as-you-go instances of the same instance type.
  • Resource reservation is supported. A specific number of pay-as-you-go instances of the specified instance types are reserved within the term. This ensures that you can create pay-as-you-go instances anytime in the specified zone.
Operating system A regional reserved instance can match only one or more pay-as-you-go instances that have the same operating system. A zonal reserved instance can match only one or more pay-as-you-go instances that have the same operating system.

Examples of regional reserved instances

Regional reserved instances and pay-as-you-go instances must meet the following requirements to match:
  • They must reside within the same region.
  • They must use instances types that belong to the same instance family. Within the same instance family, regional reserved instances of small instance types can match pay-as-you-go instances of large instance types or regional reserved instances of large instance types can match pay-as-you-go instances of small instance types.
    Note You can evaluate the usage of reserved instances based on the computing power when the instance sizes of the reserved instance and the pay-as-you-go instance are different.
  • They must run the same operating system.
The following table describes examples of regional reserved instances.
Scenario Pay-as-you-go instance Regional reserved instance How bills are offset
Regional reserved instances of small instance types match pay-as-you-go instances of large instance types You have a pay-as-you-go instance that has the following configurations:
  • Qingdao Zone B
  • ecs.g5.4xlarge
  • Linux
You have an active regional reserved instance that has the following attributes:
  • All zones in China (Qingdao)
  • ecs.g5.xlarge
  • Linux
  • Instance quantity: 2
The normalization factors are as follows:
  • The normalization factor of ecs.g5.4xlarge is 16.
  • The normalization factor of ecs.g5.xlarge is 4.
Comparison between the delivered and consumed computing powers:
  • Pay-as-you-go instance: The pay-as-you-go instance consumes 16 units of computing power per hour (1 instance × 16).
  • Reserved instance: The reserved instance delivers 8 units of computing power per hour (2 instances × 4).

Therefore, the reserved instance offsets 50% of the pay-as-you-go instance bills per hour.

Regional reserved instance of large instance types match pay-as-you-go instances of small instance types You have six pay-as-you-go instances.
  • The configurations of one of the pay-as-you-go instances are as follows:
    • Qingdao Zone B
    • ecs.g5.xlarge
    • Linux
  • The configurations of the other five pay-as-you-go instance are as follows:
    • Qingdao Zone C
    • ecs.g5.xlarge
    • Linux
You have two active regional reserved instances.
  • The attributes of one of the reserved instances are as follows:
    • All zones in China (Qingdao)
    • ecs.g5.4xlarge
    • Linux
    • Instance quantity: 1
  • The attributes of the other reserved instance are as follows:
    • All zones in China (Qingdao)
    • ecs.g5.2xlarge
    • Linux
    • Instance quantity: 1
The normalization factors are as follows:
  • The normalization factor of ecs.g5.xlarge is 4.
  • The normalization factor of ecs.g5.2xlarge is 8.
Comparison between the delivered and consumed computing powers:
  • Pay-as-you-go instance: The six pay-as-you-go instances consume 24 units of computing power per hour (6 instances × 4).
  • Reserved instance: One reserved instance delivers 16 units of computing power (1 instance × 16) per hour and the other reserved instance delivers 8 units of computing power (1 instance × 8) per hour.
Therefore, the two reserved instances offset 100% of the six pay-as-you-go instances bills per hour.
Note The pay-as-you-go instances belong to two zones. However, the regional reserved instances support the zone flexibility and can offset bills of pay-as-you-go instances across zones.
Failed to match You have two pay-as-you-go instances.
  • The configurations of one of the pay-as-you-go instances are as follows:
    • Qingdao Zone B
    • ecs.g5.xlarge
    • Windows
  • The configurations of the other pay-as-you-go instance are as follows:
    • Hangzhou Zone B
    • ecs.c5.xlarge
    • Linux
You have two active regional reserved instances.
  • The attributes of one of the reserved instances are as follows:
    • All zones in China (Qingdao)
    • ecs.g5.4xlarge
    • Linux
    • Instance quantity: 1
  • The attributes of the other reserved instance are as follows:
    • All zones in China (Qingdao)
    • ecs.g5.xlarge
    • Linux
    • Instance quantity: 1
The reserved instances failed to match the pay-as-you-go instances due to the following causes:
  • The operating system of one of the pay-as-you-go instances is Windows.
  • One of the pay-as-you-go instances resides in China (Hangzhou) and uses an instance type that belongs to the c5 instance family.

Therefore, the reserved instances remain idle and continue to be charged. The bills of pay-as-you-go instances are paid by account balance.

