>>> On 6/27/2014 at 07:48 PM, in message
<20140627114806.gm12...@stefanha-thinkpad.muc.redhat.com>, Stefan Hajnoczi
<stefa...@redhat.com> wrote:
> 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 <cy...@suse.com>
> > ---
> > 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:
Right. For those calling bdrv_create_file to create file, it's not necessary
to add NOCOW option to their .create_opts. Adding NOCOW to raw-posix.c
is enough. There will be no difference to users when they do:
qemu-img create -f fmt name size -o nocow=on
or
qemu-img create -f fmt name size -o ?
>
> 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;
> }
> }
>
