Add description of the cuIO GDS integration (#8293)

Adds a document to describe cuIO behavior with respect to the GDS library use.
Also includes a disclaimer about the current state of the integration.

Authors:
  - Vukasin Milovanovic (https://github.com/vuule)

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)
  - Keith Kraus (https://github.com/kkraus14)

URL: #8293

docs/cudf/source/io-gds-integration.rst

@@ -0,0 +1,22 @@
+GPUDirect Storage Integration
+=============================
+
+Many IO APIs can use GPUDirect Storage (GDS) library to optimize IO operations. 
+GDS enables a direct data path for direct memory access (DMA) transfers between GPU memory and storage, which avoids a bounce buffer through the CPU. 
+GDS also has a compatibility mode that allows the library to fall back to copying through a CPU bounce buffer. 
+The SDK is available for download `here <https://developer.nvidia.com/gpudirect-storage>`_.
+
+Use of GPUDirect Storage in cuDF is disabled by default, and can be enabled through environment variable ``LIBCUDF_CUFILE_POLICY``. 
+This variable also controls the GDS compatibility mode. There are two special values for the environment variable:
+
+- "GDS": Use of GDS is enabled; GDS compatibility mode is *off*.
+- "ALWAYS": Use of GDS is enabled; GDS compatibility mode is *on*.
+
+Any other value (or no value set) will keep the GDS disabled for use in cuDF and IO will be done using cuDF's CPU bounce buffers.
+
+This environment variable also affects how cuDF treats GDS errors.
+When ``LIBCUDF_CUFILE_POLICY`` is set to "GDS" and a GDS API call fails for any reason, cuDF falls back to the internal implementation with bounce buffers.
+When ``LIBCUDF_CUFILE_POLICY`` is set to "ALWAYS" and a GDS API call fails for any reason (unlikely, given that the compatibility mode is on), 
+cuDF throws an exception to propagate the error to te user.
+
+NOTE: current GDS integration is not fully optimized and enabling GDS will not lead to performance improvements in all cases.

docs/cudf/source/io.rst

@@ -8,4 +8,5 @@ This page contains Input / Output related APIs in cuDF.
    :maxdepth: 2
    :caption: Contents:
 
-   io-supported-types.rst
+   io-supported-types.rst
+   io-gds-integration.rst