协调陈旧缓存数据的失效，以及在n层缓存中复制更新。
Coordinating the invalidation of stale cache data as well as the replication of updated across an N-tier cache.
文件系统内容通常都是跨cell复制，以便一个硬盘的失效不会损害OpenAFS客户机上的运行。
File system content is usually replicated across the cell so that failure of one hard disk would not impair working at the OpenAFS client.
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