Understanding chain security as relates to blockchain history size

On the community call this week turning on farming for 3h, Nazar mentioned that there was a pledge level set to ensure reasonable security for the network before enabling rewards. (paraphrased and I’m sure mis-remembered!)

I’m curious how the 8PiB pledge threshold was determined and how to think about that re: security. My naive mental model was considering it a function of chain history size, but 8P seems excessive – I think the blockchain history was about 45GB, so 8P would handle ~175k replicas. Even the (then-current) level of 2.2P would handle ~50k replicas - either would seem excessive to provide resonable security for 45GB of history. I suspect it’s quite a bit more complicated and has to forecast the growth that will come rapidly with rewards being enabled, but wondering how this is calculated and how to think about it.

In Subspace protocol, due to use of Proof of Archival Storage consensus, security of the protocol is directly proportional to space pledged. As more space is pledged to the network, it becomes harder and harder to mount 51% attack because you’ll need to have a lot of plotted space to do so and can’t build longer chain quickly with little space because of Proof of Time protocol that runs in parallel.

So for security reasons we would like space pledged to grow as fast as possible to as high level as possible, this makes “average” space pledged of the protocol at any point in time higher.

As for the target of 8PiB it was based on something that is attainable relatively quickly, but not instantly to give smaller farmers time to plot, etc. Yet everyone is interested to plot and start farming as soon as possible because this will both unlock rewards sooner (currently weekly allocation for farmers is paused until we reach that target in space pledged) and they will be the first ones to get rewards enjoying smaller competition (see Gemini3g space pledged dynamic - #9 by dariolina for space pledged dynamics over time and how high the peak was on 3g, we expect it to be even higher on 3h).

I hope this makes sense.