Add the ability to split large chunks

[TomAugspurger]

Currently, translating HDF5 to Zarr will result in a Zarr store with identical chunks as the source. If the source isn't chunked, this will cause worse performance when you slice a subset of the original data, since fsspec will make the full range request.

Here's a Kerchunked file

import planetary_computer
import adlfs
import kerchunk.hdf
import xarray as xr
import fsspec

credential = planetary_computer.sas.get_token("ukmeteuwest", "ukcp18").token
file = "az://ukcp18/badc/ukcp18/data/land-gcm/global/60km/rcp26/01/clt/day/v20200302/clt_rcp26_land-gcm_global_60km_01_day_18991201-19091130.nc"

storage_options = dict(account_name="ukmeteuwest", credential=credential)
file = daily_files[0]
with fsspec.open(file, **storage_options) as f:
    d = kerchunk.hdf.SingleHdf5ToZarr(f, file).translate()

store = fsspec.filesystem("reference", fo=d, remote_options=storage_options).get_mapper("")

ds = xr.open_zarr(store, consolidated=False, chunks={})
ds

Timing small reads

%time ds.clt[0, 0].compute()
CPU times: user 5.54 s, sys: 1.5 s, total: 7.04 s
Wall time: 23.3 s

Compared with the non-kerchunked version

ds2 = xr.open_dataset(fsspec.open(file, **storage_options).open(), engine="h5netcdf")

%time ds2.clt[0, 0].compute()
CPU times: user 22.2 ms, sys: 8.51 ms, total: 30.7 ms
Wall time: 70.2 ms

Having the flexibility to make smaller requests by splitting large ranges into separate chunks would be helpful, if it's feasible for the backend (which it should be for these large, contiguous buffers from HDF5).

[martindurant]

if it's feasible for the backend

NOT when that chunk is compressed with something that doesn't have clear internal blocks (e.g., gzip). Zarr does not support streaming of any sort, it only knows which blocks you want, so you need to be able to cleanly subdivide the whole thing, which is easy for uncompressed buffers, and possible for block-compressed buffers (e.g., Zstd).

I wonder if in your example it is as fast to get the last point of the array?

[martindurant]

cf https://github.com/fsspec/kerchunk/issues/134 , which is a similar concept ( @d70-t )