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 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