2013/11/14 Kevin Wolf <kw...@redhat.com>
> Am 14.11.2013 um 09:15 hat Chunyan Liu geschrieben:
> > 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
>
> ioctl() returning an error is ignored. This is probably okay because
> we're only talking about an optimisation here. Perhaps worth a word or
> two in the comment.
>
> However, while this ioctl is setting FS_NOCOW_FL, it is at the same time
> clearing all other flags. This doesn't look right.
>
Yes, strictly it should be GETFLAGS and then SETFLAGS. Here because it does
FS_IOC_SETFLAGS right after creating the file, and checking the qemu_open()
parameter, in fact no FLAGS has been set, so just setting FS_NOCOW_FL
directly.
I can revise that if it's not good.
>
> Kevin
>
>
