xenvmd ring protocol

xenvmd-ring-protocol.md

Notes:

• LVM operates in "segments" which are typically (but not always) 4MiB
• device mapper operates in sectors which are either 512 or 4096
• the system should operate in terms of segments (being bigger) and the local allocator can translate down to sectors when needed
• the types below haven't been optimised: in particular there's overlap between the structure of the dm types and the LVM types.
• suspend/ resume signalling is missing
• we should allow the ring to be poisoned, just in case someone reads a ring which has been deallocated
• we will uniquely identify LVs by their UUID, not their name. In particular when we rename an LV, the UUID will not change.
• we will uniquely identify PVs by their UUID, not their name.
• we should specify a byte ordering (little-endian?)

/* The unique id of the PV */
#define PV_ID_LENGTH 32

/* Contiguous physical extents on a single PV */
struct physical_extents {
  char pv[PV_ID_LENGTH];
  uint64_t offset; /* in extents */
  uint64_t count;  /* in extents */
}

#define LV_ID_LENGTH 32

/*************************************************************************/
/* Request to expand an LVM LV by a single segment, sent to the master   */

struct expand_volume {
  char volume[LV_ID_LENGTH];
  physical_extents new_storage;
  uint64_t virtual_extent;      /* location in the LV, in extents */
}

/*************************************************************************/

/* The maximum length of a DM device appears to be 127 chars + trailing NULL
   on Linux-3.13 */
#define DM_DEVICE_LENGTH 128

struct dm_linear_target {
  uint32_t major;
  uint32_t minor;
  uint64_t virtual_sector;
  uint64_t physical_sector;
  uint64_t sector_count;
}

struct expand_dm_device {
  physical_extents used;             /* to remove from the free pool */
  char dm_device[DM_DEVICE_LENGTH];
  dm_linear_target dm_target;
}

/*************************************************************************/
/* The local allocator's transaction                                     */

struct local_allocator_journal {
  expand_volume volume;
  expand_dm_device device;
}