Hi Andreas,
On Sun, Jul 17, 2005 at 07:50:10PM +0200, Andreas Petzold wrote:
> thanks for the comments.
>
> One more question (I've already emailed Andy, but he may be on the
> road): on the slide that shows the numbers of the timing comparison
> direct disk access, xrootd, xrootd w/ redirection I'm not sure about
> the units. In Andy's talk, he uses [us] but does it mean 10^-6s which
> would be really fast, or does it mean 10^-3s?
I had understood it to be 10^-6s, but Andy or Bill should confirm this.
Those numbers do have a tendency to confuse people, however. Andy or Bill
Weeks can explain better what they were trying to do, but I understood it to
be (roughly) an attempt to isolate and measure all pieces of the process of
redirecting and opening files in order to insure that each piece within
the xrootd daemon itself was scalable and performant. This means (again, I
think) that they were probably measuring this with memory mapped files, which
removes the latencies for finding/opening something on the disk system itself
from their measurement. This _does_ demonstrate that the overhead from the
xrootd daemon itself is minimal, but doesn't of course give one the true
number you would see opening a file from disk. A couple of people reacted
as if a fast one was being pulled on them. ;-) The numbers should be more
clearly labeled to indicate this. In practice the disk part will presumably
vary depending on what hardware one has and how many things are hitting it.
All of this is of course somewhat independent from the data server itself.
[Recall also that they were doing this in the context of measurements
for the "Big Memory Machine", so in that case there is no disk subsystem
part, but clearly for most people that has to be added on top.]
Andy and Bill, is this correct?
Pete
> Peter Elmer wrote:
> > Hi Andreas,
> >
> >On Sun, Jul 17, 2005 at 01:23:11PM +0200, Andreas Petzold wrote:
> >
> >>I gonna give a talk on xrootd at Darmstadt next week. I've put my slides
> >>on the web:
> >>
> >>http://iktp.tu-dresden.de/~petzold/work/xrootd.pdf
> >>http://iktp.tu-dresden.de/~petzold/work/xrootd.sxi
> >>
> >>If you've any comments or suggestions please let me know.
> >
> >
> > It looks very nice. I see only a couple of small things (see below).
> > When you have the final version let me know and I can add it to the
> >presentations
> >section of the xrootd webpage.
> >
> > thanks,
> > Pete
> >
> > slide 9 - the protocol should be "root" instead of "xrootd" (the clients
> > even enforce this, although strictly speaking they probably
> > don't need to do that)
> >
> > slide 23 - The BaBar Padova Tier-A also uses xrootd, plus there is some
> > set of university sites using it (e.g. for serving background
> > events for MC production)
> >
> > Objectiviy -> Objectivity (technically this is the AMS, of
> > course)
> >
> > One of the other things that the system has been able to
> > achieve is that the basic system (even a load balanced
> > system)
> > is fairly easy to setup.
> >
> > slide 24 - I just realized Derek Feichtinger wasn't on the list. He has
> > been contributing code, so I've added him... I'd call the
> > list "core collaborators".
> >
> > Also, Priceton -> Princeton ;-)
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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
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