[BBLISA] Does read only really mean it?

[Alex Aminoff]

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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