從來沒在用 RDS 的 Multi AZ，所以根本沒注意到居然沒這個功能：「New Multi-AZ deployment option for Amazon RDS for PostgreSQL and for MySQL; increased read capacity, lower and more consistent write transaction latency, and shorter failover time (Preview)」。
看起來 (加上印象中) 之前的 Multi AZ 是另外一台機器先開著但不能用：
In the case of an infrastructure failure, Amazon RDS performs an automatic failover to the standby, so that database operations resume as soon as the failover is complete.
現在則是開著的機器可以跑 readonly 模式：
The standby DB instances act as automatic failover targets and can also serve read traffic to increase throughput without needing to attach additional read replica DB instances.
這樣做除了省成本外，另外因為這些 instance 平常就有 query 的量，當真的遇到 failover 切換時，warmup 的時間也會短很多 (尤其是服務夠大的時候)。
不過有些限制，首先看起來只支援 Graviton2 (ARM-based) 的機種？
The readable standby option for Amazon RDS Multi-AZ deployments works with AWS Graviton2 R6gd and M6gd DB instances (with NVMe-based SSD instance storage) and Provisioned IOPS Database Storage.
然後是支援的區域：
The Preview is available in the US East (N. Virginia), US West (Oregon), and Europe (Ireland) regions.
以及夠新的版本，MySQL 8 與 PostgreSQL 13.4 才有提供：
Amazon RDS for MySQL supports the Multi-AZ readable standby option for MySQL version 8.0.26. Amazon RDS for PostgreSQL supports the Multi-AZ readable standby option for PostgreSQL version 13.4.
但看起來還不錯，畢竟這比較接近以前在地端機房時的作法...