They speak about compressing the data before "storing" it.
I don't have a lot of experience with inmemory anything, but are we talking about retaining the compressed format in server memory here? Ie, RAM is your datastore.
Then, at some point, to serve requests/queries for the data don't you have to get it "out of" RAM and uncompress it, also an inmemory operation?
Did I get this right?
Somebody implemented the algorithm in go based on the paper here: https://github.com/dgryski/go-tsz
A short answer that may work for your question: the bits that are set in RAM are xor values relative to previous values. To provide an answer as to what the value is, a series of read|xor operations are performed.
For series without periodic sampling, delta-of-deltas performs similarly to normal deltas.
I thought they were talking about healthcare data too when I first read it but seeing as this is Facebook I realized they are talking about server health not human health.