[netcdf-hdf] what the heck is fletcher32 all about, anyway?

[Elena Pourmal]

Ed,


At 10:39 AM -0700 2/21/08, Ed Hartnett wrote:
>Elena Pourmal <epourmal at hdfgroup.org> writes:
>
>>  Ed,
>>
>>  Here is some documentation on Fletcher32 
>>http://www.hdfgroup.uiuc.edu/RFC/HDF5/ARCHIVE/H5checksum/
>>
>>  This document describes the final design and implementation
>>
>>  http://www.hdfgroup.uiuc.edu/RFC/HDF5/ARCHIVE/H5checksum/EDC_spec.htm
>>
>>  Checksum is ON by default, i.e read operation will fail, but there 
>>is an API that allows you to disable checksum and get data back.
>>
>>  Elena
>>
>
>OK, thanks!
>
>Is the following correct:
>
>Turning on the fletcher32 checksum filter causes a 4 byte-checksum to
>be written with each chunk.

Yes, this is correct.

>(What happens to contiguous data?)

Any filter in HDF5 requires chunking storage; one cannot (at least 
now) apply checksum to contiguous data.

>When reading data with fletcher32 checksum enabled, any corruption of
>the chunk will cause the read operation to fail.
>
>There is a significant performance penalty when reading checksummed
>data.
>
>Is this all correct?

Yes, it is correct, but I believe "significant " will vary from 
system to system.

Elena

>Thanks!
>
>Ed
>
>
>--
>Ed Hartnett  -- ed at unidata.ucar.edu


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