[Go-essp-tech] +2Gb CMIP5 files

[Bentley, Philip]

Hi Stephen,

Yes, that's true, we did create a small number of test netCDF files in
that size range. But this was because the CMOR library we used at the
time didn't include functionality for chunking the output into smaller
files. Plus we wanted to stress-test our pipeline!

Two things have happened since then:

1. Jamie has been working with Charles at PCMDI to implement and test a
solution whereby we can limit the size of the output netCDF files
produced by CMOR.

2. We have made the local decision to limit our netCDF file sizes to 2
GB (or thereabouts) as, logistically, that will cause us less headache
moving these files around, and it should maximise the number of client
applications in which the files can be read.

IIRC, I think Balaji mentioned that the 64-bit offset format was
required for output from the gridspec toolset. I could be wrong.

Regards,
Phil

> -----Original Message-----
> From: go-essp-tech-bounces at ucar.edu 
> [mailto:go-essp-tech-bounces at ucar.edu] On Behalf Of 
> stephen.pascoe at stfc.ac.uk
> Sent: 14 May 2010 10:52
> To: go-essp-tech at ucar.edu
> Subject: [Go-essp-tech] +2Gb CMIP5 files
> 
> The latest UKMO extraction for CMIP5 has produced some files 
> in the 30Gb range.  We had discussed previously the 
> assumption that all files would be <2Gb.  Do we feel it is 
> important to enforce a <2Gb limit or should this just be a 
> recommendation on modelling centres?
>  
> To my knowledge there is two issues with +2Gb files:
>  
>  1. +2GB NetCDF files will be in 64-bit offset format. 
> Therefore NetCDF libraries prior to v3.6 will not be able to 
> read them.
>  2. Older file systems may have a 2Gb file limit. This will 
> mainly affect 32-bit systems that are a few years old. FAT32 
> has a 4Gb limit.
> 
> These are end-user issues, is there any reason why the ESG 
> software might have problems with files over 2Gb?  If we do 
> want to ensure files are <2Gb do we want to mandate the 
> modelling centres deliver that or will the data centres need 
> to split files?
> 
> Stephen.
>  
> ---
> Stephen Pascoe  +44 (0)1235 445980
> British Atmospheric Data Centre
> Rutherford Appleton Laboratory
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