在数据迁移、跨云同步与对象存储备份等场景中，juicefs sync 常用于执行大规模数据同步任务。当数据规模达到 TB 到 PB 级、对象数量达到数百万甚至数十亿级时，单次任务执行周期通常会延长到数小时甚至数天。
在这个过程中，系统运行过程中通常会逐步暴露出以下几类问题：
- 任务在网络抖动、进程异常退出或节点重启后，难以从一致状态继续执行，往往需要重新扫描或重复处理；
- 数据备份场景可能存在明文暴露风险并可能面临合规与安全要求；
- 多个同步任务并发运行时，带宽资源竞争明显，整体传输过程缺乏有效的全局控制手段。
围绕这些场景，JuiceFS 1.4 在 sync 中提供了三项能力增强：断点续传、数据加解密，以及全局流量控制。本文将围绕这三项能力，介绍其适用场景、实现机制和使用方式。
01 断点续传
在早期版本中，用户同步过程中遇到错误或者中断，重新执行任务时，juicefs sync 需要重新扫描源端和目标端，再判断哪些对象已经完成、哪些对象仍需复制。对于上亿级对象或大量大文件场景，仅重新扫描本身就可能带来显著的时间成本和对象存储请求开销。
为了解决这一问题，我们在 JuiceFS 1.4 中为 sync 引入了 断点续传机制。启用后，sync 会将任务进度保存到目标端。当任务中断后，用户只需重新执行相同命令，sync 就会自动查找并加载与当前源端、目标端及关键参数匹配的 checkpoint，从上次未完成的位置继续执行，避免从头重新处理。
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