Documentation/admin-guide/device-mapper/kcopyd.rst

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kcopyd
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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);