问题描述

Azure Redis Cluster 在增加分片数时失败，错误消息为：

ResponseBody: { 
 "error": {
  "details": [],
  "code": "SubnetIsFull",
  "message": "Subnet Redis with address prefix 12.12.12.0/28 does not have enough capacity for 2 IP addresses."
 }
}

问题分析

根据错误消息里面的Address Prefix 12.12.12.0/28 计算出子网中的IP地址数量只有15个。

（Source ： https://jodies.de/ipcalc?host=12.12.12.0&mask1=28&mask2= ）

因为Azure服务会保留5个IP地址，所以当把Redis Cluster服务集成到这个子网后，Redis服务可用的IP地址只剩下10个。而Redis集群至少包含一个分片和一个Load Balancer（负载均衡器）。每一个分片包含两个VM实例，所以需要占用3个IP地址。如此计算，上面的子网中，最多能承载的Redis集群分片数目为 4 （15-5-1- 2*4 =1)。所以剩下的一个不足以添加第五个分片，就会报出 “SubnetIsFull” 异常。

由于Azure VNET Subnet在添加后就不能在修改IP地址范围，所以无法动态扩容（添加更多IP地址），只能删除重建子网。

Azure服务保留IP介绍：

Are there any restrictions on using IP addresses within these subnets?

Yes. Azure reserves 5 IP addresses within each subnet. These are x.x.x.0-x.x.x.3 and the last address of the subnet. x.x.x.1-x.x.x.3 is reserved in each subnet for Azure services.

• x.x.x.0: Network address
• x.x.x.1: Reserved by Azure for the default gateway
• x.x.x.2, x.x.x.3: Reserved by Azure to map the Azure DNS IPs to the VNet space
• x.x.x.255: Network broadcast address for subnets of size /25 and larger. This will be a different address in smaller subnets.

参考资料

Configure virtual network support for a Premium Azure Cache for Redis instance：https://docs.microsoft.com/en-us/azure/azure-cache-for-redis/cache-how-to-premium-vnet

Azure Virtual Network frequently asked questions (FAQ) : https://docs.microsoft.com/en-us/azure/virtual-network/virtual-networks-faq#are-there-any-restrictions-on-using-ip-addresses-within-these-subnets