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- Reference counting in pnfs:
- ==========================
- The are several inter-related caches. We have layouts which can
- reference multiple devices, each of which can reference multiple data servers.
- Each data server can be referenced by multiple devices. Each device
- can be referenced by multiple layouts. To keep all of this straight,
- we need to reference count.
- struct pnfs_layout_hdr
- ----------------------
- The on-the-wire command LAYOUTGET corresponds to struct
- pnfs_layout_segment, usually referred to by the variable name lseg.
- Each nfs_inode may hold a pointer to a cache of of these layout
- segments in nfsi->layout, of type struct pnfs_layout_hdr.
- We reference the header for the inode pointing to it, across each
- outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN,
- LAYOUTCOMMIT), and for each lseg held within.
- Each header is also (when non-empty) put on a list associated with
- struct nfs_client (cl_layouts). Being put on this list does not bump
- the reference count, as the layout is kept around by the lseg that
- keeps it in the list.
- deviceid_cache
- --------------
- lsegs reference device ids, which are resolved per nfs_client and
- layout driver type. The device ids are held in a RCU cache (struct
- nfs4_deviceid_cache). The cache itself is referenced across each
- mount. The entries (struct nfs4_deviceid) themselves are held across
- the lifetime of each lseg referencing them.
- RCU is used because the deviceid is basically a write once, read many
- data structure. The hlist size of 32 buckets needs better
- justification, but seems reasonable given that we can have multiple
- deviceid's per filesystem, and multiple filesystems per nfs_client.
- The hash code is copied from the nfsd code base. A discussion of
- hashing and variations of this algorithm can be found at:
- http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809
- data server cache
- -----------------
- file driver devices refer to data servers, which are kept in a module
- level cache. Its reference is held over the lifetime of the deviceid
- pointing to it.
- lseg
- ----
- lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
- bit which holds it in the pnfs_layout_hdr's list. When the final lseg
- is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
- bit is set, preventing any new lsegs from being added.
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