-Reference counting in pnfs:
+==========================
+Reference counting in pnfs
==========================
The are several inter-related caches. We have layouts which can
reference multiple devices, each of which can reference multiple data servers.
Each data server can be referenced by multiple devices. Each device
-can be referenced by multiple layouts. To keep all of this straight,
+can be referenced by multiple layouts. To keep all of this straight,
we need to reference count.
struct pnfs_layout_hdr
-----------------------
+======================
+
The on-the-wire command LAYOUTGET corresponds to struct
pnfs_layout_segment, usually referred to by the variable name lseg.
Each nfs_inode may hold a pointer to a cache of these layout
keeps it in the list.
deviceid_cache
---------------
+==============
+
lsegs reference device ids, which are resolved per nfs_client and
layout driver type. The device ids are held in a RCU cache (struct
nfs4_deviceid_cache). The cache itself is referenced across each
deviceid's per filesystem, and multiple filesystems per nfs_client.
The hash code is copied from the nfsd code base. A discussion of
-hashing and variations of this algorithm can be found at:
-http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809
+hashing and variations of this algorithm can be found `here.
+<http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809>`_
data server cache
------------------
+=================
+
file driver devices refer to data servers, which are kept in a module
level cache. Its reference is held over the lifetime of the deviceid
pointing to it.
lseg
-----
+====
+
lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
bit which holds it in the pnfs_layout_hdr's list. When the final lseg
is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
bit is set, preventing any new lsegs from being added.
layout drivers
---------------
+==============
PNFS utilizes what is called layout drivers. The STD defines 4 basic
layout types: "files", "objects", "blocks", and "flexfiles". For each
Flexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory
blocks-layout setup
--------------------
+===================
TODO: Document the setup needs of the blocks layout driver