1 ==========================
2 Reference counting in pnfs
3 ==========================
4
5 The are several inter-related caches. We have
6 reference multiple devices, each of which can
7 Each data server can be referenced by multiple
8 can be referenced by multiple layouts. To keep
9 we need to reference count.
10
11
12 struct pnfs_layout_hdr
13 ======================
14
15 The on-the-wire command LAYOUTGET corresponds
16 pnfs_layout_segment, usually referred to by th
17 Each nfs_inode may hold a pointer to a cache o
18 segments in nfsi->layout, of type struct pnfs_
19
20 We reference the header for the inode pointing
21 outstanding RPC call that references it (LAYOU
22 LAYOUTCOMMIT), and for each lseg held within.
23
24 Each header is also (when non-empty) put on a
25 struct nfs_client (cl_layouts). Being put on
26 the reference count, as the layout is kept aro
27 keeps it in the list.
28
29 deviceid_cache
30 ==============
31
32 lsegs reference device ids, which are resolved
33 layout driver type. The device ids are held i
34 nfs4_deviceid_cache). The cache itself is ref
35 mount. The entries (struct nfs4_deviceid) the
36 the lifetime of each lseg referencing them.
37
38 RCU is used because the deviceid is basically
39 data structure. The hlist size of 32 buckets
40 justification, but seems reasonable given that
41 deviceid's per filesystem, and multiple filesy
42
43 The hash code is copied from the nfsd code bas
44 hashing and variations of this algorithm can b
45 <http://groups.google.com/group/comp.lang.c/br
46
47 data server cache
48 =================
49
50 file driver devices refer to data servers, whi
51 level cache. Its reference is held over the l
52 pointing to it.
53
54 lseg
55 ====
56
57 lseg maintains an extra reference correspondin
58 bit which holds it in the pnfs_layout_hdr's li
59 is removed from the pnfs_layout_hdr's list, th
60 bit is set, preventing any new lsegs from bein
61
62 layout drivers
63 ==============
64
65 PNFS utilizes what is called layout drivers. T
66 layout types: "files", "objects", "blocks", an
67 of these types there is a layout-driver with a
68 table which are called by the nfs-client pnfs-
69 different layout types.
70
71 Files-layout-driver code is in: fs/nfs/filelay
72 Blocks-layout-driver code is in: fs/nfs/blockl
73 Flexfiles-layout-driver code is in: fs/nfs/fle
74
75 blocks-layout setup
76 ===================
77
78 TODO: Document the setup needs of the blocks l
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