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