doc: UTXO growth projections and monitoring guidance for 1-minute blocks

[augchan42]

Context

With BTQ's 1-minute block time and Dilithium signatures, UTXO set and chain growth dynamics differ significantly from Bitcoin. The feature/utxo-optimisations branch (commits 2b44ec5..393311e) reduced the theoretical maximum from ~75 GB/day to ~310 MB/day UTXO growth at full capacity via WITNESS_SCALE_FACTOR=16 and 8 MB blocks.

This issue documents projected growth under realistic mining scenarios and recommends monitoring practices.

Mining-Only Growth Projections

Assumptions: 1 pool, batch payout every ~100 blocks (14/day), 100 Dilithium inputs per payout, miners HODLing.

Miners	UTXO growth/yr	Chain growth/yr	UTXO set @ 3 yrs
10	~5 MB	~2.1 GB	~15 MB
100	~46 MB	~2.2 GB	~138 MB
1,000	~460 MB	~2.4 GB	~1.4 GB
5,000	~2.3 GB	~3.2 GB	~6.9 GB
10,000	~4.6 GB	~4.3 GB	~13.8 GB

If miners spend/sell within ~30 days, the UTXO set stabilizes instead of growing linearly:

• 1,000 miners, 30-day turnover: ~420K UTXOs → ~38 MB steady state
• 10,000 miners, 30-day turnover: ~4.2M UTXOs → ~378 MB steady state

Key Thresholds

• ~1,000 miners: UTXO growth rate matches Bitcoin's entire rate (mining + all user txs). Worth monitoring.
• ~5,000 miners: UTXO set exceeds default dbcache (450 MB) within months. Nodes with low RAM need config changes.
• ~10,000 miners: Single payout tx to ~10,400 miners fills an entire block (7.6 MW). Hard ceiling for single-tx payouts. UTXO set approaches Bitcoin's current total (~7-10 GB) within 2 years.

Coinbase UTXO Baseline

1-minute blocks produce 1,440 coinbase UTXOs/day (vs Bitcoin's 144). This is a guaranteed UTXO growth floor regardless of network usage. Pool fan-out to miners amplifies this — even modest fan-out (1 coinbase → 10 payouts) creates ~14,400 UTXOs/day from mining alone.

Recommended Monitoring

• gettxoutsetinfo — track txouts count and disk_size over time
• getblockstats <height> — monitor utxo_increase per block and total_weight for block fullness
• getmempoolinfo — watch for dust UTXO accumulation patterns
• getblockchaininfo — size_on_disk for total chain storage

Warning Signs

• UTXO utxo_increase consistently positive and growing across blocks
• Blocks consistently >80% full with low fee pressure (cheap to bloat)
• Large accumulation of dust UTXOs below the spend-worthiness threshold
• Node sync times increasing significantly for new nodes

Future Considerations

If growth becomes a concern, potential levers include:

• Smaller block size cap (e.g., 4 MB)
• Encouraging batched/consolidated pool payouts
• Adjusting dust thresholds for Dilithium's higher spend cost
• Pruning by default for non-archival nodes
• UTXO commitments / assumeUTXO for fast node bootstrapping