-------- Original Message --------
Subject: Re: NFS Mount Option 'nofsc'
From: Myklebust, Trond <Trond.Myklebust@xxxxxxxxxx>
To: Derek McEachern <derekm@xxxxxx>
CC: "linux-nfs@xxxxxxxxxxxxxxx" <linux-nfs@xxxxxxxxxxxxxxx>
Date: Tuesday, February 07, 2012 10:55:04 PM
On Tue, 2012-02-07 at 20:45 -0600, Derek McEachern wrote:
I joined the mailing list shortly after Neil sent out a request for
volunteer to update the nfs man page documenting the 'fsc'/'nofsc'
options. I suspect this may stem from a ticket we opened with Suse
inquiring about these options.
Coming from a Solaris background we typically use the 'forcedirectio'
option for certain mounts and I was looking for the same thing in Linux.
The typically advice seems to be use 'noac' but the description in the
man page doesn't seem to match what I would expect from 'forcedirectio',
namely no buffering on the client.
Poking around the kernel I found the 'fsc'/'nofsc' options and my
question is does 'nofsc' provide 'forcedirectio' functionality?
No. There is no equivalent to the Solaris "forcedirectio" mount option
in Linux.
Applications that need to use uncached i/o are required to use the
O_DIRECT open() mode instead, since pretty much all of them need to be
rewritten to deal with the subtleties involved anyway.
Trond
So then what exact functionality if provided by the 'nofsc' option? It
would seem to me from a write perspective that between noac and the sync
option it is pretty close to forcedirectio.
From the man page describing sync "any system call that writes data to
files on that mount point causes that data to be flushed to the server
before the system call returns control to user space."
Maybe I've answered one of my questions as flushing the data to the
server before returning to user space is really what I'm after. The
userspace app should be blocked until the write has been acknowledged by
the server and if the server is an NFS appliance then I don't
necessarily care if it has committed the data to disk as I expect it to
managed its cache properly.
Though I still want to understand what 'nofsc' is doing.
Derek
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