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TB-SUPPORT  August 2011

TB-SUPPORT August 2011

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Subject:

Re: ATLAS Tier 2 usage

From:

Ewan MacMahon <[log in to unmask]>

Reply-To:

Testbed Support for GridPP member institutes <[log in to unmask]>

Date:

Wed, 24 Aug 2011 23:58:27 +0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (51 lines)

> -----Original Message-----
> From: Alastair Dewhurst [mailto:[log in to unmask]]
> Sent: 24 August 2011 15:16
> 
> I know there has been some ongoing discussion about the lack of data being
> placed on sites datadisk by ATLAS.  
>
Indeed. From the discussions so far I feel confident in saying that
we have at least one problem, possibly more. It's clearly a complicated
matter though, and I think we're some way off having a good common 
understanding of the situation. 

For now I'd like to just pick out two bits of this email to focus on:

> As I have already mentioned a few times, if you fail Hammer Cloud tests,
> your site will be set broker-off and you won't get any data. 
>
I don't understand the motivation for this. An idle resource, whether
that's network bandwidth or empty disk space, is a wasted resource.
I cannot see any advantage to ever deliberately wasting resources. It
may be that a site has reliability problems today, but it still makes
sense to start pushing data to them so that you can run jobs there 
tomorrow. There's no need to even try to decide whether it is a better
bet to send data to site 'A' or to site 'B' when you can simply send 
it to both of them. The more sites have copies of the data, the more
CPUs are available as candidates to subsequently run analysis work
on it.

> On 24 Aug 2011, at 14:04, Alexei Klimentov wrote:
> 
> 
> 	I forwarded your e-mail to ATLAS CREM chair. Borut agreed to discuss
> 	it and if CREM will approve my proposal then T2s will have 1-2
>     planned AOD copies.
> 
Does this refer to a partial return to the old planned data placement 
model, rather than PD2P driven placement? If so, I'm not sure, based
on my understanding of the data usage patterns, that that sounds like
a good idea. My understanding, based essentially on Graeme's GridPP 26
talk, is that a lot of data that was placed in a planned fashion was 
never used. PD2P seem to be a much more sensible system for a model in
which the Tier 2 disk is primarily cache, not storage.

As far as I can see the key problem is that PD2P is not very aggressive
about getting new, hot, interesting datasets replicated. I'd have thought
it would make sense to get copies of those 'hot' datasets on to as many
Tier 2 disks as possible, as quickly as possible, so as to make as
many CPUs available for analysis as possible.

Ewan

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