The Netflix Technology Blog walks us through how they do caching on SSDs:
Storing large amounts of data in volatile memory (RAM) is expensive. Modern disk technologies based on SSD are providing fast access to data but at a much lower cost when compared to RAM. Hence, we wanted to move part of the data out of memory without sacrificing availability or performance. The cost to store 1 TB of data on SSD is much lower than storing the same amount in RAM.
We observed during experimentation that RAM random read latencies were rarely higher than 1 microsecond whereas typical SSD random read speeds are between 100–500 microseconds. For EVCache our typical SLA (Service Level Agreement) is around 1 millisecond with a default timeout of 20 milliseconds while serving around 100K RPS. During our testing using the storage optimized EC2 instances (I3.2xlarge) we noticed that we were able to perform over 200K IOPS of 1K byte items thus meeting our throughput goals with latency rarely exceeding 1 millisecond. This meant that by using SSD (NVMe) we were able to meet our SLA and throughput requirements at a significantly lower cost.
NVMe isn’t as fast as RAM, but we are well beyond the days of spinning disk hard drives.