What is the ideal value size range for redis? Is 100KB too large?

Question

Is there an upper limit to the suggested size of the value stored for a particular key in Redis?

Is 100KB too large?

Answer 1

There are two things that you need to take into consideration when deciding if something is "too big".

Does Redis have support for the size of key/value object that you want to store?

The answer to this question is documented pretty well on the Redis site (https://redis.io/topics/data-types), so I won't go into detail here.

For a given key/value size, what are the consequences I need to be aware of?

This is a much more nuanced answer as it depends heavily on how you are using Redis and what behaviors are acceptable to your application and which ones are not.

For instance, larger key/value sizes can lead to fragmentation of the memory space within your server. If you aren't using all the memory in your Redis server anyway, then this may not be a big deal to you. However, if you need to squeeze all of the memory out of your Redis server you can, then you are now reducing the efficiency of how memory is allocated and you are losing access to some memory that you would otherwise have.

As another example, when you are reading these large key/value entries from Redis, it means you have to transfer more data over the network from the server to the client. Some consequences of this are:

1. It takes more time to transfer the data, so your client may need to have a higher timeout value configured to allow for this additional transfer time.

2. Requests made to the server on the same TCP connection can get stuck behind the big transfer and cause other requests to timeout. See here for an example scenario.

3. Your network buffers used to transfer this data can impact available memory on the client or server, which can aggravate the available memory issues already described around fragmentation.

4. If these large key/value items are accessed frequently, this magnifies the impacts described above as you are repeatedly transferring this data over and over again.

So, the answer is not a crisp "yes" or "no", but some things that you should consider and possibly test for your expected workload. In general, I do advise our customers to try to stay as small as possible and I have often said to try to stay below 100kb, but I have also seen plenty of customers use Redis with larger values (in the MB range). Sometimes those larger values are no big deal. In other cases, it may not be an issue until months or years later when their application changes in load or behavior.