[Sheepdog] Need inputs on performing some operations atomically

[MORITA Kazutaka]

At Sun, 12 Sep 2010 19:41:34 +0530,
Narendra Prasad Madanapalli wrote:
> 
> Hi,
> 
> I found there are two functions that are to be executed atomically in
> sheep. These functions are below:
> 
> 1. sheep/store.c:
>                         /* FIXME: need to update atomically */
> /*                      ret = verify_object(fd, NULL, 0, 1); */
> /*                      if (ret < 0) { */
> /*                              eprintf("failed to set checksum,
> %"PRIx64"\n", oid); */
> /*                              ret = SD_RES_EIO; */
> /*                              goto out; */
> /*                      } */
> 
> 2. sheep/vdi.c:
> /* TODO: should be performed atomically */
> static int create_vdi_obj(uint32_t epoch, char *name, uint32_t
> new_vid, uint64_t size,
>                           uint32_t base_vid, uint32_t cur_vid, uint32_t copies,
>                           uint32_t snapid, int is_snapshot)
> {
> 
> My understanding is that these two functions get executed in
> worker_routine() in response to queue_request() & queue_work().
> 
> Solution for verify_object()
> Since this operates on file descriptor, I think this can be performed
> with the help of file locking mechanism.

No.  Basically we don't need a lock mechanism for sheepdog objects;
all objects are categorized into the following two groups.
  - only one virtual machine can access the object
  - all virtual machines can read the object, but no one update it

What we need to do here is atomic update of the vdi objects.  For
example, if total node failure happens during updating vdi objects, we
need to roll-back to the previous right state.

> 
> Solution for create_vdi_obj()
> This can be fixed by introducing global pthread lock.

Same as above.  If a master node fails during creating a new vdi
object, the next master need to take over the work, or more easily,
delete the object and return error code to the administrator.


Thanks,

Kazutaka