Re: [PATCH v7 10/10] qcow2: Forward ZERO_WRITE flag for full preallocation

[Max Reitz]

On 24.04.20 16:27, Kevin Wolf wrote:
> The BDRV_REQ_ZERO_WRITE is currently implemented in a way that first the
> image is possibly preallocated and then the zero flag is added to all
> clusters. This means that a copy-on-write operation may be needed when
> writing to these clusters, despite having used preallocation, negating
> one of the major benefits of preallocation.
> 
> Instead, try to forward the BDRV_REQ_ZERO_WRITE to the protocol driver,
> and if the protocol driver can ensure that the new area reads as zeros,
> we can skip setting the zero flag in the qcow2 layer.
> 
> Unfortunately, the same approach doesn't work for metadata
> preallocation, so we'll still set the zero flag there.
> 
> Signed-off-by: Kevin Wolf <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>
> ---
>  block/qcow2.c              | 22 +++++++++++++++++++---
>  tests/qemu-iotests/274.out |  4 ++--
>  2 files changed, 21 insertions(+), 5 deletions(-)

Thanks for the resend! :)

Max

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