[sheepdog] [PATCH v2 2/4] dog: use nr_copies of inode object instead of command line parameter

[Hitoshi Mitake]

On Thu, May 8, 2014 at 2:43 PM, Liu Yuan <namei.unix at gmail.com> wrote:
> On Wed, May 07, 2014 at 11:38:28PM +0900, Hitoshi Mitake wrote:
>> From: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
>>
>> Current "dog vdi clone" has a possibility of creating clone volume
>> which has a different replica number from its original volume when it
>> receives -c option. This patch forbid such a situation by letting
>> vdi_clone() use inode->nr_copies instead of vdi_cmd_data.nr_copies
>> (the variable which holds a value passed with -c option).
>
> what is the problem of a clone that has different copies than its base?

If we allow different copy numbers in one VDI family, we have to run
recovery after creating clone.

e.g. assume a case of creating a clone with 4 copies from a snapshot
with 3 copies. After creating clones, sheep must create one more
copies of objects. VDI deletion, discard, COW will be more difficult.
Every object should have a maximum number of copies of every VDIs
which point the object.

Even if we have a feature for runtime changing of copy number in the
future, we have to implement the above complicated feature and we
don't have it now. So the copy number of clones must be equal to its
base volume.

Thanks,
Hitoshi