[libvirt] [PATCH 3/3] Add volume zeroing

[Paolo Bonzini]

On 02/16/2010 12:33 PM, Daniel P. Berrange wrote:
> The idea of zeroing upon delete, is that we want the currently allocated
> extents to be overwritten with zeros. If we truncate to 0 size, then
> extend to original size I imagine that would easily allow the filesystem
> to give you a new set of extents filled with zeros, leaving the old
> extents with data intact as unused space on the FS.

Yeah, as long as you use regular files as images you're safe, but you'd 
expose the old data if you destroyed the partition on which the file 
resided and used the partition as storage for a new guest.

But then in this scenario (delete file system partition and give it out 
as raw) you could expose information not only about other/old guests, 
but also about the host.  So for partitions it can be even more 
important to provide an option to zero the partition _before_ giving it 
out.  Currently you cannot do that with libvirt.

> Also, we need to make sure that this code works with physical block
> devices, as well as plain files. You can't truncate a block device so
> we'll need to write zeros in that case anyway.

A block device won't ever be sparse---I was talking about the sparse 
case only.

Paolo