[sheepdog] [PATCH v3 1/9] sheep: introduce generational reference counting for object reclaim

[Liu Yuan]

On Sun, Feb 23, 2014 at 02:28:20PM +0900, Hitoshi Mitake wrote:
> 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.
> 
> Cc: MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp>
> Cc: Valerio Pachera <sirio81 at gmail.com>
> Cc: Alessandro Bolgia <alessandro at extensys.it>
> Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
> ---
>  include/sheepdog_proto.h |  6 +++++
>  sheep/vdi.c              | 57 ++++++++++++++++++++++++++++++++++++------------
>  2 files changed, 49 insertions(+), 14 deletions(-)
> 
> diff --git a/include/sheepdog_proto.h b/include/sheepdog_proto.h
> index 1d17e8f..39d57aa 100644
> --- a/include/sheepdog_proto.h
> +++ b/include/sheepdog_proto.h
> @@ -211,6 +211,11 @@ 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];
> @@ -229,6 +234,7 @@ struct sd_inode {
>  	uint32_t child_vdi_id[MAX_CHILDREN];
>  	uint32_t data_vdi_id[SD_INODE_DATA_INDEX];
>  	uint32_t btree_counter;
> +	struct generation_reference data_ref[SD_INODE_DATA_INDEX];

data_ref -> gref looks better.
	
It seems that with this change, it would be much harder to have hyper volume
support snapshot/clone. It means we need a btree-like management to manage 
data_ref.

Or can we modulize the deletion path, and make it support two different deletion
algorithm, one is current stupid one and the other generational ref? In this way
hyper volume can make use of old algorithm and then have it support snpashot
looks easier.

Anyway, let hyper volume support snapshot/clone is a far future stuff. I am not
sure if we should consider it.

Thanks
Yuan