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From experimenting further with this and reading about <a href="http://redis.io/topics/persistence" rel="noreferrer">redis persistence</a>, I think the following observations can be made: <ul> <li>When using RDB (the default settings), redis will fork each time a <code>save</code> 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.</li> <li>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 <code>ps</code>, <code>htop</code> and the like.</li> <li>The fork itself can be quite a cpu-intensive operation, <a href="http://redis.io/topics/latency" rel="noreferrer">particularly on xen-based virtual hosts</a> (which is what we're using currently).</li> <li>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.</li> </ul> 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 <code>auto-aof-rewrite-percentage</code> and <code>auto-aof-rewrite-min-size</code> 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.
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