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. Kevin