Major Description
Identified early in the design of CDC was that there is a minimum amount of history that we can retain, e.g. 1 byte is not realistic, there is some granularity that magma operates with. The real minimum per vbucket is actually 2MiB this occurs because magma will always retain at least 1 "sstable of history", resulting in 2GiB for 1024 vbuckets. This leads to an issue in that any history_retention_bytes value under 2GiB could lead a user to question the systems behaviour. Additionally if we do permit <2GiB the current alerting is misleading, for example if I configure 1GiB of history and 180 seconds of time, the ns_server alert will trigger when my history size is 90% of the 1GiB, but the reality is that magma won't need to "trim" history until 2GiB is exceeded.
For 7.2 we should set a minimum configuration value of 2GiB and ensure documentation (Tony Hillman) explains why this exists.