Disable MSC by default; opt in via --enable-msc

megatron/core/MSC_Integration.md

@@ -125,14 +125,16 @@ python pretrain_gpt.py                \
 
 **Notes:** Only the `torch_dist` checkpoint format is currently supported when saving to or loading from MSC URLs.
 
-## Disable MSC
+## Enable MSC
 
-By default, MSC integration is automatically enabled when the `multi-storage-client` library is installed. MSC is also used for regular filesystem paths (like `/filesystem_mountpoint/path` in `--data-path`, `--save`, or `--load`) even when not using explicit MSC URLs. MSC functions as a very thin abstraction layer with negligible performance impact when used with regular paths, so there's typically no need to disable it. If you need to disable MSC, you can do so using the `--disable-msc` flag:
+MSC integration is opt-in: even when the `multi-storage-client` library is installed, MSC is **disabled by default**. To opt in, pass the `--enable-msc` flag. Once enabled, MSC is also used for regular filesystem paths (like `/filesystem_mountpoint/path` in `--data-path`, `--save`, or `--load`), not just explicit `msc://` URLs.
 
 ```bash
-python pretrain_gpt.py --disable-msc
+python pretrain_gpt.py --enable-msc
 ```
 
+> **Note:** When MSC is enabled, the dist-checkpointing loader uses `msc.torch.MultiStorageFileSystemReader` instead of `CachedMetadataFileSystemReader`. This means `ckpt_assume_constant_structure=True` (and any other path that requests `cache_metadata=True`) will be silently overridden — metadata is re-read on every load. A warning is emitted in this case.
+
 ## Performance Considerations
 
 When using object storage with MSC, there are a few important performance implications to keep in mind:

megatron/core/dist_checkpointing/strategies/torch.py

@@ -831,6 +831,14 @@ def _get_filesystem_reader(
 ) -> FileSystemReader:
     if MultiStorageClientFeature.is_enabled():
         msc = MultiStorageClientFeature.import_package()
+        if cache_metadata:
+            warnings.warn(
+                "MSC is enabled: returning msc.torch.MultiStorageFileSystemReader instead of "
+                "CachedMetadataFileSystemReader. The cache_metadata=True request "
+                "(e.g. ckpt_assume_constant_structure=True) will be ignored and metadata "
+                "will be re-read on every load. Pass --enable-msc only when this is intended.",
+                stacklevel=2,
+            )
         return msc.torch.MultiStorageFileSystemReader(checkpoint_dir, thread_count=2)
 
     if cache_metadata:

megatron/core/msc_utils.py

@@ -8,11 +8,9 @@
 try:
     import multistorageclient as msc
 
-    _msc_available = True
     logger.info('The multistorageclient package is available.')
 except ModuleNotFoundError:
     msc = None
-    _msc_available = False
 
 
 class _FeatureFlag:
@@ -41,7 +39,7 @@ def import_package(self) -> Any:
             )
         if not self.is_enabled():
             raise RuntimeError(
-                "The MSC feature is disabled. Please enable by removing the --disable-msc argument."
+                "The MSC feature is disabled. Please enable it by passing --enable-msc."
             )
         return msc
 
@@ -54,7 +52,7 @@ def __setstate__(self, state):
         self._enabled = state['_enabled']
 
 
-MultiStorageClientFeature = _FeatureFlag(_msc_available)
+MultiStorageClientFeature = _FeatureFlag(default=False)
 
 
 def open_file(*args, **kwargs):

megatron/training/arguments.py

@@ -148,11 +148,27 @@ def parse_args(extra_args_provider=None, ignore_unknown_args=False):
     args.rank = int(os.getenv('RANK', '0'))
     args.world_size = int(os.getenv("WORLD_SIZE", '1'))
 
-    # Args to disable MSC
-    if not args.enable_msc:
+    # Args to enable MSC (opt-in: disabled by default)
+    if args.disable_msc_deprecated:
+        warn_rank_0(
+            '--disable-msc is deprecated and will be removed in a future release. '
+            'MSC is now disabled by default; pass --enable-msc to opt in.'
+        )
+        # Preserve legacy semantics: --disable-msc forces MSC off, even if
+        # --enable-msc was also passed.
+        args.enable_msc = False
+
+    if args.enable_msc:
+        MultiStorageClientFeature.enable()
+        if not MultiStorageClientFeature.is_enabled():
+            raise RuntimeError(
+                "--enable-msc was passed but the multistorageclient package is not "
+                "installed. Install it with `pip install multi-storage-client`."
+            )
+        warn_rank_0('The MSC feature is enabled.')
+    else:
         MultiStorageClientFeature.disable()
         assert MultiStorageClientFeature.is_enabled() is False
-        warn_rank_0('The MSC feature is disabled.')
 
     return args
 
@@ -3383,8 +3399,13 @@ def _add_experimental_args(parser):
 
 def _add_msc_args(parser):
     group = parser.add_argument_group(title="msc")
-    group.add_argument('--disable-msc', default=True, action='store_false', dest='enable_msc',
-                       help='Disable the usage of Multi-Storage Client (MSC) in Megatron Core.')
+    group.add_argument('--enable-msc', default=False, action='store_true', dest='enable_msc',
+                       help='Enable the usage of Multi-Storage Client (MSC) in Megatron Core. '
+                            'Disabled by default; pass this flag to opt in.')
+    group.add_argument('--disable-msc', default=False, action='store_true',
+                       dest='disable_msc_deprecated',
+                       help='[DEPRECATED] MSC is disabled by default; this flag is a no-op '
+                            'and will be removed in a future release.')
     return parser
 
 def _add_kitchen_quantization_arguments(parser: argparse.ArgumentParser):