From 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 27 Apr 2024 12:05:51 +0200 Subject: Adding upstream version 5.10.209. Signed-off-by: Daniel Baumann --- Documentation/filesystems/nfs/pnfs.rst | 78 ++++++++++++++++++++++++++++++++++ 1 file changed, 78 insertions(+) create mode 100644 Documentation/filesystems/nfs/pnfs.rst (limited to 'Documentation/filesystems/nfs/pnfs.rst') diff --git a/Documentation/filesystems/nfs/pnfs.rst b/Documentation/filesystems/nfs/pnfs.rst new file mode 100644 index 000000000..7c470ecdc --- /dev/null +++ b/Documentation/filesystems/nfs/pnfs.rst @@ -0,0 +1,78 @@ +========================== +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 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 `here. +`_ + +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 4 basic +layout types: "files", "objects", "blocks", and "flexfiles". 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/filelayout/.. directory +Blocks-layout-driver code is in: fs/nfs/blocklayout/.. directory +Flexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory + +blocks-layout setup +=================== + +TODO: Document the setup needs of the blocks layout driver -- cgit v1.2.3