9p: implement optional loose read cache

While cacheing is generally frowned upon in the 9p world, it has its
place -- particularly in situations where the remote file system is
exclusive and/or read-only.  The vacfs views of venti content addressable
store are a real-world instance of such a situation.  To facilitate higher
performance for these workloads (and eventually use the fscache patches),
we have enabled a "loose" cache mode which does not attempt to maintain
any form of consistency on the page-cache or dcache.  This results in over
two orders of magnitude performance improvement for cacheable block reads
in the Bonnie benchmark.  The more aggressive use of the dcache also seems
to improve metadata operational performance.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/Documentation/filesystems/9p.txt b/Documentation/filesystems/9p.txt
index 4d075a4558f9..bbd8b28c13de 100644
--- a/Documentation/filesystems/9p.txt
+++ b/Documentation/filesystems/9p.txt
@@ -40,6 +40,10 @@ OPTIONS
   aname=name    aname specifies the file tree to access when the server is
                 offering several exported file systems.
 
+  cache=mode    specifies a cacheing policy.  By default, no caches are used.
+                        loose = no attempts are made at consistency,
+                                intended for exclusive, read-only mounts
+
   debug=n       specifies debug level.  The debug level is a bitmask.
                         0x01 = display verbose error messages
                         0x02 = developer debug (DEBUG_CURRENT)