Re: [Qemu-devel] [PATCH V3] qemu-img create: add 'nocow' option

[Stefan Hajnoczi]

On Mon, Jun 23, 2014 at 05:17:02PM +0800, Chunyan Liu wrote:
> Add 'nocow' option so that users could have a chance to set NOCOW flag to
> newly created files. It's useful on btrfs file system to enhance performance.
> 
> Btrfs has low performance when hosting VM images, even more when the guest
> in those VM are also using btrfs as file system. One way to mitigate this bad
> performance is to turn off COW attributes on VM files. Generally, there are
> two ways to turn off NOCOW on btrfs: a) by mounting fs with nodatacow, then
> all newly created files will be NOCOW. b) per file. Add the NOCOW file
> attribute. It could only be done to empty or new files.
> 
> This patch tries the second way, according to the option, it could add NOCOW
> per file.
> 
> For most block drivers, since the create file step is in raw-posix.c, so we
> can do setting NOCOW flag ioctl in raw-posix.c only.
> 
> But there are some exceptions, like block/vpc.c and block/vdi.c, they are
> creating file by calling qemu_open directly. For them, do the same setting
> NOCOW flag ioctl work in them separately.
> 
> Signed-off-by: Chunyan Liu <address@hidden>
> ---
> Changes to v2:
>   * based on QemuOpts instead of old QEMUOptionParameters
>   * add nocow description in man page and html doc
> 
>   Old v2 is here:
>   http://lists.gnu.org/archive/html/qemu-devel/2013-11/msg02429.html
> 
> ---
>  block/cow.c               |  5 +++++
>  block/qcow.c              |  5 +++++
>  block/qcow2.c             |  5 +++++
>  block/qed.c               | 11 ++++++++---
>  block/raw-posix.c         | 25 +++++++++++++++++++++++++
>  block/vdi.c               | 29 +++++++++++++++++++++++++++++
>  block/vhdx.c              |  5 +++++
>  block/vmdk.c              | 11 ++++++++---
>  block/vpc.c               | 29 +++++++++++++++++++++++++++++
>  include/block/block_int.h |  1 +
>  qemu-doc.texi             | 16 ++++++++++++++++
>  qemu-img.texi             | 16 ++++++++++++++++
>  12 files changed, 152 insertions(+), 6 deletions(-)

Are you sure it's necessary to touch all image formats in order to pass
through the nocow option?  Looking at bdrv_img_create() I think it will
work without touching all image formats since both drv and
proto_drv->create_opts are appended:

void bdrv_img_create(const char *filename, const char *fmt,
                     const char *base_filename, const char *base_fmt,
                     char *options, uint64_t img_size, int flags,
                     Error **errp, bool quiet)
{
    QemuOptsList *create_opts = NULL;
...
    create_opts = qemu_opts_append(create_opts, drv->create_opts);
    create_opts = qemu_opts_append(create_opts, proto_drv->create_opts);

    /* Create parameter list with default values */
    opts = qemu_opts_create(create_opts, NULL, 0, &error_abort);
    qemu_opt_set_number(opts, BLOCK_OPT_SIZE, img_size);

    /* Parse -o options */
    if (options) {
        if (qemu_opts_do_parse(opts, options, NULL) != 0) {
            error_setg(errp, "Invalid options for file format '%s'", fmt);
            goto out;
        }
    }

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