Under abnormal circumstances, it can happen that a transaction is accepted
into the mempool paying a feerate below 1 sat per byte (or below
1 sat per virtualbyte).
We want that these transactions can be eventually mined, since they may have
unconfirmed children which are being relied on (and the unconfirmed children
might not pay enough for CPFP). Unfortunately if the default is (as now)
-blockmintxfee= 1000 sat/kB then we can guess that no miner will mine them:
some miners increase -blockmintxfee above the default, but I haven't heard
of any that decrease it voluntarily.
Assuming such transactions are well distributed in the network, including
them won't introduce much orphan risk due to the fast block propagation
mechanisms and tx validation caching. And since the circumstances of
low-fee transactions are rare, this doesn't introduce significant damage
to miner profitability, but it can help the network function better.
Here are some possible circumstances why a lowfee tx could be in mempool:
- When a reorg occurs all transactions get thrown back into the mempool, bypassing the normal fee limits. This may include some lowfee transactions.
- Virtualsize may be introduced in a way that does not impact the relaying threshold (1 sat/byte). As a result, nodes may accept transactions below 1 sat/vbyte, provided they are at least 1 sat/byte. If -blockmintxfee refers to the sat/vbyte threshold, then they might not ever get mined.
- Some kind of new mechanism may be introduced where < 1 sat/byte transactions can be pulled out of the orphan pool, provided their children pay enough fee. This does not exist right now, but if the children were later invalidated then the lowfee parent may be all alone in the mempool.
- Mempool reconciliation algorithms may be introduced that result in a node accepting a low fee transaction just because of peer pressure, even though it wouldn't normally do so.