| Fred Isaman | 02c35fc | 2010-10-20 00:17:59 -0400 | [diff] [blame] | 1 | Reference counting in pnfs: |
| 2 | ========================== |
| 3 | |
| 4 | The are several inter-related caches. We have layouts which can |
| 5 | reference multiple devices, each of which can reference multiple data servers. |
| 6 | Each data server can be referenced by multiple devices. Each device |
| 7 | can be referenced by multiple layouts. To keep all of this straight, |
| 8 | we need to reference count. |
| 9 | |
| 10 | |
| 11 | struct pnfs_layout_hdr |
| 12 | ---------------------- |
| 13 | The on-the-wire command LAYOUTGET corresponds to struct |
| 14 | pnfs_layout_segment, usually referred to by the variable name lseg. |
| 15 | Each nfs_inode may hold a pointer to a cache of of these layout |
| 16 | segments in nfsi->layout, of type struct pnfs_layout_hdr. |
| 17 | |
| 18 | We reference the header for the inode pointing to it, across each |
| 19 | outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN, |
| 20 | LAYOUTCOMMIT), and for each lseg held within. |
| 21 | |
| 22 | Each header is also (when non-empty) put on a list associated with |
| 23 | struct nfs_client (cl_layouts). Being put on this list does not bump |
| 24 | the reference count, as the layout is kept around by the lseg that |
| 25 | keeps it in the list. |
| 26 | |
| 27 | deviceid_cache |
| 28 | -------------- |
| 29 | lsegs reference device ids, which are resolved per nfs_client and |
| 30 | layout driver type. The device ids are held in a RCU cache (struct |
| 31 | nfs4_deviceid_cache). The cache itself is referenced across each |
| 32 | mount. The entries (struct nfs4_deviceid) themselves are held across |
| 33 | the lifetime of each lseg referencing them. |
| 34 | |
| 35 | RCU is used because the deviceid is basically a write once, read many |
| 36 | data structure. The hlist size of 32 buckets needs better |
| 37 | justification, but seems reasonable given that we can have multiple |
| 38 | deviceid's per filesystem, and multiple filesystems per nfs_client. |
| 39 | |
| 40 | The hash code is copied from the nfsd code base. A discussion of |
| 41 | hashing and variations of this algorithm can be found at: |
| 42 | http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809 |
| 43 | |
| 44 | data server cache |
| 45 | ----------------- |
| 46 | file driver devices refer to data servers, which are kept in a module |
| 47 | level cache. Its reference is held over the lifetime of the deviceid |
| 48 | pointing to it. |
| Fred Isaman | 80fe2b1 | 2011-03-01 01:34:23 +0000 | [diff] [blame] | 49 | |
| 50 | lseg |
| 51 | ---- |
| 52 | lseg maintains an extra reference corresponding to the NFS_LSEG_VALID |
| 53 | bit which holds it in the pnfs_layout_hdr's list. When the final lseg |
| 54 | is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED |
| 55 | bit is set, preventing any new lsegs from being added. |