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. +.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 -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
