Version3 header first parallel initial block download is now restartable
after a controlled shutdown. Also...

I have been informed by a customer who has been evaluating various node
implementations that version3 downloads 400,000 mainnet blocks in about
45 minutes on a 32 GB RAM machine. That's about twice as fast as on my
old 8 GB machine.

So the < 1.0 hour goal has been achieved!

I expect increasing speed up to 100GB RAM (full chain and indexes in
RAM). I can't say how many core will be fully utilized, as there is some
overhead in parallelization that will produce diminishing returns, but I
assume increasing speed up to around 32 cores. At some point I'll pick
up a new machine with these specs and test it out. My new goal is 30
minutes for the full chain :).

I am presently in the process of reintegrating post-initial block
download chain and mempool management back into version3 for a release
target in August.

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