1Reference counting in pnfs:
2==========================
3
4The are several inter-related caches.  We have layouts which can
5reference multiple devices, each of which can reference multiple data servers.
6Each data server can be referenced by multiple devices.  Each device
7can be referenced by multiple layouts.  To keep all of this straight,
8we need to reference count.
9
10
11struct pnfs_layout_hdr
12----------------------
13The on-the-wire command LAYOUTGET corresponds to struct
14pnfs_layout_segment, usually referred to by the variable name lseg.
15Each nfs_inode may hold a pointer to a cache of these layout
16segments in nfsi->layout, of type struct pnfs_layout_hdr.
17
18We reference the header for the inode pointing to it, across each
19outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN,
20LAYOUTCOMMIT), and for each lseg held within.
21
22Each header is also (when non-empty) put on a list associated with
23struct nfs_client (cl_layouts).  Being put on this list does not bump
24the reference count, as the layout is kept around by the lseg that
25keeps it in the list.
26
27deviceid_cache
28--------------
29lsegs reference device ids, which are resolved per nfs_client and
30layout driver type.  The device ids are held in a RCU cache (struct
31nfs4_deviceid_cache).  The cache itself is referenced across each
32mount.  The entries (struct nfs4_deviceid) themselves are held across
33the lifetime of each lseg referencing them.
34
35RCU is used because the deviceid is basically a write once, read many
36data structure.  The hlist size of 32 buckets needs better
37justification, but seems reasonable given that we can have multiple
38deviceid's per filesystem, and multiple filesystems per nfs_client.
39
40The hash code is copied from the nfsd code base.  A discussion of
41hashing and variations of this algorithm can be found at:
42http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809
43
44data server cache
45-----------------
46file driver devices refer to data servers, which are kept in a module
47level cache.  Its reference is held over the lifetime of the deviceid
48pointing to it.
49
50lseg
51----
52lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
53bit which holds it in the pnfs_layout_hdr's list.  When the final lseg
54is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
55bit is set, preventing any new lsegs from being added.
56
57layout drivers
58--------------
59
60PNFS utilizes what is called layout drivers. The STD defines 4 basic
61layout types: "files", "objects", "blocks", and "flexfiles". For each
62of these types there is a layout-driver with a common function-vectors
63table which are called by the nfs-client pnfs-core to implement the
64different layout types.
65
66Files-layout-driver code is in: fs/nfs/filelayout/.. directory
67Objects-layout-deriver code is in: fs/nfs/objlayout/.. directory
68Blocks-layout-deriver code is in: fs/nfs/blocklayout/.. directory
69Flexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory
70
71objects-layout setup
72--------------------
73
74As part of the full STD implementation the objlayoutdriver.ko needs, at times,
75to automatically login to yet undiscovered iscsi/osd devices. For this the
76driver makes up-calles to a user-mode script called *osd_login*
77
78The path_name of the script to use is by default:
79	/sbin/osd_login.
80This name can be overridden by the Kernel module parameter:
81	objlayoutdriver.osd_login_prog
82
83If Kernel does not find the osd_login_prog path it will zero it out
84and will not attempt farther logins. An admin can then write new value
85to the objlayoutdriver.osd_login_prog Kernel parameter to re-enable it.
86
87The /sbin/osd_login is part of the nfs-utils package, and should usually
88be installed on distributions that support this Kernel version.
89
90The API to the login script is as follows:
91	Usage: $0 -u <URI> -o <OSDNAME> -s <SYSTEMID>
92	Options:
93		-u		target uri e.g. iscsi://<ip>:<port>
94				(allways exists)
95				(More protocols can be defined in the future.
96				 The client does not interpret this string it is
97				 passed unchanged as received from the Server)
98		-o		osdname of the requested target OSD
99				(Might be empty)
100				(A string which denotes the OSD name, there is a
101				 limit of 64 chars on this string)
102		-s 		systemid of the requested target OSD
103				(Might be empty)
104				(This string, if not empty is always an hex
105				 representation of the 20 bytes osd_system_id)
106
107blocks-layout setup
108-------------------
109
110TODO: Document the setup needs of the blocks layout driver
111