New Feature: Switch to AssumeUTXO

[BenWestgate]

When Bitcoin Core 26.0 is released, switch Bails to use AssumeUTXO.

This will reduce initial time to use from hours or days to minutes, even on slower computers or flash drives.

It will however require a higher minimum USB size than 16GB as there will be two copies of the chainstate folder (now up to 7GB), 32GB will be the new minimum size to support this feature. It won't be worth allowing 16GB to sync without it as they have less than 1GB of free space for blocks at this time and also, 32GB USB flash drives are cheaper than 16GB on Amazon.

[BenWestgate]

Marked high priority as I've had two testers need more than week to sync due to 4GB of RAM and a slow USB stick.

[BenWestgate]

https://github.com/bitcoin/bitcoin/blob/master/doc/design/assumeutxo.md#generating-a-snapshot

[BenWestgate]

@epiccurious have you tried the new commands in v26.0 to sync with AssumeUTXO, it involves hardcoding some chainstate snapshot mirrors or onion services. (I guess I could host one, to keep track of how many people are using this.) Currently there's no P2P for distributing the snapshot. But the checksum of the snapshot is hardcoded into Bitcoin Core so any option works.

Since it's a huge download I would probably want to display a progress bar for the users since it is happening outside of Bitcoin Core.

[BenWestgate]

I consider this a Beta required feature. Otherwise dozens of people are going to be downloading for days and maybe most wont finish and certainly most won't recommend this to others due to the long time until being useful.

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[BenWestgate]

This will be the first of the 3 headline BETA features worked on.

Remember to account for double the assumed_chainstate size when initially setting prune.

After the node catches up, we can increase the prune setting at that time for better future performance.

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