On Jul 10, 2009, at 1:20 PM, J. Bruce Fields wrote:
On Thu, Jul 09, 2009 at 04:11:46PM -0400, Chuck Lever wrote:
See kernel commit 7973c1f1.
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
Trond-
Noted the absense of a description in nfs(8) of the new lookupcache
mount option. Cobbled together this explanation. Before I submit
this to Steve, any comments?
utils/mount/nfs.man | 86 +++++++++++++++++++++++++++++++++++++++++
++++++++++
1 files changed, 86 insertions(+), 0 deletions(-)
diff --git a/utils/mount/nfs.man b/utils/mount/nfs.man
index 13de524..8f1b393 100644
--- a/utils/mount/nfs.man
+++ b/utils/mount/nfs.man
@@ -422,6 +422,44 @@ NFS mount points allowed on a client, but NFS
servers must be configured
to allow clients to connect via non-privileged source ports.
.IP
Refer to the SECURITY CONSIDERATIONS section for important details.
+.TP 1.5i
+.BI lookupcache= mode
+Specifies how the NFS client manages its cache of directory entries.
+.I mode
+can be one of
+.BR all ,
+.BR none ,
+.BR pos ,
+or
+.BR positive .
+This option is supported in kernels 2.6.28 and later.
+.IP
+The NFS client caches the result of all LOOKUP requests,
+including results that indicate that a requested directory entry
+does not exist on the server.
+If this option is not specified, or if
+.B all
+is specified, the client
+assumes its directory entry cache is valid
+until the parent directory's cached attributes expire.
+.IP
+If
+.BR pos " or " positive
+is specified,
+the client always revalidates entries in its directory cache
+that indicate that the requested file does not exist on the server
+before an application can use them.
I find this sentence hard to parse (e.g. I've lost track of what
"them"
refers to by the time I get to the end of the sentence).
"them" = entries
entries is the only plural noun in the sentence.
I'll try to clean this one up.
How about
explicitly defining "negative directory cache entry", and then saying
"the client always revalidates negative directory cache entries before
using them."?
--b.
+.IP
+If
+.B none
+is specified,
+the client always revalidates all cached entries in its directory
+cache before an application can use them,
+which permits quick detection of files that were created or removed
+by other clients, but can impact application and server performance.
+.IP
+The DATA AND METADATA COHERENCE section contains a
+detailed discussion of these trade-offs.
.SS "Valid options for the nfs file system type"
Use these options, along with the options in the above subsection,
for mounting the
@@ -1017,6 +1055,54 @@ If absolute cache coherence among clients is
required,
applications should use file locking. Alternatively, applications
can also open their files with the O_DIRECT flag
to disable data caching entirely.
+.SS "Directory entry caching"
+The Linux NFS client caches results of LOOKUP requests
+to reduce traffic to servers.
+These days, the client caches lookup results
+even if the requested directory entry
+in a LOOKUP request does not exist on the server
+(in other words, if the server returns ENOENT).
+This is known as a
+.IR "cached negative lookup result" .
+.P
+The Linux NFS client watches a directory's mtime
+to detect when directory entries have been added or removed
+on the server.
+If the client detects a change in a directory's mtime,
+the client drops all cached LOOKUP results for that directory.
+Since the directory's mtime is a cached attribute, it may
+take some time before a client notices it has changed.
+See the descriptions of the
+.BR acdirmin ", " acdirmax ", and " noac
+mount options for more information about
+how long a directory's mtime is cached.
+.P
+This caching behavior improves the performance of applications that
+do not share files with applications on other clients.
+Using cached information about directories can interfere
+with applications that run concurrently on multiple clients and
+need to detect the creation or removal of files quickly, however.
+The
+.B lookupcache
+mount option allows some tuning of directory entry caching behavior.
+.P
+Previously, the Linux NFS client tracked only successful lookup
+results, not negative results, with its lookup cache.
+This permits applications to detect new directory entries
+created by other clients quickly.
+If an application depends on the previous lookup caching behavior
+of the Linux NFS client, you can use
+.BR lookupcache=positive .
+.P
+If the client ignores its cache and validates every application
+lookup request with the server,
+that client can immediately detect when a new directory
+entry has been either created or removed by another client.
+You can specify this behavior using
+.BR lookupcache=none .
+However, the extra NFS requests needed if the client does not
+cache directory entries can exact a performance penalty.
+.P
.SS "The sync mount option"
The NFS client treats the
.B sync
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--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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