1======
2kcopyd
3======
4
5Kcopyd provides the ability to copy a range of sectors from one block-device
6to one or more other block-devices, with an asynchronous completion
7notification. It is used by dm-snapshot and dm-mirror.
8
9Users of kcopyd must first create a client and indicate how many memory pages
10to set aside for their copy jobs. This is done with a call to
11kcopyd_client_create()::
12
13   int kcopyd_client_create(unsigned int num_pages,
14                            struct kcopyd_client **result);
15
16To start a copy job, the user must set up io_region structures to describe
17the source and destinations of the copy. Each io_region indicates a
18block-device along with the starting sector and size of the region. The source
19of the copy is given as one io_region structure, and the destinations of the
20copy are given as an array of io_region structures::
21
22   struct io_region {
23      struct block_device *bdev;
24      sector_t sector;
25      sector_t count;
26   };
27
28To start the copy, the user calls kcopyd_copy(), passing in the client
29pointer, pointers to the source and destination io_regions, the name of a
30completion callback routine, and a pointer to some context data for the copy::
31
32   int kcopyd_copy(struct kcopyd_client *kc, struct io_region *from,
33                   unsigned int num_dests, struct io_region *dests,
34                   unsigned int flags, kcopyd_notify_fn fn, void *context);
35
36   typedef void (*kcopyd_notify_fn)(int read_err, unsigned int write_err,
37				    void *context);
38
39When the copy completes, kcopyd will call the user's completion routine,
40passing back the user's context pointer. It will also indicate if a read or
41write error occurred during the copy.
42
43When a user is done with all their copy jobs, they should call
44kcopyd_client_destroy() to delete the kcopyd client, which will release the
45associated memory pages::
46
47   void kcopyd_client_destroy(struct kcopyd_client *kc);
48