I've spent one day to find solution for a nfs mounting problem with Oracle11g installation. Finally, I've found that the missing options was
forcedirectio
My question is, why this options is mandatory when we talk about datafiles installation?
tnk
gb
One of the reasons is outlined in the following MOS note (and it's not only on HP, as the note title may seem to imply),
I don't know if it applies to your case:
Potential Lost Writes: RAC on NAS using NFS may not use DIRECTIO or HP on NFS (Doc ID 566545.1)
A bit of late reply, but I tried to stay away, as much as I could, from NFS overall, when we talk about large scales, RAC in our case.
From the manpage of mount_nfs:
Thanks guys,
but both did not answer my question 
I asked whe it is mandatory.
As sysgate said, mount command indicates it as a performance options, so I asked myself... is it a really mandatory option?
The Oracle 11g documentation Checking NFS Mount and Buffer Size Parameters for Oracle Clusterware points to My Oracle Support bulletin 359515.1 which states:
Purpose
The purpose of this bulletin is to document the options with which the NAS systems should be mounted.
This note does not cover the new dNFS feature that was introduced in 11g
They use the verb "should" (not "must"). From Request for Comments: 2119:
3. SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
That said, it's worth noting that all this applies to GI and RAC (it's not specified if it applies to a regular databases as well).
Hope this helps.