The child disk, which is created with a snapshot, is a sparse disk. Sparse disks employ the copy-on-write (COW) mechanism, in which the virtual disk contains no data in places, until copied there by a write. This optimization saves storage space. The grain is the unit of measure in which the sparse disk uses the copy-on-write mechanism. Each grain is a block of sectors containing virtual disk data. The default size is 128 sectors or 64 KB.
我也没有在它处看到更多关于VMware快照COW的更详细解释。相比之下，EMC CLARiiON快照的COFW (Copy-On-First-Write)就清楚多了。