1========================== 2Reference counting in pnfs 3========================== 4 5The are several inter-related caches. We have layouts which can 6reference multiple devices, each of which can reference multiple data servers. 7Each data server can be referenced by multiple devices. Each device 8can be referenced by multiple layouts. To keep all of this straight, 9we need to reference count. 10 11 12struct pnfs_layout_hdr 13====================== 14 15The on-the-wire command LAYOUTGET corresponds to struct 16pnfs_layout_segment, usually referred to by the variable name lseg. 17Each nfs_inode may hold a pointer to a cache of these layout 18segments in nfsi->layout, of type struct pnfs_layout_hdr. 19 20We reference the header for the inode pointing to it, across each 21outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN, 22LAYOUTCOMMIT), and for each lseg held within. 23 24Each header is also (when non-empty) put on a list associated with 25struct nfs_client (cl_layouts). Being put on this list does not bump 26the reference count, as the layout is kept around by the lseg that 27keeps it in the list. 28 29deviceid_cache 30============== 31 32lsegs reference device ids, which are resolved per nfs_client and 33layout driver type. The device ids are held in a RCU cache (struct 34nfs4_deviceid_cache). The cache itself is referenced across each 35mount. The entries (struct nfs4_deviceid) themselves are held across 36the lifetime of each lseg referencing them. 37 38RCU is used because the deviceid is basically a write once, read many 39data structure. The hlist size of 32 buckets needs better 40justification, but seems reasonable given that we can have multiple 41deviceid's per filesystem, and multiple filesystems per nfs_client. 42 43The hash code is copied from the nfsd code base. A discussion of 44hashing and variations of this algorithm can be found `here. 45<http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809>`_ 46 47data server cache 48================= 49 50file driver devices refer to data servers, which are kept in a module 51level cache. Its reference is held over the lifetime of the deviceid 52pointing to it. 53 54lseg 55==== 56 57lseg maintains an extra reference corresponding to the NFS_LSEG_VALID 58bit which holds it in the pnfs_layout_hdr's list. When the final lseg 59is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED 60bit is set, preventing any new lsegs from being added. 61 62layout drivers 63============== 64 65PNFS utilizes what is called layout drivers. The STD defines 4 basic 66layout types: "files", "objects", "blocks", and "flexfiles". For each 67of these types there is a layout-driver with a common function-vectors 68table which are called by the nfs-client pnfs-core to implement the 69different layout types. 70 71Files-layout-driver code is in: fs/nfs/filelayout/.. directory 72Blocks-layout-driver code is in: fs/nfs/blocklayout/.. directory 73Flexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory 74 75blocks-layout setup 76=================== 77 78TODO: Document the setup needs of the blocks layout driver 79