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Re: THREDDS Aggregation limits





Roy Mendelssohn wrote:
Hi John:

Some questions on THREDDS aggregation.

1. Is there a limit to the number of files that can be aggregated over?

nope


2. Can aggregation occur over sub-directories of a directory structure?

using "scan" i assume?

supposedly you can have multiple scan directives within the aggregation, but its not well tested. But I would try this if you need this feature.

the scan directive is still pretty primitive, we will continue to improve it, adding a 
"recurse" tag might be one way.

remember that the aggregated files need to be pretty much homogenous.


3. For a lot of time periods, when aggregating fields over time. do you have any feel for the trade-off in speed of aggregation for the size of the netcdf file versus the number of files aggregated over (i.e. if we have 6-hourly data, should we produce 6-hourly files, daily files, weekly files, monthly files - and what would be the likely speed tradeoff if we want to extract a time series of a relatively small region?).

My intuition is that you want to create fewer large files, not lots of little files. It costs the same to open a big or a little file. My current rule of thumb is try to write files that are 50 - 200 Mbytes.

In the future, we may add a feature that will try to open all the needed files 
in different threads. So that may argue for smaller file sizes, but its 
theoretical at this point.



TIA,

-Roy


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