struct pnfs_layout_hdr¶

The on-the-wire command LAYOUTGET corresponds to structpnfs_layout_segment, usually referred to by the variable name lseg.Each nfs_inode may hold a pointer to a cache of these layoutsegments in nfsi->layout, of type struct pnfs_layout_hdr.

We reference the header for the inode pointing to it, across eachoutstanding 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 withstruct nfs_client (cl_layouts). Being put on this list does not bumpthe reference count, as the layout is kept around by the lseg thatkeeps it in the list.

deviceid_cache¶

lsegs reference device ids, which are resolved per nfs_client andlayout driver type. The device ids are held in a RCU cache (structnfs4_deviceid_cache). The cache itself is referenced across eachmount. The entries (struct nfs4_deviceid) themselves are held acrossthe lifetime of each lseg referencing them.

RCU is used because the deviceid is basically a write once, read manydata structure. The hlist size of 32 buckets needs betterjustification, but seems reasonable given that we can have multipledeviceid’s per filesystem, and multiple filesystems per nfs_client.

The hash code is copied from the nfsd code base. A discussion ofhashing and variations of this algorithm can be foundhere.

data server cache¶

file driver devices refer to data servers, which are kept in a modulelevel cache. Its reference is held over the lifetime of the deviceidpointing to it.

lseg¶

lseg maintains an extra reference corresponding to the NFS_LSEG_VALIDbit which holds it in the pnfs_layout_hdr’s list. When the final lsegis removed from the pnfs_layout_hdr’s list, the NFS_LAYOUT_DESTROYEDbit is set, preventing any new lsegs from being added.

layout drivers¶

PNFS utilizes what is called layout drivers. The STD defines 4 basiclayout types: “files”, “objects”, “blocks”, and “flexfiles”. For eachof these types there is a layout-driver with a common function-vectorstable which are called by the nfs-client pnfs-core to implement thedifferent layout types.

Files-layout-driver code is in: fs/nfs/filelayout/.. directoryBlocks-layout-driver code is in: fs/nfs/blocklayout/.. directoryFlexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory

blocks-layout setup¶

TODO: Document the setup needs of the blocks layout driver