Bitcoin Core removes -persistmempoolv1, the option kept for downgrades to v26
Victoria Shinder2 min readBitcoin Core merged pull request #36294 on 13 October 2026, removing the -persistmempoolv1 startup option from the development branch. The change affects how a node writes the mempool.dat file it uses to save unconfirmed transactions across restarts.
What the option was for. According to the pull request, -persistmempoolv1 was introduced in #28207 as a temporary measure. That earlier change began writing mempool.dat in an XOR-obfuscated format, which releases v26 and earlier cannot read. The option let an operator keep writing the old v1 format so that a node could still be downgraded to one of those versions without losing its saved mempool. The documentation had always described it as a "temporary option that will be removed in the future".
Why now. The author, contributor fjahr, argues that v26 has been end-of-life for some time, so every version the option could help downgrade to is now unsupported. On that basis the pull request removes the option without a separate deprecation cycle, reasoning that it serves no maintained release.
What still works. Removal concerns writing the old format only. Reading v1 files remains supported, so a node upgraded directly from an old version still loads the mempool it persisted. The compatibility test mempool_compatibility.py now uses v28.2 as its old node — the oldest release in the project's previous-releases helper that writes the current format.
Who should care. For most operators nothing changes: the default has been the new format since the obfuscation was introduced. The users affected are those who still pass -persistmempoolv1 in a configuration file or startup script, typically to keep a fallback to a pre-v27 binary. Once a release containing this change ships, that flag will no longer be accepted, and a downgrade to v26 or earlier means starting with an empty mempool. The change was merged to master; the 32.0 release is being prepared separately and reached release candidate 4 on 12 October (#36498).
Source: Bitcoin Core, pull request #36294 "mempool: Remove -persistmempoolv1 option" — https://github.com/bitcoin/bitcoin/pull/36294