Copy-on-first- write

关于快照，关于copy－on－first－write。

移动的是原有的数据，而不是更新的数据。

SnapView snapshots use a pointer-and-copy-based design; a memory map keeps track of chunks (groups of blocks) of data. As write requests are made to the active LUN (the source LUN), the chunks are copied to a reserved area consisting of nonvolatile space on private LUNs (the reserved LUN pool1), and the memory map is updated with the new location of each of these chunks. This process, referred to as copy on first write, occurs only when the first write request is made to any chunk on the source LUN, since once the original data is safely copied to the reserved LUN, any number of writes can be made to the same chunk without losing the original view of the data.