Three sneaky changes in Bitcoin Core v30 are confusing node operators
A vocal community of node operators who prefer traditional limits on the amount of arbitrary data that can accompany a bitcoin (BTC) transaction is infuriated with an upcoming software release for at least three reasons.
Bitcoin Core version 30 (v30) is the upcoming version of the network’s most popular full node software. It will drastically increase the amount of data unrelated to the on-chain movement of BTC that nodes will accept by default into their mempool.
However, the release is also confusing Bitcoin Core users who’ve grown accustomed to a data filter on OP_RETURN outputs which has operated since 2011.
Specifically, users who want to limit the amount of data unrelated to the on-chain movement of BTC will have to jump through confusing hoops, including a rewritten config option that changes the effect of a simple number that has done the same thing for more than a decade.
In v30, Core devs will suddenly nerf “datacarriersize=” by about 88%.
In all, there are three major changes from Bitcoin Core version 29.0, and the upcoming v30 scheduled for release in October 2025.
Bitcoin Core v30: More data, more confusion about how to limit it
Three changes will come into effect with Bitcoin Core v30 in October.
First, v30 will, for the first time in more than a decade, allow transactions into a node’s default mempool with more than one OP_RETURN output.
New standardness rules in v30 accept multiple OP_RETURN outputs within transactions into the default mempool of Core nodes.
Second, Core developers have rewritten the v30 configurability setting “datacarriersize=” from signifying what it has meant for years.
This user-configurable number used to specify the number of allowable bytes of data that a node’s mempool would accept within one OP_RETURN output.
In v30, this same number and setting will now permit nine times more data than that same number would have allowed in v29 and prior versions.
Knots developer Luke Dashjr, whose software has displaced Core on about 16% of the reachable nodes across Bitcoin’s network, illustrated this confusing change with an example of datacarriersize=83.
In v29 and prior, any Core node operator specifying the number 83 here would have limited OP_RETURN arbitrary data to 92 bytes per transaction.
In v30, however, any user specifying the same figure will suddenly allow 830 bytes of arbitrary data.
This is because Core developers have rewritten the configurability setting “datacarriersize=” entirely to accommodate v30’s new standardness rules to allow multiple OP_RETURN outputs instead of just one.
He calls the slick change a “trick” by “bad actors.”
Deprecating user configurability of OP_RETURN storage limits
Finally, Core developers are resetting the default filter from a few bytes to nearly 4MB. They’re also marking this user-configurable filter for deprecation entirely as a way to advise users to not rely on the configurability setting “datacarriersize=” in upcoming releases.
Although Core developers will technically allow v30 node operators users to lower the data storage number via the setting “datacarriersize=”, they’ve not only changed the meaning of datacarriersize but also told users that this rejiggered configurability option will be deprecated soon anyway.
As a result of these changes, Knots leaders are complaining about Bitcoin Core v30 having “malicious code” because it will deprecate the user configurability of datacarriersize and permanently enshrine a new default that accommodates arbitrary data into mempools at a scale never before seen in Bitcoin’s history.
Read more: Knots ‘warning’ escalates Bitcoin OP_RETURN war
A GitHub post by Instagibbs attempts to clarify that Bitcoin Core developers still retained datacarrier arguments, although they are marked as deprecated.
As a result, Bitcoin Core developers haven’t promised to keep these variables forever.
Bitcoin Core developers at Chaincode Labs seem intent on pushing all of these changes through with v30 despite their drastic impact and confusing implementation for average node operators.
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