void markCopyStart() {
    CycleLog cycleLog = getCurrentCycleLog();
    if (!cycleLog.getPhase().equals(Phase.PREPARE)) {
      throw new IllegalStateException("Should be in PREPARE phase to transition to COPY phase");
    }
    cycleLog.copyStartTime = time.milliseconds();
    flush();
    logger.trace("{}: Marked copy as started for {}", file, cycleLog.compactionDetails);
  }
Marks the start of the copy phase.
ambry-store/src/main/java/com/github/ambry/store/CompactionLog.java