76 lines
3.2 KiB
Plaintext
76 lines
3.2 KiB
Plaintext
dm-io
|
|
=====
|
|
|
|
Dm-io provides synchronous and asynchronous I/O services. There are three
|
|
types of I/O services available, and each type has a sync and an async
|
|
version.
|
|
|
|
The user must set up an io_region structure to describe the desired location
|
|
of the I/O. Each io_region indicates a block-device along with the starting
|
|
sector and size of the region.
|
|
|
|
struct io_region {
|
|
struct block_device *bdev;
|
|
sector_t sector;
|
|
sector_t count;
|
|
};
|
|
|
|
Dm-io can read from one io_region or write to one or more io_regions. Writes
|
|
to multiple regions are specified by an array of io_region structures.
|
|
|
|
The first I/O service type takes a list of memory pages as the data buffer for
|
|
the I/O, along with an offset into the first page.
|
|
|
|
struct page_list {
|
|
struct page_list *next;
|
|
struct page *page;
|
|
};
|
|
|
|
int dm_io_sync(unsigned int num_regions, struct io_region *where, int rw,
|
|
struct page_list *pl, unsigned int offset,
|
|
unsigned long *error_bits);
|
|
int dm_io_async(unsigned int num_regions, struct io_region *where, int rw,
|
|
struct page_list *pl, unsigned int offset,
|
|
io_notify_fn fn, void *context);
|
|
|
|
The second I/O service type takes an array of bio vectors as the data buffer
|
|
for the I/O. This service can be handy if the caller has a pre-assembled bio,
|
|
but wants to direct different portions of the bio to different devices.
|
|
|
|
int dm_io_sync_bvec(unsigned int num_regions, struct io_region *where,
|
|
int rw, struct bio_vec *bvec,
|
|
unsigned long *error_bits);
|
|
int dm_io_async_bvec(unsigned int num_regions, struct io_region *where,
|
|
int rw, struct bio_vec *bvec,
|
|
io_notify_fn fn, void *context);
|
|
|
|
The third I/O service type takes a pointer to a vmalloc'd memory buffer as the
|
|
data buffer for the I/O. This service can be handy if the caller needs to do
|
|
I/O to a large region but doesn't want to allocate a large number of individual
|
|
memory pages.
|
|
|
|
int dm_io_sync_vm(unsigned int num_regions, struct io_region *where, int rw,
|
|
void *data, unsigned long *error_bits);
|
|
int dm_io_async_vm(unsigned int num_regions, struct io_region *where, int rw,
|
|
void *data, io_notify_fn fn, void *context);
|
|
|
|
Callers of the asynchronous I/O services must include the name of a completion
|
|
callback routine and a pointer to some context data for the I/O.
|
|
|
|
typedef void (*io_notify_fn)(unsigned long error, void *context);
|
|
|
|
The "error" parameter in this callback, as well as the "*error" parameter in
|
|
all of the synchronous versions, is a bitset (instead of a simple error value).
|
|
In the case of an write-I/O to multiple regions, this bitset allows dm-io to
|
|
indicate success or failure on each individual region.
|
|
|
|
Before using any of the dm-io services, the user should call dm_io_get()
|
|
and specify the number of pages they expect to perform I/O on concurrently.
|
|
Dm-io will attempt to resize its mempool to make sure enough pages are
|
|
always available in order to avoid unnecessary waiting while performing I/O.
|
|
|
|
When the user is finished using the dm-io services, they should call
|
|
dm_io_put() and specify the same number of pages that were given on the
|
|
dm_io_get() call.
|
|
|