[BBLISA] Does read only really mean it?

[Dean Anderson]

Hi folks,

Its been a while. I've been lurking tho.

There is no mystery: The nfs server itself doesn't have the filesystem 
mounted read-only.  So the client should respect the RO on nfs mount. 
However, actually reading the file causes the nfs server itself to 
access the file, and since that system isn't mounting its own disk read 
only, it updates atime--as it should.

Try mounting the exported filesystem readonly on the filesever and see 
what happens...

 		--Dean

On Dec 4, 2013, at 10:37 PM, Alex Aminoff <alex at basespace.net> wrote:

On 12/4/2013 10:21 PM, Matt Simmons wrote:
My knowledge is somewhat limited to the Linux world, but in my
experience I've never seen a mount be set to 'ro' and have anything
updated. I hate to use the term 'flabbergasted', but I'm pretty sure
that if I saw an implementation that didn't respect the 'ro' flag, I'd
be at the very least 'put out', and perhaps even vitriolic.

Yeah, flabbergasted is a good description of how I felt.

Nevertheless, I tested it and unless I messed up my test, an NFS mount
with -o ro, you read a file on the mounted FS, and the access time is
updated.

For the test the server was a NetApp, the client was Linux.

There is a mount flag -o noatime that does what I want. But I would
argue that this is not right. The simplest behavior - nothing is ever
written period - should be what you get by default, and then there could
be a flag that enables exceptional behavior, that is updating the access
time.

I can squint and see why it would be the way it is. One perspective is
that the naive assumption is that reading off a RO filesystem should be
just like reading any other way; when you read, the OS conveniently
remembers when you did. The inconsistency of "writing" to a read-only
thing is less important than the inconsistency of not updating the
access time when the file is read.

But what if the underlying device is not capable of recording access
times, like a CD-ROM? Can you look at the mount options and see that a
CDROM is read-only? But then you can't know whether access times will be
updated unless you use some other method to find out what the underlying
device is. So that's an abstraction violation. Bother, I don't have a
unix box easily to hand where I can check what the mount options on a
CDROM look like.

I'm not sure if this is just grousing, or flame bait, or a gotcha that
every sysadmin should know because there is no way to anticipate it.

  - Alex

--Matt



On Tue, Dec 3, 2013 at 2:21 PM, Alex Aminoff <alex at basespace.net
<mailto:alex at basespace.net>> wrote:


    Hi folks. I encountered something odd.

    Suppose you mount a file system read only. You read a file from
    it. Does
    the access time of that file get updated?

    In one place I found documentation saying no. But other places seem to
    imply that it does.

    Does the answer change if it is an NFS mount?

    I have deliberately left details of what OS I'm using out, because it
    seems to me that the answer should be consistent, and if it is not, it
    should be documented publicly.

      - Alex Aminoff
        BaseSpace.net, NBER




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