Hi Kostas,
I'm not even sure if rfio will be using readahead by default. If it is,
I also don't know what the default buffer size is (probably 1MB). I'm
currently trying to find out.
http://grid-deployment.web.cern.ch/grid-deployment/documentation/LFC_DPM/dpm/html/rfiosetopt.html
For dCache, the default readahead is 1MB unless it is overwritten by the
variable DCACHE_RA_BUFFER. By default, the readahead option is enabled
if the file has been opened with the O_RDONLY flag or the DCACHE_RAHEAD
variable is defined in the environment.
http://www.dcache.org/manuals/libdcap.shtml
Cheers,
Greig
Kostas Georgiou wrote, On 25/11/08 17:01:
> On Tue, Nov 25, 2008 at 02:11:10PM +0100, Graeme Stewart wrote:
>
>
>> This means we have saturated our i/o capacity with a cluster which is
>> 15% full of analysis jobs.
>>
>> I think we need more network cards and bigger switches. I am astonished.
>>
>
> Just out of curiosity what is the value of your rfio/dcap readahead? In
> the past I've seen some CMS jobs here spending most of their time
> downloading data because they read root files in a semi random way[1].
> Since their reads are in ~4K blocks with a readahead of 1MB or so they
> ended up downloading each file a few hundred times or so.
>
> Cheers,
> Kostas
>
> [1] http://cmsdoc.cern.ch/cms/cpt/Computing/Technical/documents/20080422/iosummary/iosummary.html
>
>
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