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Support use of xrootd by HEP experiments

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Subject:
From:
Chad William Seys <[log in to unmask]>
Reply To:
Support use of xrootd by HEP experiments <[log in to unmask]>
Date:
Tue, 13 Oct 2020 11:33:03 -0500
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Hi all,
   We have a cluster "CMS T2_US_Wisconsin" on which (almost) every 
compute node is also an xrootd server and has uniform access to shared 
storage (HDFS).
   When a compute job request a file via xrootd  it contacts an xrootd 
redirector, which chooses a xrootd server based on default settings. 
(AFAIK randomly chosen from least loaded servers.)
   Since every compute node is also an xrootd server, there would be 
less network traffic if a job requested files from the local xrootd 
server.  Files would go directly to the compute node needing them, 
rather than first to an xrootd server on a randomly choseb node and then 
to the requesting compute node.
   I was hoping someone would have an idea of how to first request the 
file from the localhost xrootd.

   One idea is to set in the xrootd server config file:
xrd.allow host <FQDN_or_IPs_of_local_host>
   I believe this would cause the redirector to tell the xrootd client 
to fetch the file from the same server as the requesting compute job as 
desired.
   Unfortunately these xrootd servers also must serve to computers 
outside of the local cluster.  Since xrd.allow doesn't take IP ranges 
and there is no xrd.deny satisfying this requirement doesn't appear easy.

   I am also investigating how to do this using LFN to PFN rules as 
implemented in CERN's storage.xml files.  They are loaded like this:
oss.namelib /usr/lib64/libXrdCmsTfc.so 
file:/etc/xrootd/storage.xml?protocol=hadoop
I don't know how or if a fallback mechanism is implemented if the first 
matching LFC does not result in a successful read.

If anyone has any ideas, let me know!
Thanks,
Chad.

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