# Puzzling S3 xarray.open\_zarr latency

**URL:** <https://discourse.pangeo.io/t/puzzling-s3-xarray-open-zarr-latency/1074>\
**Category:** Data\
**Created:** [December 3, 2020, 10:52pm UTC](https://discourse.pangeo.io/t/puzzling-s3-xarray-open-zarr-latency/1074 "2020-12-03T22:52:00Z")\
**Posts on this page:** 1\
**Showing post:** 3

<div class="post-metadata">

**Author:** ![martindurant](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.pangeo.io/martindurant/32/388_2.png) [@martindurant](https://discourse.pangeo.io/u/martindurant)\
**Post date:** [December 11, 2020, 6:14pm UTC](https://discourse.pangeo.io/t/puzzling-s3-xarray-open-zarr-latency/1074/3 "2020-12-11T18:14:31Z")

</div>

Xarray loads coordinates eagerly.  
If you do the following, you will see the calls that s3fs is actually making:

```auto
import s3fs
s3fs.core.setup_logging("DEBUG")

```

You will see that the first call required 9 files to be downloaded, but the second version required _MANY_, hundreds of keys in “swe\_run\_a-geo.zarr/time”.

How did you create this data? Ideally, coordinates should not be chunked at all!

---

_[View the full topic](https://discourse.pangeo.io/t/puzzling-s3-xarray-open-zarr-latency/1074)._