49 lines
2.0 KiB
Plaintext
49 lines
2.0 KiB
Plaintext
|
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.
|