I was reminded recently that NFS treats file atime time stamps differently than other filesystems. It also ignores the generic *atime mount options because it cannot support the atime semantics of local filesystems. We should document that somewhere. nfs(5) seems like a logical place for it. Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx> --- utils/mount/nfs.man | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 59 insertions(+) diff --git a/utils/mount/nfs.man b/utils/mount/nfs.man index 67031b5..2250963 100644 --- a/utils/mount/nfs.man +++ b/utils/mount/nfs.man @@ -1227,6 +1227,65 @@ 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 "File timestamp maintainence" +NFS servers are responsible for managing file and directory timestamps +.RB ( atime , +.BR ctime ", and" +.BR mtime ). +When a file is accessed or updated on an NFS server, +the file's timestamps are updated just like they would be on a filesystem +local to an application. +.P +NFS clients cache file attributes, including timestamps. +A file's timestamps are updated on NFS clients when its attributes +are retrieved from the NFS server. +Thus there may be some delay before timestamp updates +on an NFS server appear to applications on NFS clients. +.P +To comply with the POSIX filesystem standard, the Linux NFS client +relies on NFS servers to keep a file's +.B mtime +and +.B ctime +timestamps properly up to date. +It does this by flushing local data changes to the server +before reporting +.B mtime +to applications via system calls such as +.BR stat (2). +.P +The Linux client handles +.B atime +updates more loosely, however. +NFS clients maintain good performance by caching data, +but that means that application reads, which normally update +.BR atime , +are not reflected to the server where a file's +.B atime +is actually maintained. +.P +Because of this caching behavior, +the Linux NFS client does not support generic atime-related mount options. +See +.BR mount (8) +for details on these options. +.P +In particular, the +.BR atime / noatime , +.BR diratime / nodiratime , +.BR relatime / norelatime , +and +.BR strictatime / nostrictatime +mount options have no effect on NFS mounts. +.P +.I /proc/mounts +may report that the +.B relatime +mount option is set on NFS mounts, but in fact the +.B atime +semantics are always as described here, and are not like +.B relatime +semantics. .SS "Directory entry caching" The Linux NFS client caches the result of all NFS LOOKUP requests. If the requested directory entry exists on the server, -- 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
