Re: [Qemu-block] [PATCH v3] qcow2: fix preallocation with metadata on bare block device

[Kevin Wolf]

Am 11.05.2018 um 17:36 hat Ivan Ren geschrieben:
> Create a qcow2 directly on bare block device with
> "-o preallocation=metadata" option. When read this qcow2, it will
> return pre-existing data on block device. This patch add
> QCOW_OFLAG_ZERO flag (supported in qcow_version >= 3) for
> preallocated l2 entry to avoid this problem.
> 
> Signed-off-by: Ivan Ren <address@hidden>

Doesn't this defeat the purpose of preallocation? Usually, the idea with
preallocation is that you don't need to update any metadata on the first
write, but if you set QCOW_OFLAG_ZERO, we do need a metadata update
again.

So what's the advantage compared to not preallocating at all?

Kevin