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Fred Isaman02c35fc2010-10-20 00:17:59 -04001Reference 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 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.
Fred Isaman80fe2b12011-03-01 01:34:23 +000049
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.