[SAGA-RG] missing(?) method reporting last modification time

[John Shalf]

It just makes problems with an undefined consistency model clearer  
because ordering of requests may result in incorrect timestamps.  The  
problem is more difficult when dealing with timestamps because that  
hits the metadata server, which is a different data path than data  
storage. Users will assume POSIX-like consistency where
	a=check_timestamp(file foo)
	write(file foo)
	b=check _timestamp(file foo)
that b will always be greater than a.  For remote connections, this is  
more difficult to guarantee unless you are explicit about the  
underlying consistency model.  Are you going to claim POSIX  
consistency?  If so, then it doesn't have anything to say about the  
async case.

I brought this up years ago, but reading the current spec I don't see  
that info. I'm not sure if it has been fully addressed.  It will be  
easier to end up with absurd or seemingly incorrect situations with  
relying on timestamp data.

-john

On May 29, 2009, at 6:29 AM, Thilo Kielmann wrote:
> John,
>
> what would be the special thing with a timestamp and everything else,
> like file size, content, permissions... that we all have already?
>
> Thilo
>
> On Fri, May 29, 2009 at 06:21:40AM -0700, John Shalf wrote:
>> Cc: saga-rg at ogf.org
>> From: John Shalf <jshalf at lbl.gov>
>> To: Thilo Kielmann <kielmann at cs.vu.nl>
>> Subject: Re: [SAGA-RG] missing(?) method reporting last  
>> modification time
>>
>> For the async version of the SAGA interface, what consistency model  
>> to
>> you propose for the modification time information?  POSIX semantics  
>> do
>> not address this, which is precisely why POSIX is so damned slow on
>> distributed/remote filesystems.  It seems we'd need to at least
>> propose an unambiguous consistency model for the time-stamps and how
>> this would interact with concurrent async read/write calls that might
>> be in progress.
>>
>> (just making the consistency model based on current state at the
>> remote side is fine, but from the client side, you might end up with
>> absurd situations when you do an async timestamp request concurrent
>> with an async file open for example.)
>>
>> -john
>>
>> On May 29, 2009, at 6:15 AM, Thilo Kielmann wrote:
>>> Folks,
>>>
>>> within our group we are currently delving into issues with accessing
>>> remote file systems. What strikes us is that such access is SLOW.
>>> As such, it would be very beneficial if one could find out when (and
>>> thus whether) a remote file or directory has been modified.
>>>
>>> While returning this piece of information sounds to be "trivial", it
>>> strikes
>>> us that the SAGA spec has no such call in the name space package
>>> (where files
>>> reside).
>>>
>>> In POSIX terms, this is the info returned by the stat system call
>>> (see: man 2 stat), with the st_mtime parameter.
>>>
>>> In Java, files have a method lastmodified().
>>>
>>> Both POSIX and Java report the time in milliseconds since 01/01/1970
>>> (epoch).
>>>
>>>
>>> Of course, it looks like nobody has ever been thinking about such a
>>> use case,
>>> but here we are! Our feeling is that the last modification time is
>>> very
>>> essential meta data about files, so such a call should certainly be
>>> there.
>>> With our current problem certainly not being the only use case for
>>> finding
>>> out how old/new a given file or directory is...
>>>
>>> Our favourite proposal is to add a method that returns the last
>>> modification
>>> time to both ns_entry and ns_directory as this makes sense with
>>> physical as
>>> well as with logical (replicated) files.
>>>
>>>
>>> Any reactions/objections ???
>>>
>>>
>>> Thilo Kielmann
>>> -- 
>>> Thilo Kielmann
>>> http://www.cs.vu.nl/~kielmann/
>>> --
>>> saga-rg mailing list
>>> saga-rg at ogf.org
>>> http://www.ogf.org/mailman/listinfo/saga-rg
>>
>
>
>
> -- 
> Thilo Kielmann                                 http://www.cs.vu.nl/~kielmann/