John
Below are the answers to your questions -- let me know if it's not enough
info.
Lauren
======================================
Lauren E. Hay, Ph.D. Tel: (303) 236-7279
U.S. Geological Survey Fax: (303) 236-5034
Box 25046, MS 412, DFC Email: lhay@xxxxxxxx
Lakewood, CO 80225
======================================
From:
John Caron <caron@xxxxxxxxxxxxxxxx>
To:
Rich Signell <rsignell@xxxxxxxx>
Cc:
netcdf-java <netcdf-java@xxxxxxxxxxxxxxxx>, Roland Viger
<rviger@xxxxxxxx>, Steven Markstrom <markstro@xxxxxxxx>, Lauren E Hay
<lhay@xxxxxxxx>, Nate Booth <nlbooth@xxxxxxxx>
Date:
01/25/2010 10:21 AM
Subject:
Re: [netcdf-java] point data
Hi Rich and all:
This is a interesting challenge on such a large datasets to get good read
response.
First, you have to decide what kinds of queries you want to support and
what kind of response time is needed. I have generally used the
assumption that the common queries that you want to optimize are:
1) get data over a time range for all stations in a lat/lon box.
2) get data for a single station over a time range, or for all time.
3) get data with a specified list of stations
Usually I would break the data into multiple files based on time range,
aiming for a file size of 50-500 Mb. I also use a different format for
current vs archived data, so that the current dataset can be added to
dynamically, while the archived data is rewritten (once) for speed of
retrieval.
Again, all depends on what queries you want to optimize so ill wait for
your thoughts on that.
We ran into this problem in the past so we made a separate file for each
station and each variable. Is there a problem with having too many files?
Can we have a file by year that only contains stations with data for that
year? Or -- if we don't care how many files -- 1 file for each station for
each variable for each year. It does not matter to me. The current project
will have data that has a set time period. We hope to use this structure
for other projects that will have file updates as new data is collected.
Another question is what clients need to access this data. Are you writing
your own web service, do you just want remote access from IDV, or ??
We anticipate that our web serivces will use the OpenDAP API. I'm not the
person to answer this one.
I would think that if we're careful, we can get netcdf-4 sizes that are
similar to compressed text, but we'll have to experiment. The data appears
to be integer or float with a fixed dynamic range, which is amenable to
storing as an integer with scale/offset. integer data compresses much
better than floating point due to the noise in the low bits of the
mantissa. So one task you should get started on is to examine each field
and decide its data type. if floating point, decide on its range and the
number of significant bits.
