Re: [PATCH 1/4] qcow2: Add keep_data_file command-line option

[Kevin Wolf <kwolf@redhat.com>]

Am 30.05.2025 um 10:44 hat Hanna Czenczek geschrieben:
> Add a command-line-only option to prevent overwriting the file specified
> as external data file.
> 
> This option is only available on the qemu-img create command line, not
> via blockdev-create, as it makes no sense there: That interface
> separates file creation and formatting, so where the external data file
> attached to a newly formatted qcow2 node comes from is completely up to
> the user.
> 
> Implementation detail: Enabling this option will not only not overwrite
> the external data file, but also assume it already exists, for two
> reasons:
> - It is simpler than checking whether the file exists, and only skipping
>   creating it when it does not.  It is therefore also less error-prone,
>   i.e. we can never accidentally overwrite an existing file because we
>   made some mistake in checking whether it exists.
> - I think it makes sense from a user's perspective: You set this option
>   when you want to use an existing data file, and you unset it when you
>   want a new one.  Getting an error when you expect to use an existing
>   data file seems to me a nice warning that something is not right.
> 
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

> @@ -4004,6 +4029,40 @@ qcow2_co_create_opts(BlockDriver *drv, const char *filename, QemuOpts *opts,
>          goto finish;
>      }
>  
> +    qcow2_opts = &create_options->u.qcow2;
> +
> +    if (!qcow2_opts->has_preallocation) {
> +        qcow2_opts->preallocation = PREALLOC_MODE_OFF;
> +    }
> +    if (!qcow2_opts->has_data_file_raw) {
> +        qcow2_opts->data_file_raw = false;
> +    }

These checks are technically redundant, QAPI already gives us a zeroed
object. At least for the enum, it might still be nice not to rely on the
order. I don't think there's a reason for the bool one, though.

Kevin