Hi Jean-Yves,

  I didn't realize that you were offering a read/write service to the D0
people, is that really correct?

  As you know we only recently (last fall) brought up the read/write output
buffer for skimming (which isn't backed by MSS, but a temp buffer), and
Wilko and I are just now planning to bring up the /store/users read/write 
system (backed by MSS) at SLAC in the next weeks. That was supposed to have 
the stat check to MSS when a new file was being opened for write (only) to 
verify that it does not already exist (i.e. this is the oss.check config
directive, I think).

  As Andy has pointed out, it was not foreseen to allow _updates_ to files 
via any other mechanism than via xrootd itself (if that is what is going 
on). While a mechanism such as the one you suggest could be added to cover 
such "somebody came in through the backdoor to update the file", it seems to 
me like it would it would be difficult and costly to do it properly and 

  What alternate door did the user use to update the file in HPSS?


On Wed, Mar 02, 2005 at 08:52:33PM +0100, Jean-Yves Nief wrote:
>          one of the D0 user accessing files via xrootd encountered the 
> following issue: after having accessed a file via xrootd (so after being 
> staged from the "master" copy stored in HPSS), he modified the master 
> copy in HPSS and wanted to access the modified file via xrootd: but as 
> the old version of the file was already on the disk cache, no staging 
> occured of course (but that is the expected behavior obviously) and he 
> grabbed the old one version, which is not what he wanted. Well as an 
> emergency solution and as it was the first time it happened, I've 
> deleted the old version on the disk cache so he could proceed.
> However, I think it would be nice to have some control on the validity 
> of the cache: one solution would be to add the following test: in case 
> the file is already in the cache, compare the creation time on the cache 
> disk (t1) with the last modified time of the file stored in HPSS (t2): 
> if t1<t2 then restage the file.
> it will be a very small overhead to the mechanism, each time a file is 
> accessed: it have just to issue a "statx" request to the MSS.
> or maybe there is a more simple solution.
> cheers,
> JY

Peter Elmer     E-mail: [log in to unmask]      Phone: +41 (22) 767-4644
Address: CERN Division PPE, Bat. 32 2C-14, CH-1211 Geneva 23, Switzerland