2010-10-20 00:17:59 -04:00
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Reference counting in pnfs:
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==========================
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The are several inter-related caches. We have layouts which can
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reference multiple devices, each of which can reference multiple data servers.
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Each data server can be referenced by multiple devices. Each device
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can be referenced by multiple layouts. To keep all of this straight,
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we need to reference count.
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struct pnfs_layout_hdr
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----------------------
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The on-the-wire command LAYOUTGET corresponds to struct
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pnfs_layout_segment, usually referred to by the variable name lseg.
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Each nfs_inode may hold a pointer to a cache of of these layout
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segments in nfsi->layout, of type struct pnfs_layout_hdr.
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We reference the header for the inode pointing to it, across each
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outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN,
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LAYOUTCOMMIT), and for each lseg held within.
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Each header is also (when non-empty) put on a list associated with
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struct nfs_client (cl_layouts). Being put on this list does not bump
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the reference count, as the layout is kept around by the lseg that
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keeps it in the list.
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deviceid_cache
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--------------
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lsegs reference device ids, which are resolved per nfs_client and
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layout driver type. The device ids are held in a RCU cache (struct
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nfs4_deviceid_cache). The cache itself is referenced across each
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mount. The entries (struct nfs4_deviceid) themselves are held across
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the lifetime of each lseg referencing them.
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RCU is used because the deviceid is basically a write once, read many
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data structure. The hlist size of 32 buckets needs better
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justification, but seems reasonable given that we can have multiple
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deviceid's per filesystem, and multiple filesystems per nfs_client.
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The hash code is copied from the nfsd code base. A discussion of
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hashing and variations of this algorithm can be found at:
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http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809
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data server cache
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-----------------
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file driver devices refer to data servers, which are kept in a module
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level cache. Its reference is held over the lifetime of the deviceid
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pointing to it.
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2011-03-01 01:34:23 +00:00
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lseg
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----
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lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
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bit which holds it in the pnfs_layout_hdr's list. When the final lseg
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is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
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bit is set, preventing any new lsegs from being added.
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