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why copy on write (CoW) save storage space?

  • 1.  why copy on write (CoW) save storage space?

    Posted Dec 20, 2017 03:59 PM

    Hello,

    I read the virtual disk type doc Virtual Disk Types .

    It says this "Sparse disks employ the copy-on-write (COW) mechanism, in which virtual disk contains no data in places, until copied there by a write. This optimization saves storage space."

    If I write some thing to disk for multiple times, so according to "copied there (to the disk) by a write", the disk will maintain several copies of previous disk data.

    The storage will have duplicated old data, so why it says "This optimization saves storage space." ?

    anyone can tell me why? thanks in advance.