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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.
-
-layout drivers
---------------
-
-PNFS utilizes what is called layout drivers. The STD defines 3 basic
-layout types: "files" "objects" and "blocks". For each of these types
-there is a layout-driver with a common function-vectors table which
-are called by the nfs-client pnfs-core to implement the different layout
-types.
-
-Files-layout-driver code is in: fs/nfs/nfs4filelayout.c && nfs4filelayoutdev.c
-Objects-layout-deriver code is in: fs/nfs/objlayout/.. directory
-Blocks-layout-deriver code is in: fs/nfs/blocklayout/.. directory
-
-objects-layout setup
---------------------
-
-As part of the full STD implementation the objlayoutdriver.ko needs, at times,
-to automatically login to yet undiscovered iscsi/osd devices. For this the
-driver makes up-calles to a user-mode script called *osd_login*
-
-The path_name of the script to use is by default:
- /sbin/osd_login.
-This name can be overridden by the Kernel module parameter:
- objlayoutdriver.osd_login_prog
-
-If Kernel does not find the osd_login_prog path it will zero it out
-and will not attempt farther logins. An admin can then write new value
-to the objlayoutdriver.osd_login_prog Kernel parameter to re-enable it.
-
-The /sbin/osd_login is part of the nfs-utils package, and should usually
-be installed on distributions that support this Kernel version.
-
-The API to the login script is as follows:
- Usage: $0 -u <URI> -o <OSDNAME> -s <SYSTEMID>
- Options:
- -u target uri e.g. iscsi://<ip>:<port>
- (allways exists)
- (More protocols can be defined in the future.
- The client does not interpret this string it is
- passed unchanged as received from the Server)
- -o osdname of the requested target OSD
- (Might be empty)
- (A string which denotes the OSD name, there is a
- limit of 64 chars on this string)
- -s systemid of the requested target OSD
- (Might be empty)
- (This string, if not empty is always an hex
- representation of the 20 bytes osd_system_id)
-
-blocks-layout setup
--------------------
-
-TODO: Document the setup needs of the blocks layout driver