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The OS doesn’t actually map/allocate the pages to you until you touch them, so if you preallocate 30 GB and only use one row, you’d only be using 4 KB (or whatever your page size is). This is the same as if you’d individually malloc’d the single row, since deep down malloc just calls mmap.

Joran from the TigerBeetle team here!

We have a secret plan for this too. ;)

The article differentiates between "memory" and "on disk" storage, which is where the rows in the database would live.

This is pretty much how all DBMSs work right now. You have to tell it how much memory it can use for its internal buffer pool.