所以，我来到了一个地方，我想把我存储在redis的数据分割成单独的数据库，因为我有时需要在一种特定的数据上使用键命令，并想把它分开，以使其更快。
If I segment into multiple databases, everything is still single threaded, and I still only get to use one core. If I just launch another instance of Redis on the same box, I get to use an extra core. On top of that, I can't name Redis databases, or give them any sort of more logical identifier. So, with all of that said, why/when would I ever want to use multiple Redis databases instead of just spinning up an extra instance of Redis for each extra database I want? And relatedly, why doesn't Redis try to utilize an extra core for each extra database I add? What's the advantage of being single threaded across databases?
I don't really know any benefits of having multiple databases on a single instance. I guess it's useful if multiple services use the same database server(s), so you can avoid key collisions.
I would not recommend building around using the KEYS command, since it's O(n) and that doesn't scale well. What are you using it for that you can accomplish in another way? Maybe redis isn't the best match for you if functionality like KEYS is vital.
I think they mention the benefits of a single threaded server in their FAQ, but the main thing is simplicity - you don't have to bother with concurrency in any real way. Every action is blocking, so no two things can alter the database at the same time. Ideally you would have one (or more) instances per core of each server, and use a consistent hashing algorithm (or a proxy) to divide the keys among them. Of course, you'll loose some functionality - piping will only work for things on the same server, sorts become harder etc.
甚至Salvatore Sanfilippo (Redis的创造者)也认为在Redis中使用多个db是一个坏主意。点击这里查看他的评论:
https://groups.google.com/d/topic/redis-db/vS5wX8X4Cjg/discussion
I understand how this can be useful, but unfortunately I consider
Redis multiple database errors my worst decision in Redis design at
all... without any kind of real gain, it makes the internals a lot
more complex. The reality is that databases don't scale well for a
number of reason, like active expire of keys and VM. If the DB
selection can be performed with a string I can see this feature being
used as a scalable O(1) dictionary layer, that instead it is not.
With DB numbers, with a default of a few DBs, we are communication
better what this feature is and how can be used I think. I hope that
at some point we can drop the multiple DBs support at all, but I think
it is probably too late as there is a number of people relying on this
feature for their work.
