issue_comments: 399495668

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https://github.com/pydata/xarray/issues/2242#issuecomment-399495668 https://api.github.com/repos/pydata/xarray/issues/2242 399495668 MDEyOklzc3VlQ29tbWVudDM5OTQ5NTY2OA== 1554921 2018-06-22T16:10:45Z 2018-06-22T16:10:45Z CONTRIBUTOR

True, I would expect some performance hit due to writing chunk-by-chunk, however that same performance hit is present in both of the test cases.

In addition to the snippet @shoyer mentioned, I found that xarray also intentionally uses autoclose=True when writing chunks to netCDF: https://github.com/pydata/xarray/blob/73b476e4db6631b2203954dd5b138cb650e4fb8c/xarray/backends/netCDF4_.py#L45-L48

However, ensure_open only uses autoclose if the file isn't already open:

https://github.com/pydata/xarray/blob/73b476e4db6631b2203954dd5b138cb650e4fb8c/xarray/backends/common.py#L496-L503

So if the file is already open before getting to BaseNetCDF4Array__setitem__, it will remain open. If the file isn't yet opened, it will be opened, but then immediately closed after writing the chunk. I suspect this is what's happening in the delayed version - the starting state of NetCDF4DataStore._isopen is False for some reason, and so it is doomed to re-close itself for each chunk processed.

If I remove the autoclose=True from BaseNetCDF4Array__setitem__, the file remains open and performance is comparable between the two tests.

 
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