Re: [Qemu-devel] [PATCH] qemu-img create: add -o nocow option

[Chunyan Liu]

2013/11/20 Stefan Hajnoczi <address@hidden>

On Wed, Nov 20, 2013 at 04:50:29PM +0800, Chunyan Liu wrote:
>  block/cow.c               |   22 ++++++++++++++++++++++
>  block/qcow.c              |   22 ++++++++++++++++++++++
>  block/qcow2.c             |   22 ++++++++++++++++++++++

I think you can avoid modifying all the image formats:

.bdrv_create() functions pass options to bdrv_create_file().  Therefore
an image format like qcow2 does not need to parse the nocow option
itself.  Only raw-posix.c:.bdrv_create() needs to know about the nocow
option.

In existing code, options passed to bdrv_create_file contains no option in fact.

And if we pass all options to bdrv_create_file directly, raw-posix.c:
.bdrv_create() will get NOCOW option but at the same time get SIZE option, it
will create a file with total size. For cow/qcow/qcow2, I suppose it's not
expected? In current code, bdrv_create_file will create a zero-sized image for
cow/qcow/qcow2.

 

The exception is the block drivers that currently use open(2) directly
instead of bdrv_create_file().  These should be converted to use
bdrv_*() APIs instead of POSIX I/O.  Please either convert them or skip
them (someone will get around to fixing them eventually).

> +    if (nocow) {
> +        QEMUOptionParameter list[] = {
> +            {
> +                .name = BLOCK_OPT_NOCOW,
> +                .type = OPT_FLAG,
> +                .value.n = 1,
> +                .help = "No copy-on-write",
> +                .assigned = true
> +            },
> +            { NULL }
> +        };
> +        options = list;
> +    }

This doesn't look safe to me.  list[] is now out-of-scope (the compiler
can reuse the stack space) and options is a dangling pointer.