diff --git a/scripts/update_payload/update_metadata_pb2.py b/scripts/update_payload/update_metadata_pb2.py
index 9aef9f2..a8f1bff 100644
--- a/scripts/update_payload/update_metadata_pb2.py
+++ b/scripts/update_payload/update_metadata_pb2.py
@@ -20,7 +20,7 @@
   package='chromeos_update_engine',
   syntax='proto2',
   serialized_options=_b('H\003'),
-  serialized_pb=_b('\n\x15update_metadata.proto\x12\x16\x63hromeos_update_engine\"1\n\x06\x45xtent\x12\x13\n\x0bstart_block\x18\x01 \x01(\x04\x12\x12\n\nnum_blocks\x18\x02 \x01(\x04\"\x9f\x01\n\nSignatures\x12@\n\nsignatures\x18\x01 \x03(\x0b\x32,.chromeos_update_engine.Signatures.Signature\x1aO\n\tSignature\x12\x13\n\x07version\x18\x01 \x01(\rB\x02\x18\x01\x12\x0c\n\x04\x64\x61ta\x18\x02 \x01(\x0c\x12\x1f\n\x17unpadded_signature_size\x18\x03 \x01(\x07\"+\n\rPartitionInfo\x12\x0c\n\x04size\x18\x01 \x01(\x04\x12\x0c\n\x04hash\x18\x02 \x01(\x0c\"\x8f\x01\n\tImageInfo\x12\x11\n\x05\x62oard\x18\x01 \x01(\tB\x02\x18\x01\x12\x0f\n\x03key\x18\x02 \x01(\tB\x02\x18\x01\x12\x13\n\x07\x63hannel\x18\x03 \x01(\tB\x02\x18\x01\x12\x13\n\x07version\x18\x04 \x01(\tB\x02\x18\x01\x12\x19\n\rbuild_channel\x18\x05 \x01(\tB\x02\x18\x01\x12\x19\n\rbuild_version\x18\x06 \x01(\tB\x02\x18\x01\"\xee\x03\n\x10InstallOperation\x12;\n\x04type\x18\x01 \x02(\x0e\x32-.chromeos_update_engine.InstallOperation.Type\x12\x13\n\x0b\x64\x61ta_offset\x18\x02 \x01(\x04\x12\x13\n\x0b\x64\x61ta_length\x18\x03 \x01(\x04\x12\x33\n\x0bsrc_extents\x18\x04 \x03(\x0b\x32\x1e.chromeos_update_engine.Extent\x12\x12\n\nsrc_length\x18\x05 \x01(\x04\x12\x33\n\x0b\x64st_extents\x18\x06 \x03(\x0b\x32\x1e.chromeos_update_engine.Extent\x12\x12\n\ndst_length\x18\x07 \x01(\x04\x12\x18\n\x10\x64\x61ta_sha256_hash\x18\x08 \x01(\x0c\x12\x17\n\x0fsrc_sha256_hash\x18\t \x01(\x0c\"\xad\x01\n\x04Type\x12\x0b\n\x07REPLACE\x10\x00\x12\x0e\n\nREPLACE_BZ\x10\x01\x12\x0c\n\x04MOVE\x10\x02\x1a\x02\x08\x01\x12\x0e\n\x06\x42SDIFF\x10\x03\x1a\x02\x08\x01\x12\x0f\n\x0bSOURCE_COPY\x10\x04\x12\x11\n\rSOURCE_BSDIFF\x10\x05\x12\x0e\n\nREPLACE_XZ\x10\x08\x12\x08\n\x04ZERO\x10\x06\x12\x0b\n\x07\x44ISCARD\x10\x07\x12\x11\n\rBROTLI_BSDIFF\x10\n\x12\x0c\n\x08PUFFDIFF\x10\t\"\xcf\x01\n\x11\x43owMergeOperation\x12<\n\x04type\x18\x01 \x01(\x0e\x32..chromeos_update_engine.CowMergeOperation.Type\x12\x32\n\nsrc_extent\x18\x02 \x01(\x0b\x32\x1e.chromeos_update_engine.Extent\x12\x32\n\ndst_extent\x18\x03 \x01(\x0b\x32\x1e.chromeos_update_engine.Extent\"\x14\n\x04Type\x12\x0c\n\x08\x43OW_COPY\x10\x00\"\xc8\x06\n\x0fPartitionUpdate\x12\x16\n\x0epartition_name\x18\x01 \x02(\t\x12\x17\n\x0frun_postinstall\x18\x02 \x01(\x08\x12\x18\n\x10postinstall_path\x18\x03 \x01(\t\x12\x17\n\x0f\x66ilesystem_type\x18\x04 \x01(\t\x12M\n\x17new_partition_signature\x18\x05 \x03(\x0b\x32,.chromeos_update_engine.Signatures.Signature\x12\x41\n\x12old_partition_info\x18\x06 \x01(\x0b\x32%.chromeos_update_engine.PartitionInfo\x12\x41\n\x12new_partition_info\x18\x07 \x01(\x0b\x32%.chromeos_update_engine.PartitionInfo\x12<\n\noperations\x18\x08 \x03(\x0b\x32(.chromeos_update_engine.InstallOperation\x12\x1c\n\x14postinstall_optional\x18\t \x01(\x08\x12=\n\x15hash_tree_data_extent\x18\n 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\x01(\x08\x12\x1e\n\x16vabc_compression_param\x18\x04 \x01(\t\"c\n\x08\x41pexInfo\x12\x14\n\x0cpackage_name\x18\x01 \x01(\t\x12\x0f\n\x07version\x18\x02 \x01(\x03\x12\x15\n\ris_compressed\x18\x03 \x01(\x08\x12\x19\n\x11\x64\x65\x63ompressed_size\x18\x04 \x01(\x03\"C\n\x0c\x41pexMetadata\x12\x33\n\tapex_info\x18\x01 \x03(\x0b\x32 .chromeos_update_engine.ApexInfo\"\x9e\x07\n\x14\x44\x65ltaArchiveManifest\x12H\n\x12install_operations\x18\x01 \x03(\x0b\x32(.chromeos_update_engine.InstallOperationB\x02\x18\x01\x12O\n\x19kernel_install_operations\x18\x02 \x03(\x0b\x32(.chromeos_update_engine.InstallOperationB\x02\x18\x01\x12\x18\n\nblock_size\x18\x03 \x01(\r:\x04\x34\x30\x39\x36\x12\x19\n\x11signatures_offset\x18\x04 \x01(\x04\x12\x17\n\x0fsignatures_size\x18\x05 \x01(\x04\x12\x42\n\x0fold_kernel_info\x18\x06 \x01(\x0b\x32%.chromeos_update_engine.PartitionInfoB\x02\x18\x01\x12\x42\n\x0fnew_kernel_info\x18\x07 \x01(\x0b\x32%.chromeos_update_engine.PartitionInfoB\x02\x18\x01\x12\x42\n\x0fold_rootfs_info\x18\x08 \x01(\x0b\x32%.chromeos_update_engine.PartitionInfoB\x02\x18\x01\x12\x42\n\x0fnew_rootfs_info\x18\t \x01(\x0b\x32%.chromeos_update_engine.PartitionInfoB\x02\x18\x01\x12=\n\x0eold_image_info\x18\n \x01(\x0b\x32!.chromeos_update_engine.ImageInfoB\x02\x18\x01\x12=\n\x0enew_image_info\x18\x0b \x01(\x0b\x32!.chromeos_update_engine.ImageInfoB\x02\x18\x01\x12\x18\n\rminor_version\x18\x0c \x01(\r:\x01\x30\x12;\n\npartitions\x18\r \x03(\x0b\x32\'.chromeos_update_engine.PartitionUpdate\x12\x15\n\rmax_timestamp\x18\x0e \x01(\x03\x12T\n\x1a\x64ynamic_partition_metadata\x18\x0f \x01(\x0b\x32\x30.chromeos_update_engine.DynamicPartitionMetadata\x12\x16\n\x0epartial_update\x18\x10 \x01(\x08\x12\x33\n\tapex_info\x18\x11 \x03(\x0b\x32 .chromeos_update_engine.ApexInfoB\x02H\x03')
+  serialized_pb=_b('\n\x15update_metadata.proto\x12\x16\x63hromeos_update_engine\"1\n\x06\x45xtent\x12\x13\n\x0bstart_block\x18\x01 \x01(\x04\x12\x12\n\nnum_blocks\x18\x02 \x01(\x04\"\x9f\x01\n\nSignatures\x12@\n\nsignatures\x18\x01 \x03(\x0b\x32,.chromeos_update_engine.Signatures.Signature\x1aO\n\tSignature\x12\x13\n\x07version\x18\x01 \x01(\rB\x02\x18\x01\x12\x0c\n\x04\x64\x61ta\x18\x02 \x01(\x0c\x12\x1f\n\x17unpadded_signature_size\x18\x03 \x01(\x07\"+\n\rPartitionInfo\x12\x0c\n\x04size\x18\x01 \x01(\x04\x12\x0c\n\x04hash\x18\x02 \x01(\x0c\"\x8f\x01\n\tImageInfo\x12\x11\n\x05\x62oard\x18\x01 \x01(\tB\x02\x18\x01\x12\x0f\n\x03key\x18\x02 \x01(\tB\x02\x18\x01\x12\x13\n\x07\x63hannel\x18\x03 \x01(\tB\x02\x18\x01\x12\x13\n\x07version\x18\x04 \x01(\tB\x02\x18\x01\x12\x19\n\rbuild_channel\x18\x05 \x01(\tB\x02\x18\x01\x12\x19\n\rbuild_version\x18\x06 \x01(\tB\x02\x18\x01\"\xee\x03\n\x10InstallOperation\x12;\n\x04type\x18\x01 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 )
 
