awslabs · TingDaoK · Jun 24, 2026 · May 27, 2026 · Jun 24, 2026 · Jun 24, 2026
diff --git a/awscrt/s3.py b/awscrt/s3.py
@@ -185,10 +185,19 @@ class S3FileIoOptions:
     direct_io: bool = False
     """
     Enable direct IO to bypass the OS cache. Helpful when the disk I/O outperforms the kernel cache.
-    Notes:
-    - Only supported on linux for now.
-    - Only supports upload for now.
+
+    - Currently only Linux supports direct I/O. On unsupported platforms, the request transparently
+      falls back to buffered I/O and a warning is logged.
+    - Supported for both upload (send_filepath) and download (recv_filepath).
+    - For upload, part_size MUST be page-aligned when direct_io is enabled (hard fail otherwise).
+      The client controls part_size and is responsible for ensuring alignment.
+    - For download, this is a best-effort optimization. The request transparently falls back to
+      buffered I/O (logging a warning) if any of the following preconditions are not met:
+      part_size is not page-aligned, recv_file_position is not page-aligned (WRITE_TO_POSITION),
+      existing file size is not page-aligned (CREATE_OR_APPEND), or the last part has an
+      unaligned length (only that part falls back).
     - Check NOTES for O_DIRECT for additional info https://man7.org/linux/man-pages/man2/openat.2.html
+
     In summary, O_DIRECT is a potentially powerful tool that should be used with caution.
     """
 

diff --git a/crt/aws-c-common b/crt/aws-c-common
diff --git a/crt/aws-c-s3 b/crt/aws-c-s3
diff --git a/crt/aws-c-sdkutils b/crt/aws-c-sdkutils
diff --git a/crt/aws-lc b/crt/aws-lc
+13 −0		include/aws/common/cbor.h
+41 −0		include/aws/common/file.h
+13 −0		source/cbor.c
+111 −0		source/linux/file_direct_io.c
+15 −0		source/platform_fallback_stubs/file_direct_io.c
+2 −0		tests/CMakeLists.txt
+46 −0		tests/cbor_test.c
+147 −4		tests/file_test.c
+38 −0		.github/workflows/ci.yml
+3 −1		docs/memory_aware_request_execution.md
+17 −1		include/aws/s3/private/s3_meta_request_impl.h
+2 −1		include/aws/s3/private/s3_parallel_input_stream.h
+16 −1		include/aws/s3/private/s3_request_messages.h
+7 −5		include/aws/s3/s3_buffer_pool.h
+15 −2		include/aws/s3/s3_client.h
+10 −0		source/s3_auto_ranged_get.c
+0 −7		source/s3_client.c
+6 −3		source/s3_copy_object.c
+368 −125		source/s3_meta_request.c
+9 −14		source/s3_parallel_input_stream.c
+48 −9		source/s3_request_messages.c
+13 −0		tests/CMakeLists.txt
+268 −2		tests/s3_asyncwrite_tests.c
+949 −81		tests/s3_data_plane_tests.c
+10 −0		tests/s3_tester.c
+4 −0		tests/s3_tester.h
+17 −17		.github/workflows/ci.yml
+1 −1		.github/workflows/codecov.yml
+2 −2		.github/workflows/handle-stale-discussions.yml
+1 −1		.github/workflows/stale_issue.yml
+485 −0		compiler/bdd-bytecoder.py
+68 −0		include/aws/sdkutils/endpoints_bdd_engine.h
+153 −12		include/aws/sdkutils/private/endpoints_types_impl.h
+1 −0		include/aws/sdkutils/private/endpoints_util.h
+3 −1		source/aws_profile.c
+778 −0		source/endpoints_bdd_loader.c
+77 −422		source/endpoints_rule_engine.c
+147 −61		source/endpoints_ruleset.c
+196 −49		source/endpoints_standard_lib.c
+366 −21		source/endpoints_types_impl.c
+13 −2		source/endpoints_util.c
+3 −0		tests/CMakeLists.txt
+5 −5		tests/aws_profile_tests.c
+77 −0		tests/endpoints_bdd_loader_tests.c
+10 −2		tests/endpoints_rule_engine_tests.c
+ −		tests/resources/bdd_test.bin
+40 −0		tests/resources/simple_bdd_model.json
+ −		tests/resources/simple_bdd_test.bin