There are two place that dead space can be introduced:
- VMFS — When an administrator deletes a virtual disk or an entire virtual machine (or moves it to another datastore) the space that used to store that virtual disk or virtual machine is now dead on the array. The array does not know that the space has been freed up, therefore, turning those blocks into dead space.
- In-guest — When a file has been moved or deleted from a guest files system inside of a virtual machine on a virtual disk, the underlying VMFS does not know that a block is no longer in use by the virtual disk and consequently neither does the array. So that space is now also dead space.
So dead space can be accumulated in two ways. Fortunately, VMware has methods for dealing with both, that leverage the UNMAP feature support of the FlashArray.