[sheepdog] [PATCH v2 03/11] sheep: introduce generational reference counting for object reclaim

[Liu Yuan]

On Mon, Jul 01, 2013 at 08:46:22AM +0900, MORITA Kazutaka wrote:
> At Fri, 28 Jun 2013 23:53:04 -0700,
> Kai Zhang wrote:
> > 
> > MORITA Kazutaka <morita.kazutaka at gmail.com> writes:
> > 
> > > From: MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp>
> > >
> > > Generational reference counting is an algorithm to reclaim data
> > > efficiently without race conditions on distributed system.  This
> > > extends vdi objects structure to store generational reference counts,
> > > and increments the counts when creating snapshots.
> > >
> > > Signed-off-by: MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp>
> > > ---
> > >  include/sheepdog_proto.h |    9 +++++++--
> > >  sheep/vdi.c              |    8 +++++++-
> > >  2 files changed, 14 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/sheepdog_proto.h b/include/sheepdog_proto.h
> > > index beef0a6..bb0f253 100644
> > > --- a/include/sheepdog_proto.h
> > > +++ b/include/sheepdog_proto.h
> > > @@ -101,8 +101,7 @@
> > >  #define SD_MAX_VDI_SIZE (SD_DATA_OBJ_SIZE * MAX_DATA_OBJS)
> > >
> > >  #define SD_INODE_SIZE (sizeof(struct sd_inode))
> > > -#define SD_INODE_HEADER_SIZE (sizeof(struct sd_inode) - \
> > > -			      sizeof(uint32_t) * MAX_DATA_OBJS)
> > > +#define SD_INODE_HEADER_SIZE offsetof(struct sd_inode, data_vdi_id)
> > >  #define SD_ATTR_OBJ_SIZE (sizeof(struct sheepdog_vdi_attr))
> > >  #define CURRENT_VDI_ID 0
> > >
> > > @@ -193,6 +192,11 @@ struct sd_rsp {
> > >	};
> > >  };
> > >
> > > +struct generation_reference {
> > > +	int32_t generation;
> > > +	int32_t count;
> > > +};
> > > +
> > >  struct sd_inode {
> > >	char name[SD_MAX_VDI_LEN];
> > >	char tag[SD_MAX_VDI_TAG_LEN];
> > > @@ -209,6 +213,7 @@ struct sd_inode {
> > >	uint32_t parent_vdi_id;
> > >	uint32_t child_vdi_id[MAX_CHILDREN];
> > >	uint32_t data_vdi_id[MAX_DATA_OBJS];
> > > +	struct generation_reference data_ref[MAX_DATA_OBJS];
> > >  };
> > >
> > 
> > Does that mean it will incur an incompatable change?
> > Does we need to clean up all data before we upgrade to a new version?
> 
> Basically the old format can be still usable, but the existing objects
> can be reclaimed wrongly since the old inode doesn't contain correct
> reference counts.  In the next version, I think of adding a check to
> avoid this problem and a command to update the object reference counts
> for the existing users.
> 

then QEMU driver need update and old QEMU can't work with new sheep?

Thanks
Yuan