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Re: 20010227: LDM Question

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  • Subject: Re: 20010227: LDM Question
  • Date: Tue, 27 Feb 2001 10:10:37 -0700
Unidata Support wrote:
> 
> ------- Forwarded Message
> 
> >From: "Bruce Marshak" <address@hidden>
> >Organization: NWS Southern Region
> >Keywords: 200102270644.f1R6i9L27981 LDM
> 
> We have an LDM question.
> 
> When our # 1 LDM box connects with an UPSTREAM box for meteorological
> data, the process should be:
> 
> Our # 1 box sends a "feedme" request to the upstream box, and the
> upstream box replies by sending the data.  All configuration and
> permissions are set properly in both boxes.
> 
> Now, our # 1 box has the data.  If we configure another box locally
> (LDM box # 2) to receive the same identical data from the # 1 LDM box
> that it just requested from the upstream box, the question is:
> 
> When # 2 LDM box generates a feedme request for the same identical data
> from the # 1 LDM box as just received, does:
> 
> The data first get written to the hard drive on the # 1 LDM box and
> then the feedme request from the # 2 box is serviced by reading the
> data off the # 1 LDM box hard drive and sent along or
> 
> Is the data for the # 2 LDM box serviced from a memory cache (que)
> before the data is written to the disk on the # 1 LDM box.
> 
> We are moving large amounts of data and it appears that disk I/O may be
> a problem.  Any help would be appreciated.
> 
> Thanks,
> 
> Bruce Marshak
> Regional Systems Analyst
> NWS Southern Region
> Climate, Weather, and Water Division
> Dissemination Enhancement Team
> 819 Taylor Street, Room 10E09
> Ft. Worth, TX 76102
> address@hidden
> (817) 978-1100 x146 Voice
> (817) 978-2020 FAX
> 
> ------- End of Forwarded Message

Hi Bruce,

When box #1 gets the data, it is put into the queue.  The queue is a
memory mapped file.  That means that it resides both on the disk and in
memory.  So when you're writing to the queue, you're also writing to the
disk.  And, when you're reading from the queue you may be reading from
the disk if that particular page doesn't happen to be in memory at that
time.  So, a write from the queue always involves disk I/O, and a read
from the queue, e.g., a request from box #2, may involve disk I/O. 
(Note that this is not the same as filing data to the disk, which
involves further I/O.)

Does this answer your question?

Anne
-- 
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Anne Wilson                     UCAR Unidata Program            
address@hidden                 P.O. Box 3000
                                  Boulder, CO  80307
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