[BBLISA] Does read only really mean it?

[John Miller]

I'd be curious to see if this is a function of how NetApp handles NFS, 
or if it's deeper than that.  I'd like to try this out on a bog-standard 
ext4 fs and see if the behavior's still the same.  Likewise for the 
native Linux NFS server.

John



On 12/04/2013 10:37 PM, 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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