Will all data be 0 for the img created by prealloction=full

[Han Han]

Hi,

I'd like to create a file filled with 0. I am not sure if `qemu-img create FILENAME SIZE -o prealloction=full` will work.

I checked the manual of qemu-img. The description for zeroing is not clear. It says:

```

full mode preallocates space  for  image  by writing data to underlying storage.  This data may or may not be zero, depending on the storage location.

```

What is the meaning of "depending on the storage location"? Which location will work for zeroing, and which will not?