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.
Roger_Wu
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2017年1月10日 21:00
可能不同公司对COW、ROW的定义有不同吧，KB原文里的解释是这样的：
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。
Roger_Wu
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2017年1月10日 18:00
就是COW，对于增量磁盘文件的这段文字描述，与COW不矛盾，就是没讲清楚。IBM那个示例就很清楚了。
walke420
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2017年1月10日 18:00
Hi, Roger
谢谢你的答复。
对于Delta disk files存储的就是变化的数据，这个是ROW技术，这个如何来理解是COW技术呢？你能帮忙解惑一下吗？
另外IBM实例中也是同样的问题，第一次快照后，快照空间存储的是改变的数据E和R，那怎么来理解这是COW呢？
Thanks
Walker