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1=====2dm-io3=====4 5Dm-io provides synchronous and asynchronous I/O services. There are three6types of I/O services available, and each type has a sync and an async7version.8 9The user must set up an io_region structure to describe the desired location10of the I/O. Each io_region indicates a block-device along with the starting11sector 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 19Dm-io can read from one io_region or write to one or more io_regions. Writes20to multiple regions are specified by an array of io_region structures.21 22The first I/O service type takes a list of memory pages as the data buffer for23the 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 37The second I/O service type takes an array of bio vectors as the data buffer38for the I/O. This service can be handy if the caller has a pre-assembled bio,39but 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 48The third I/O service type takes a pointer to a vmalloc'd memory buffer as the49data buffer for the I/O. This service can be handy if the caller needs to do50I/O to a large region but doesn't want to allocate a large number of individual51memory 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 58Callers of the asynchronous I/O services must include the name of a completion59callback 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 63The "error" parameter in this callback, as well as the `*error` parameter in64all of the synchronous versions, is a bitset (instead of a simple error value).65In the case of an write-I/O to multiple regions, this bitset allows dm-io to66indicate success or failure on each individual region.67 68Before using any of the dm-io services, the user should call dm_io_get()69and specify the number of pages they expect to perform I/O on concurrently.70Dm-io will attempt to resize its mempool to make sure enough pages are71always available in order to avoid unnecessary waiting while performing I/O.72 73When the user is finished using the dm-io services, they should call74dm_io_put() and specify the same number of pages that were given on the75dm_io_get() call.76