ceph: document ioctls

...after some prodding by Christoph.
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/ioctl.h

@@ -6,7 +6,31 @@
 
 #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;
@@ -21,6 +45,8 @@ struct ceph_ioctl_layout {
 				   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.
  */
@@ -39,7 +65,34 @@ struct ceph_ioctl_dataloc {
 #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