We keep it there for a short duration, until the changes are confirmed to also be written to S3. Writing to 3 instances in 3 availability zones is considered p

We keep it there for a short duration, until the changes are confirmed to also be written to S3. Writing to 3 instances in 3 availability zones is considered persistent enough while also maintaining a high performance, and even though it does not provide the 11 9s of durability that S3 has, 3 availability zones dropping out with loss of all instance-local storage is considered rare enough that we do not think that it will impact our availability and durability guarantees.