Kcopyd provides the ability to copy a range of sectors from one block-device to one or more other block-devices, with an asynchronous completion notification. It is used by dm-snapshot and dm-mirror.

Users of kcopyd must first create a client and indicate how many memory pages to set aside for their copy jobs. This is done with a call to kcopyd_client_create():

int kcopyd_client_create(unsigned int num_pages,
                         struct kcopyd_client **result);

To start a copy job, the user must set up io_region structures to describe the source and destinations of the copy. Each io_region indicates a block-device along with the starting sector and size of the region. The source of the copy is given as one io_region structure, and the destinations of the copy are given as an array of io_region structures:

struct io_region {
   struct block_device *bdev;
   sector_t sector;
   sector_t count;

To start the copy, the user calls kcopyd_copy(), passing in the client pointer, pointers to the source and destination io_regions, the name of a completion callback routine, and a pointer to some context data for the copy:

int kcopyd_copy(struct kcopyd_client *kc, struct io_region *from,
                unsigned int num_dests, struct io_region *dests,
                unsigned int flags, kcopyd_notify_fn fn, void *context);

typedef void (*kcopyd_notify_fn)(int read_err, unsigned int write_err,
                                 void *context);

When the copy completes, kcopyd will call the user’s completion routine, passing back the user’s context pointer. It will also indicate if a read or write error occurred during the copy.

When a user is done with all their copy jobs, they should call kcopyd_client_destroy() to delete the kcopyd client, which will release the associated memory pages:

void kcopyd_client_destroy(struct kcopyd_client *kc);