Redis persistence demystified

Part of my work on Redis is reading blog posts, forum messages, and the twitter time line for the "Redis" search. It is very important for a developer to have a feeling about what the community of users, and the community of non users, think about the product he is developing. And my feeling is that there is no Redis feature that is as misunderstood as its persistence. 

In this blog post I'll do an effort to be truly impartial: no advertising of Redis, no attempt to skip the details that may put Redis in a bad light. All I want is simply to provide a clear, understandable picture of how Redis persistence works, how much reliable is, and how it compares to other database systems.

The OS and the disk

The first thing to consider is what we can expect from a database in terms of durability. In order to do so we can visualize what happens during a simple write operation:

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