ceph: document ioctls

#define CEPH_IOCTL_MAGIC 0x97

/* just use u64 to align sanely on all archs */

/*

 * CEPH_IOC_GET_LAYOUT - get file layout or dir layout policy

 * CEPH_IOC_SET_LAYOUT - set file layout

 * CEPH_IOC_SET_LAYOUT_POLICY - set dir layout policy

 *

 * The file layout specifies how file data is striped over objects in

 * the distributed object store, which object pool they belong to (if

 * it differs from the default), and an optional 'preferred osd' to

 * store them on.

 *

 * Files get a new layout based on the policy set on the containing

 * directory or one of its ancestors.  The GET_LAYOUT ioctl will let

 * you examine the layout for a file or the policy on a directory.

 *

 * SET_LAYOUT will let you set a layout on a newly created file.  This

 * only works immediately after the file is created and before any

 * data is written to it.

 *

 * SET_LAYOUT_POLICY will let you set a layout policy (default layout)

 * on a directory that will apply to any new files created in that

 * directory (or any child directory that doesn't specify a layout of

 * its own).

 */

/* use u64 to align sanely on all archs */

struct ceph_ioctl_layout {

	__u64 stripe_unit, stripe_count, object_size;

	__u64 data_pool;

				   struct ceph_ioctl_layout)

/*

 * CEPH_IOC_GET_DATALOC - get location of file data in the cluster

 *

 * Extract identity, address of the OSD and object storing a given

 * file offset.

 */

#define CEPH_IOC_GET_DATALOC _IOWR(CEPH_IOCTL_MAGIC, 3,	\

				   struct ceph_ioctl_dataloc)

/*

 * CEPH_IOC_LAZYIO - relax consistency

 *

 * Normally Ceph switches to synchronous IO when multiple clients have

 * the file open (and or more for write).  Reads and writes bypass the

 * page cache and go directly to the OSD.  Setting this flag on a file

 * descriptor will allow buffered IO for this file in cases where the

 * application knows it won't interfere with other nodes (or doesn't

 * care).

 */

#define CEPH_IOC_LAZYIO _IO(CEPH_IOCTL_MAGIC, 4)

/*

 * CEPH_IOC_SYNCIO - force synchronous IO

 *

 * This ioctl sets a file flag that forces the synchronous IO that

 * bypasses the page cache, even if it is not necessary.  This is

 * essentially the opposite behavior of IOC_LAZYIO.  This forces the

 * same read/write path as a file opened by multiple clients when one

 * or more of those clients is opened for write.

 *

 * Note that this type of sync IO takes a different path than a file

 * opened with O_SYNC/D_SYNC (writes hit the page cache and are

 * immediately flushed on page boundaries).  It is very similar to

 * O_DIRECT (writes bypass the page cache) excep that O_DIRECT writes

 * are not copied (user page must remain stable) and O_DIRECT writes

 * have alignment restrictions (on the buffer and file offset).

 */

#define CEPH_IOC_SYNCIO _IO(CEPH_IOCTL_MAGIC, 5)

#endif