If you write the DB on disk sequentially, every 5 minutes, the I/O you perform is a fixed amount regardless of the amount of writes you have against the dataset. For instance using pipelining Redis can easily peak 400k operations per second, and you can have a few instances in the same box. In this setup 5 minutes of data loss may be acceptable, if you are writing 2 millions of records per second, and RDB make this possible. The I/O performed will always be proportional to the number of keys, it is not proportional to the operations the instances are receiving per second. With Redis AOF, and generally with most other databases, it is unlikely that you have an operational mode where the I/O is simply proportional to the size of the data set, and not to the amount of reads/writes.