[BBLISA] Does read only really mean it?

[Dave Allan]

What you're describing is (if Wikipedia is to be believed) the
behavior specified by POSIX:

http://en.wikipedia.org/wiki/Stat_%28system_call%29#Criticism_of_atime

Dave

On Wed, Dec 04, 2013 at 10:37:27PM -0500, Alex Aminoff 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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> >
> >
> >
> >
> > -- 
> > "Today, vegetables... Tomorrow, the world!"
> >
> >
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