Hi Volodymyr,

  For the posix access, how difficult would it be for dCache to expose 
its cache to xrootd (on the same servers) via a server-local soft link 
namespace to files on disk, for example? (Such a thing _does_ work as the 
Wisconsin people did this at one point to serve data in dCache via xrootd, 
although they had to create this server-local soft link namespace by hand.)

                                   Pete

On Mon, Sep 19, 2005 at 04:26:50PM -0700, Andy Hanushevsky wrote:
> Hi Volodymyr ,
> 
> Hmmm, seems that when I go down this path, there appears to be a disconnect 
> between the SRM and a data access system concepts. What xrootd provides to 
> clients is low latency high performance random access on par with the best 
> commercial systems out there. That can't be done with the SRM protocol 
> (i.e., delivering random access data to client analysis jobs). On the other 
> hand, xrootd relies on some other entity to manage the disk cache. All that 
> xrootd requires is physical access to that cache and some amount of 
> predictability in its management (though it's reselient to crazed 
> management). To date, xrootd interfaces to the mps system which can be 
> customized to talk to a general purpose MSS (it can actually talk to an 
> SRM/DRM combination as well if you don't mind double caching).
> 
> The general integration of SRM calls for using SRM facilities to bring 
> files into some disk cache when needed. Access to data in those files uses 
> either the root framework or the Posix calls (which in turn use the 
> xrootd). Where that fits in depends on what whether the SRM implementation 
> provides some disk cache management features or not. So, from an SE 
> perspective, the SE provides an SRM interface to the outside world but 
> internally provides xrootd access to applications.
> 
> Andy
> 
> ----- Original Message ----- 
> From: "Volodymyr Kapchynskyy" <[log in to unmask]>
> To: "Andy Hanushevsky" <[log in to unmask]>; <[log in to unmask]>; 
> "Patrick Fuhrmann" <[log in to unmask]>
> Sent: Monday, September 19, 2005 1:28 AM
> Subject: Re: dCache wants to communicate with XRootd
> 
> 
> >
> >Hi Andy,
> >
> >Thanks for the detailed reply.
> >We will make ourselves familiar with the descriped
> >mechanisms.
> >There is just some question left about integrating
> >SRM into the xrootd service.
> >Did I understand correctly that
> >"to integrate xrootd with a generalized SRM" means that
> >instead of having oss/ofs and/or MPS you would prefer to talk
> >SRM client protocol and with that have a general interface
> >to mass storage systems (disk/tape) ?
> >BTW: are there any plans to let xrootd talk the SRM
> >server protocol so that it can serve as a Storage Element ?
> >
> >dCache Team
> >



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Peter Elmer     E-mail: [log in to unmask]      Phone: +41 (22) 767-4644
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