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GRIDPP-STORAGE  August 2009

GRIDPP-STORAGE August 2009

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Subject:

Re: quick agenda for today

From:

John Bland <[log in to unmask]>

Reply-To:

John Bland <[log in to unmask]>

Date:

Wed, 19 Aug 2009 15:23:20 +0100

Content-Type:

text/plain

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text/plain (55 lines)

Ewan MacMahon wrote:
>> -----Original Message-----
>> From: GRIDPP2: Deployment and support of SRM and local storage
>> management [mailto:[log in to unmask]] On Behalf Of John
>>
>> Sorry I wasn't at the meeting, missed the email. I noticed in the
>> minutes the subject of moving from rfcp to RFIO with 4kB buffers.
> We've
>> been looking into the problems of rfcp at Liverpool and have come to
>> some conclusions about the lack of performance and ways of improving
>> efficiency. 
>>
> I'm going to take some time to think this over properly, but I have a
> couple
> of quick questions:
>  
>> Following system activity during analysis shows that while there is a
>> lot of read access to the WN disk the major disk activity (at least
>> twice as much and often more) is *write* access. See this plot for a
> WN
>> disk
>>
>> http://hep.ph.liv.ac.uk/~jbland/device_i_o_transfers__dev9_0.png
>  
> - That's a nice plot, how did you generate it?

Data generated by sysstat on the node, plot produced by sarplot 
(http://sourceforge.net/projects/sarplot/). You get far more info than 
from ganglia but it's not as convenient to monitor multiple machines.

> - Was this during one of the current round of rfcp file stager based 
> tests? If it was then I wonder how much of the write traffic is the
> input files being staged in, as opposed to actual results being written
> out? Were we to move back to rfio posix access I'd expect the former
> to be eliminated/pushed out to the disk pools.

The results are from the rfcp hammer cloud tests. Hard to separate the 
two input/output contributions but I would expect the file staging to be 
by far the biggest contributor, not least because we just don't see this 
level of disk activity when using RFIO and output files are relatively 
small compared to the gigs of input.

I've also done tests on our local systems with a homebrew atlas test, 
where input was 10s of GBs, output was a few meg/job and the disparity 
between read and write was similar.

John

-- 
Dr John Bland, Systems Administrator
Room 220, Oliver Lodge
Particle Physics Group, University of Liverpool
Mail: [log in to unmask]
Tel : 0151 794 2911
"I canna change the laws of physics, Captain!"

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