On Wed, Feb 29, 2012 at 1:37 PM, Paolo Bonzini <pbonz...@redhat.com> wrote: > From: Marcelo Tosatti <mtosa...@redhat.com> > > Mirrored writes are used by live block copy. > > The blkmirror driver is for internal use only, because it requires > bdrv_append to set up a backing_hd for it. It relies on a quirk > of bdrv_append, which leaves the old image open for writes. > > The source is hardcoded as the backing_hd for the destination, so that > copy-on-write functions properly. Since the source is not yet available > at the time blkmirror_open is called, the backing_hd is set later. > > Signed-off-by: Marcelo Tosatti <mtosa...@redhat.com> > Signed-off-by: Federico Simoncelli <fsimo...@redhat.com> > Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> > --- > This version of the driver is almost entirely rewritten to > use bs->backing_hd and bs->file. This is necessary in order > to share as much code as possible with group snapshots. > > Makefile.objs | 2 +- > block/blkmirror.c | 153 > ++++++++++++++++++++++++++++++++++++++++++++++++++++ > docs/blkmirror.txt | 16 ++++++ > 3 files changed, 170 insertions(+), 1 deletions(-) > create mode 100644 block/blkmirror.c > create mode 100644 docs/blkmirror.txt
Mostly happy here, I just recommend tweaking the name of this block driver and documenting clearly that this is not a general-purpose mirroring driver, given that it points image B's backing file at image A's backing file. I see this driver as internal functionality and it's fairly easy for users to misuse it and be surprised by the results. > +static int blkmirror_co_writev(BlockDriverState *bs, > + int64_t sector_num, int nb_sectors, > + QEMUIOVector *qiov) > +{ > + int ret; > + > + /* bs->backing_hd is set after initialization. */ > + bs->file->backing_hd = bs->backing_hd; > + > + ret = bdrv_co_writev(bs->backing_hd, sector_num, nb_sectors, qiov); > + if (ret >= 0) { > + ret = bdrv_co_writev(bs->file, sector_num, nb_sectors, qiov); > + } > + > + return ret; > +} Have you done performance tests? It seems suboptimal to use .bdrv_co_writev() and perform writes sequentially, even with cache=unsafe.