Root/Documentation/device-mapper/dm-io.txt

1dm-io
2=====
3
4Dm-io provides synchronous and asynchronous I/O services. There are three
5types of I/O services available, and each type has a sync and an async
6version.
7
8The user must set up an io_region structure to describe the desired location
9of the I/O. Each io_region indicates a block-device along with the starting
10sector and size of the region.
11
12   struct io_region {
13      struct block_device *bdev;
14      sector_t sector;
15      sector_t count;
16   };
17
18Dm-io can read from one io_region or write to one or more io_regions. Writes
19to multiple regions are specified by an array of io_region structures.
20
21The first I/O service type takes a list of memory pages as the data buffer for
22the I/O, along with an offset into the first page.
23
24   struct page_list {
25      struct page_list *next;
26      struct page *page;
27   };
28
29   int dm_io_sync(unsigned int num_regions, struct io_region *where, int rw,
30                  struct page_list *pl, unsigned int offset,
31                  unsigned long *error_bits);
32   int dm_io_async(unsigned int num_regions, struct io_region *where, int rw,
33                   struct page_list *pl, unsigned int offset,
34                   io_notify_fn fn, void *context);
35
36The second I/O service type takes an array of bio vectors as the data buffer
37for the I/O. This service can be handy if the caller has a pre-assembled bio,
38but wants to direct different portions of the bio to different devices.
39
40   int dm_io_sync_bvec(unsigned int num_regions, struct io_region *where,
41                       int rw, struct bio_vec *bvec,
42                       unsigned long *error_bits);
43   int dm_io_async_bvec(unsigned int num_regions, struct io_region *where,
44                        int rw, struct bio_vec *bvec,
45                        io_notify_fn fn, void *context);
46
47The third I/O service type takes a pointer to a vmalloc'd memory buffer as the
48data buffer for the I/O. This service can be handy if the caller needs to do
49I/O to a large region but doesn't want to allocate a large number of individual
50memory pages.
51
52   int dm_io_sync_vm(unsigned int num_regions, struct io_region *where, int rw,
53                     void *data, unsigned long *error_bits);
54   int dm_io_async_vm(unsigned int num_regions, struct io_region *where, int rw,
55                      void *data, io_notify_fn fn, void *context);
56
57Callers of the asynchronous I/O services must include the name of a completion
58callback routine and a pointer to some context data for the I/O.
59
60   typedef void (*io_notify_fn)(unsigned long error, void *context);
61
62The "error" parameter in this callback, as well as the "*error" parameter in
63all of the synchronous versions, is a bitset (instead of a simple error value).
64In the case of an write-I/O to multiple regions, this bitset allows dm-io to
65indicate success or failure on each individual region.
66
67Before using any of the dm-io services, the user should call dm_io_get()
68and specify the number of pages they expect to perform I/O on concurrently.
69Dm-io will attempt to resize its mempool to make sure enough pages are
70always available in order to avoid unnecessary waiting while performing I/O.
71
72When the user is finished using the dm-io services, they should call
73dm_io_put() and specify the same number of pages that were given on the
74dm_io_get() call.
75
76

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