Summary: They used to cache things immediately onto SSDs. This causes lots of wear because you end up writing lots of data that it turns out you didn't want to.

Summary: They used to cache things immediately onto SSDs. This causes lots of wear because you end up writing lots of data that it turns out you didn't want to. To fix this, they first stage things in memory. If an item is popular enough and worth caching, they store it onto disk (hence reducing the number of writes on the SSD). Both with the old and new approaches, they rely on the OS page cache to keep hot items in memory.