 
@@ -93,11 +93,19 @@
       name='COW_COPY', index=0, number=0,
       serialized_options=None,
       type=None),
+    _descriptor.EnumValueDescriptor(
+      name='COW_XOR', index=1, number=1,
+      serialized_options=None,
+      type=None),
+    _descriptor.EnumValueDescriptor(
+      name='COW_REPLACE', index=2, number=2,
+      serialized_options=None,
+      type=None),
   ],
   containing_type=None,
   serialized_options=None,
-  serialized_start=1138,
-  serialized_end=1158,
+  serialized_start=1158,
+  serialized_end=1208,
 )
 _sym_db.RegisterEnumDescriptor(_COWMERGEOPERATION_TYPE)
 
@@ -435,6 +443,13 @@
       message_type=None, enum_type=None, containing_type=None,
       is_extension=False, extension_scope=None,
       serialized_options=None, file=DESCRIPTOR),
+    _descriptor.FieldDescriptor(
+      name='src_offset', full_name='chromeos_update_engine.CowMergeOperation.src_offset', index=3,
+      number=4, type=4, cpp_type=4, label=1,
+      has_default_value=False, default_value=0,
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
   ],
   extensions=[
   ],
@@ -449,7 +464,7 @@
   oneofs=[
   ],
   serialized_start=951,
-  serialized_end=1158,
+  serialized_end=1208,
 )
 
 
@@ -605,8 +620,8 @@
   extension_ranges=[],
   oneofs=[
   ],
-  serialized_start=1161,
-  serialized_end=2001,
+  serialized_start=1211,
+  serialized_end=2051,
 )
 
 
@@ -650,8 +665,8 @@
   extension_ranges=[],
   oneofs=[
   ],
-  serialized_start=2003,
-  serialized_end=2079,
+  serialized_start=2053,
+  serialized_end=2129,
 )
 
 
@@ -690,6 +705,20 @@
       message_type=None, enum_type=None, containing_type=None,
       is_extension=False, extension_scope=None,
       serialized_options=None, file=DESCRIPTOR),
+    _descriptor.FieldDescriptor(
+      name='cow_version', full_name='chromeos_update_engine.DynamicPartitionMetadata.cow_version', index=4,
+      number=5, type=13, cpp_type=3, label=1,
+      has_default_value=False, default_value=0,
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
+    _descriptor.FieldDescriptor(
+      name='vabc_optional', full_name='chromeos_update_engine.DynamicPartitionMetadata.vabc_optional', index=5,
+      number=6, type=8, cpp_type=7, label=1,
+      has_default_value=False, default_value=False,
+      message_type=None, enum_type=None, containing_type=None,
+      is_extension=False, extension_scope=None,
+      serialized_options=None, file=DESCRIPTOR),
   ],
   extensions=[
   ],
@@ -702,8 +731,8 @@
   extension_ranges=[],
   oneofs=[
   ],
-  serialized_start=2082,
-  serialized_end=2251,
+  serialized_start=2132,
+  serialized_end=2345,
 )
 
 
@@ -754,8 +783,8 @@
   extension_ranges=[],
   oneofs=[
   ],
-  serialized_start=2253,
-  serialized_end=2352,
+  serialized_start=2347,
+  serialized_end=2446,
 )
 
 
@@ -785,8 +814,8 @@
   extension_ranges=[],
   oneofs=[
   ],
-  serialized_start=2354,
-  serialized_end=2421,
+  serialized_start=2448,
+  serialized_end=2515,
 )
 
 
@@ -928,8 +957,8 @@
   extension_ranges=[],
   oneofs=[
   ],
-  serialized_start=2424,
-  serialized_end=3350,
+  serialized_start=2518,
+  serialized_end=3444,
 )
 
 _SIGNATURES_SIGNATURE.containing_type = _SIGNATURES
diff --git a/update_metadata.proto b/update_metadata.proto
index 93e4e2e..533f6f0 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -233,12 +233,22 @@
 // read after write, similar to the inplace update schema.
 message CowMergeOperation {
   enum Type {
-    COW_COPY = 0;  // identical blocks
+    COW_COPY = 0;     // identical blocks
+    COW_XOR = 1;      // used when src/dst blocks are highly similar
+    COW_REPLACE = 2;  // Raw replace operation
   }
   optional Type type = 1;
 
   optional Extent src_extent = 2;
   optional Extent dst_extent = 3;
+  // For COW_XOR, source location might be unaligned, so this field is in range
+  // [0, block_size), representing how much should the src_extent shift toward
+  // larger block number. If this field is non-zero, then src_extent will
+  // include 1 extra block in the end, as the merge op actually references the
+  // first |src_offset| bytes of that extra block. For example, if |dst_extent|
+  // is [10, 15], |src_offset| is 500, then src_extent might look like [25, 31].
+  // Note that |src_extent| contains 1 extra block than the |dst_extent|.
+  optional uint32 src_offset = 4;
 }
 
 // Describes the update to apply to a single partition.