Because changes aren't reflected across the contentdelivery networks immediately, ghost copies of files tend to linger for days or weeks.
由于更改不会立即在内容交付网络中反映出来，文件的影子副本往往会持续几天或几周。
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While your personal computer only keeps one copy of a file, large-scale services like Facebook rely on what are called contentdelivery networks to manage data and distribution.