Para manter a eficiência, as cópias de sombra são realmente armazenadas no mesmo volume. É assim que os instantâneos funcionam. É o mesmo com o ZFS.
The copy-on-write method creates shadow copies that are differential rather than full copies of the original data. Like the clone method of creating shadow copies, the copy-on-write method can produce shadow copies using either software or hardware solutions. This method makes a copy of the original data before it is overwritten with new changes, as shown in the following table. When a change to the original volume occurs, but before it is written to disk, the block about to be modified is read and then written to a “differences area”, which preserves a copy of the data block before it is overwritten with the change. Using the blocks in the differences area and unchanged blocks in the original volume, a shadow copy can be logically constructed that represents the shadow copy at the point in time in which it was created.
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Isso significa que, inicialmente, uma cópia de sombra é apenas um sinalizador. À medida que os dados começam a mudar, os dados alterados são copiados para diferentes locais no disco para que os dados originais permaneçam disponíveis.