Re: [Qemu-devel] [PATCH] qemu-img: set nocow flag to new file

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Nov 14, 2013 at 04:15:28PM +0800, Chunyan Liu wrote:
> Set NOCOW flag to newly created images to solve performance issues on btrfs.
> 
> Btrfs has terrible 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 (since having copy on
> write for this kind of data is not useful).
> 
> Signed-off-by: Chunyan Liu <cyliu@suse.com>
> ---
>  block/raw-posix.c     |    6 ++++++
>  block/vdi.c           |    7 +++++++
>  block/vmdk.c          |    7 +++++++
>  include/qemu-common.h |    9 +++++++++
>  4 files changed, 29 insertions(+), 0 deletions(-)
> 
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index f6d48bb..4a3e9d0 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -1072,6 +1072,12 @@ static int raw_create(const char *filename, QEMUOptionParameter *options,
>          result = -errno;
>          error_setg_errno(errp, -result, "Could not create file");
>      } else {
> +#ifdef __linux__
> +        /* set NOCOW flag to solve performance issue on fs like btrfs */
> +        int attr;
> +        attr = FS_NOCOW_FL;
> +        ioctl(fd, FS_IOC_SETFLAGS, &attr);
> +#endif

This should be optional and I'm not sure it should be the default.

Rationale: If you're on btrfs you probably expect the copy-on-write and
snapshot features of the file system.  We shouldn't silently disable
that unless the user asks for it.

Stefan