noactimeo and actimeo option on nfs

http://dsstos.blogspot.com/2008/01/nfs-and-oracle-mount-options-noac.html

noac

Suppress data and attribute caching. The data cach-
ing that is suppressed is the write-behind. The
local page cache is still maintained, but data
copied into it is immediately written to the server.

Setting the noac option also disables attribute caching, but has the further effect of disabling client write caching. While this guarantees that data written by an application is written directly to a server, where it can be viewed immediately by other clients, it has a significant adverse effect on client write performance. Data written into memory-mapped file pages (mmap(2)) are not written directly to this server.

actimeo=n

Set min and max times for regular files and direc-
tories to n seconds. See "File Attributes," below,
for a description of the effect of setting this
option to 0.

Setting actimeo=0 disables attribute caching on the client.
This means that every reference to attributes is satisfied
directly from the server though file data is still cached.
While this guarantees that the client always has the latest
file attributes from the server, it has an adverse effect on
performance through additional latency, network load, and
server load.