Re: [PATCH v5 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Max Reitz]

On 22.04.20 17:21, Kevin Wolf wrote:
> If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
> qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
> undo any previous preallocation, but just adds the zero flag to all
> relevant L2 entries. If an external data file is in use, a write_zeroes
> request to the data file is made instead.
> 
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block/qcow2.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 9cfbdfc939..bd632405d1 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c

[...]

> @@ -4214,6 +4215,35 @@ static int coroutine_fn 
> qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
>          g_assert_not_reached();
>      }
>  
> +    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
> +        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
> +        uint64_t zero_end = QEMU_ALIGN_UP(offset, s->cluster_size);
> +
> +        /* Use zero clusters as much as we can */
> +        ret = qcow2_cluster_zeroize(bs, zero_start, zero_end - zero_start, 
> 0);

It’s kind of a pity that this changes the cluster mappings that were
established when using falloc/full preallocation already (i.e., they
become preallocated zero clusters then, so when writing to them, we need
COW again).

But falloc/full preallocation do not guarantee that the new data is
zero, so I suppose this is the only thing we can reasonably do.

I personally don’t really care about whether zero_end is aligned or not,
so either way:

Reviewed-by: Max Reitz <address@hidden>

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