...标志位的组合，取决于具体情况； vfork的优点：fork需要拷贝父进程的页表项,而vfork不需要； 写时拷贝(COW-Copy On Write): COW是一种可以推迟甚至免除拷贝数据的技术。
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This is accomplished by having an exception list that is updated every time something changes between the LVs (formally known as CoW, Copy-on-Write).
快照由一个例外列表(exception list)来实现，每当LV之间出现差异时就会更新这个列表(正式的说法是CoW, Copy - on - Write)。
Copy-on-write (or COW) is a technique to delay or altogether prevent copying of the data.
即写即拷(或牛)技术完全延迟或防止复制的数据。
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