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Documentation / filesystems / nfs / pnfs.txt

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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	Reference counting in pnfs:
2	==========================
4	The are several inter-related caches.  We have layouts which can
5	reference multiple devices, each of which can reference multiple data servers.
6	Each data server can be referenced by multiple devices.  Each device
7	can be referenced by multiple layouts.  To keep all of this straight,
8	we need to reference count.
11	struct pnfs_layout_hdr
12	----------------------
13	The on-the-wire command LAYOUTGET corresponds to struct
14	pnfs_layout_segment, usually referred to by the variable name lseg.
15	Each nfs_inode may hold a pointer to a cache of these layout
16	segments in nfsi->layout, of type struct pnfs_layout_hdr.
18	We reference the header for the inode pointing to it, across each
19	outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN,
20	LAYOUTCOMMIT), and for each lseg held within.
22	Each header is also (when non-empty) put on a list associated with
23	struct nfs_client (cl_layouts).  Being put on this list does not bump
24	the reference count, as the layout is kept around by the lseg that
25	keeps it in the list.
27	deviceid_cache
28	--------------
29	lsegs reference device ids, which are resolved per nfs_client and
30	layout driver type.  The device ids are held in a RCU cache (struct
31	nfs4_deviceid_cache).  The cache itself is referenced across each
32	mount.  The entries (struct nfs4_deviceid) themselves are held across
33	the lifetime of each lseg referencing them.
35	RCU is used because the deviceid is basically a write once, read many
36	data structure.  The hlist size of 32 buckets needs better
37	justification, but seems reasonable given that we can have multiple
38	deviceid's per filesystem, and multiple filesystems per nfs_client.
40	The hash code is copied from the nfsd code base.  A discussion of
41	hashing and variations of this algorithm can be found at:
42	http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809
44	data server cache
45	-----------------
46	file driver devices refer to data servers, which are kept in a module
47	level cache.  Its reference is held over the lifetime of the deviceid
48	pointing to it.
50	lseg
51	----
52	lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
53	bit which holds it in the pnfs_layout_hdr's list.  When the final lseg
54	is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
55	bit is set, preventing any new lsegs from being added.
57	layout drivers
58	--------------
60	PNFS utilizes what is called layout drivers. The STD defines 4 basic
61	layout types: "files", "objects", "blocks", and "flexfiles". For each
62	of these types there is a layout-driver with a common function-vectors
63	table which are called by the nfs-client pnfs-core to implement the
64	different layout types.
66	Files-layout-driver code is in: fs/nfs/filelayout/.. directory
67	Blocks-layout-driver code is in: fs/nfs/blocklayout/.. directory
68	Flexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory
70	blocks-layout setup
71	-------------------
73	TODO: Document the setup needs of the blocks layout driver
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