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 <cy...@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
