private void logProgress(final BlockHeader blockHeader) {
    if (blockHeader.getNumber() == lastPoWBlockNumber) {
      LOG.info("Pre-merge headers import completed at block {}", blockHeader.toLogString());
    } else {
      long blockNumber = blockHeader.getNumber();
      if (blockNumber % LOG_PROGRESS_INTERVAL == 0) {
        double importPercent = (double) (100 * blockNumber) / lastPoWBlockNumber;
        throttledLog(
            LOG::info,
            String.format(
                "Pre-merge headers import progress: %d of %d (%.2f%%)",
                blockNumber, lastPoWBlockNumber, importPercent),
            shouldLog,
            LOG_REPEAT_DELAY_SECONDS);
      }
    }
  }
A step in the synchronization process that saves historical block headers.
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/SavePreMergeHeadersStep.java