redis memory and cpu spikes

Question

We use redis for some data in our app, and it\'s totally great. I noticed however occasional cpu and memory spikes on the redis-server process.

Answer 1

From experimenting further with this and reading about redis persistence, I think the following observations can be made:

• When using RDB (the default settings), redis will fork each time a save operation is triggered, which (by default) is set to once every 15 minutes as a minimum. When more writes to Redis are performed, then RDB writes are as frequent as once every 60 seconds.
• Each fork will use a "copy-on-write" memory allocation, which means that whilst memory won't actually double - it will appear so on tools like ps, htop and the like.
• The fork itself can be quite a cpu-intensive operation, particularly on xen-based virtual hosts (which is what we're using currently).
• The write operation seems to completely overwrite the existing RDB file. It does not write only the changes, but rather dumps the entire dataset to disk.

So on a modest virtual host with 4Gb RAM and data set of around 750Mb (at the time I posted the question), this starts to become rather "expensive". We observed those CPU/Memory spikes, as well as increased IO, even under fairly moderate load / redis usage.

So to answer my own question - this does seem to be the "expected" behaviour.

As for improving the situation, we opted to switch our configuration to use a combination of RDB and AOF. AOF (Append Only File), does appear to only write changes to disk. You can (and should) still configure the AOF file to rewrite (using auto-aof-rewrite-percentage and auto-aof-rewrite-min-size settings). It is also advisable to still use RDB for snapshots. In this configuration however, you can probably do full rewrites / snapshots less frequently and still maintain pretty-good performance and even-better durability.