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:
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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