Examples of zonal reserved instances

Zonal reserved instances and pay-as-you-go instances must meet the following requirements to match:
  • They must reside within the same region and zone.
  • They must use instance types that belong to the same instance family and have the same size.
  • They must run the same operating system.
The following table describes examples of zonal reserved instances.
Scenario Pay-as-you-go instance Zonal reserved instance Deduction
Matched You have five pay-as-you-go instances that have the following configurations:
  • Qingdao Zone B
  • ecs.g5.xlarge
  • Windows
You have an active zonal reserved instance that has the following attributes:
  • Qingdao Zone B
  • ecs.g5.xlarge
  • Windows
  • Instance quantity: 5
The reserved instance matches the pay-as-you-go instances. The reserved instance offsets 100% of the five pay-as-you-go instances bills per hour.
Resource reservation You have no pay-as-you-go instances. You have an active zonal reserved instance that has the following attributes:
  • Qingdao Zone B
  • ecs.g5.2xlarge
  • Linux
  • Instance quantity: 10
The reserved instance is idle and continues to be charged. However, 10 pay-as-you-go instances of the ecs.g5.2xlarge instance type are reserved for you within the term of the reserved instance. This ensures that you can create pay-as-you-go instances anytime in Qingdao Zone B.
Failed to match You have two pay-as-you-go instances.
  • The configurations of one of the pay-as-you-go instances are as follows:
    • Qingdao Zone B
    • ecs.g5.xlarge
    • Windows
  • The configurations of the other pay-as-you-go instance are as follows:
    • Qingdao Zone C
    • ecs.g5.4xlarge
    • Linux
You have two active zonal reserved instances.
  • The attributes of one of the reserved instances are as follows:
    • Qingdao Zone B
    • ecs.g5.xlarge
    • Linux
    • Instance quantity: 1
  • The attributes of the other reserved instance are as follows:
    • Qingdao Zone B
    • ecs.g5.xlarge
    • Linux
    • Instance quantity: 1
The reserved instances failed to match the pay-as-you-go instances due to the following causes:
  • The operating system of one of the pay-as-you-go instances is Windows.
  • One of the pay-as-you-go instances resides in Qingdao Zone C and uses the ecs.g5.4xlarge instance type.

Therefore, the reserved instances remain idle and continue to be charged. The bills of pay-as-you-go instances are paid by account balance.

Match between a single reserved instance and multiple pay-as-you-go instances

A single regional or zonal reserved instance can match multiple pay-as-you-go instances. However, you cannot shorten the term of a reserved instance to deliver more computing power.

The following table describes an example that involves six pay-as-you-go instances and an active reserved instance.
Six pay-as-you-go instances An active reserved instance
The configurations of each pay-as-you-go instance are as follows:
  • Qingdao Zone B
  • ecs.g5.6xlarge
  • Linux
  • Normalization factor of the instance type: 24
The attributes of the reserved instance are as follows:
  • Qingdao Zone B
  • ecs.g5.6xlarge
  • Linux
  • Normalization factor of the instance type: 24
  • Instance quantity: 1
  • Term: one year
The six pay-as-you-go instances all match the reserved instance. The following table describes examples of how the reserved instances offset bills of the pay-as-you-go instances based on how long each pay-as-you-go instance exists.
Six pay-as-you-go instances A reserved instance How bills are offset
The consumed computing power is as follows:
  • Duration of each pay-as-you-go instance: one hour (each consumes 24 units of computing power)
  • Computing power consumed by the six pay-as-you-go instances per hour: 144 (6 instances × 24)
The delivered computing power is as follows:

Computing power delivered by the reserved instance per hour: 24 (1 instance × 24)

The computing power delivered by the reserved instance is equal to that consumed by a pay-as-you-go instance. The reserved instance offsets the bills of a pay-as-you-go instance randomly. You cannot shorten the term of the reserved instance to two months to offset bills of the six pay-as-you-go instances at the same time.
The consumed computing power is as follows:
  • Duration of each pay-as-you-go instance: 10 minutes (each consumes 24/6 units of computing power)
  • Computing power consumed by the six pay-as-you-go instances per hour: 24 (6 instances × 24/6)
The delivered computing power is as follows:

Computing power delivered by the reserved instance per hour: 24 (1 instance × 24)

The computing power delivered by the reserved instance is equal to that consumed by the six pay-as-you-go instances. The reserved instance offsets bills of the six pay-as-you-go instances.
The consumed computing power is as follows:
  • Duration of each pay-as-you-go instance: 15 minutes (each consumes 24/4 units of computing power)
  • Computing power consumed by the six pay-as-you-go instances per hour: 36 (6 instances × 24/4)
The delivered computing power is as follows:

Computing power delivered by the reserved instance per hour: 24 (1 instance × 24)

The computing power delivered by the reserved instance is less than that consumed by the six pay-as-you-go instances. The reserved instance offsets bills of the six pay-as-you-go instances for an hour. The deducted amount of each pay-as-you-go instance is random.