Re: [Qemu-devel] [Qemu-block] [PATCH 0/3] block: Make various formats' block_status recurse again

[John Snow]

On 7/25/19 11:55 AM, Max Reitz wrote:
> Hi,
> 
> 69f47505ee66afaa513305de0c1895a224e52c45 changed block_status so that it
> would only go down to the protocol layer if the format layer returned
> BDRV_BLOCK_RECURSE, thus indicating that it has no sufficient
> information whether a given range in the image is zero or not.
> Generally, this is because the image is preallocated and thus all ranges
> appear as zeroes.
> 
> However, it only implemented this preallocation detection for qcow2.
> There are more formats that support preallocation, though: vdi, vhdx,
> vmdk, vpc.  (Funny how they all start with “v”.)
> 
> For vdi, vmdk, and vpc, the fix is rather simple, because they really
> have different subformats depending on whether an image is preallocated
> or not.  This makes the check very simple.
> 
> vhdx is more like qcow2, where after the image has been created, it
> isn’t clear whether it’s been preallocated or everything is allocated
> because everything was already written to.  69f47505ee added a heuristic
> to qcow2 to get around this, but I think that’s too much for vhdx.  I
> just left it unfixed, because I don’t care that much, honestly (and I
> don’t think anyone else does).
> 

What's the practical outcome of that, and is the limitation documented
somewhere?

(I'm fine with not fixing it, I just want it documented somehow.)

> 
> Max Reitz (3):
>   vdi: Make block_status recurse for fixed images
>   vmdk: Make block_status recurse for flat extents
>   vpc: Do not return RAW from block_status
> 
>  block/vdi.c  | 3 ++-
>  block/vmdk.c | 3 +++
>  block/vpc.c  | 2 +-
>  3 files changed, 6 insertions(+), 2 deletions(-)
>