diff --git a/fs_mgr/liblp/builder.cpp b/fs_mgr/liblp/builder.cpp
index da86d75..699b9e7 100644
--- a/fs_mgr/liblp/builder.cpp
+++ b/fs_mgr/liblp/builder.cpp
@@ -578,6 +578,12 @@
     CHECK_NE(sectors_per_block, 0);
     CHECK(sectors_needed % sectors_per_block == 0);
 
+    if (IsABDevice() && !IsRetrofitDevice() && GetPartitionSlotSuffix(partition->name()) == "_b") {
+        // Allocate "a" partitions top-down and "b" partitions bottom-up, to
+        // minimize fragmentation during OTA.
+        free_regions = PrioritizeSecondHalfOfSuper(free_regions);
+    }
+
     // Find gaps that we can use for new extents. Note we store new extents in a
     // temporary vector, and only commit them if we are guaranteed enough free
     // space.
@@ -621,6 +627,40 @@
     return true;
 }
 
+std::vector<MetadataBuilder::Interval> MetadataBuilder::PrioritizeSecondHalfOfSuper(
+        const std::vector<Interval>& free_list) {
+    const auto& super = block_devices_[0];
+    uint64_t first_sector = super.first_logical_sector;
+    uint64_t last_sector = super.size / LP_SECTOR_SIZE;
+    uint64_t midpoint = first_sector + (last_sector - first_sector) / 2;
+
+    // Choose an aligned sector for the midpoint. This could lead to one half
+    // being slightly larger than the other, but this will not restrict the
+    // size of partitions (it might lead to one extra extent if "B" overflows).
+    midpoint = AlignSector(super, midpoint);
+
+    std::vector<Interval> first_half;
+    std::vector<Interval> second_half;
+    for (const auto& region : free_list) {
+        // Note: deprioritze if not the main super partition. Even though we
+        // don't call this for retrofit devices, we will allow adding additional
+        // block devices on non-retrofit devices.
+        if (region.device_index != 0 || region.end <= midpoint) {
+            first_half.emplace_back(region);
+            continue;
+        }
+        if (region.start < midpoint && region.end > midpoint) {
+            // Split this into two regions.
+            first_half.emplace_back(region.device_index, region.start, midpoint);
+            second_half.emplace_back(region.device_index, midpoint, region.end);
+        } else {
+            second_half.emplace_back(region);
+        }
+    }
+    second_half.insert(second_half.end(), first_half.begin(), first_half.end());
+    return second_half;
+}
+
 void MetadataBuilder::ShrinkPartition(Partition* partition, uint64_t aligned_size) {
     partition->ShrinkTo(aligned_size);
 }
@@ -930,5 +970,9 @@
     return android::base::GetBoolProperty("ro.build.ab_update", false);
 }
 
+bool MetadataBuilder::IsRetrofitDevice() const {
+    return GetBlockDevicePartitionName(block_devices_[0]) != LP_METADATA_DEFAULT_PARTITION_NAME;
+}
+
 }  // namespace fs_mgr
 }  // namespace android
diff --git a/fs_mgr/liblp/builder_test.cpp b/fs_mgr/liblp/builder_test.cpp
index 926fe12..7833a25 100644
--- a/fs_mgr/liblp/builder_test.cpp
+++ b/fs_mgr/liblp/builder_test.cpp
@@ -720,3 +720,41 @@
     ASSERT_EQ(builder->AddPartition("system", 0), nullptr);
     ASSERT_NE(builder->AddPartition("system_a", 0), nullptr);
 }
+
+TEST_F(BuilderTest, ABExtents) {
+    BlockDeviceInfo device_info("super", 10_GiB, 768 * 1024, 0, 4096);
+
+    // A and B slots should be allocated from separate halves of the partition,
+    // to mitigate allocating too many extents. (b/120433288)
+    MetadataBuilder::OverrideABForTesting(true);
+    auto builder = MetadataBuilder::New(device_info, 65536, 2);
+    ASSERT_NE(builder, nullptr);
+    Partition* system_a = builder->AddPartition("system_a", 0);
+    ASSERT_NE(system_a, nullptr);
+    Partition* system_b = builder->AddPartition("system_b", 0);
+    ASSERT_NE(system_b, nullptr);
+    ASSERT_TRUE(builder->ResizePartition(system_a, 2_GiB));
+    ASSERT_TRUE(builder->ResizePartition(system_b, 2_GiB));
+
+    builder->RemovePartition("system_a");
+    system_a = builder->AddPartition("system_a", 0);
+    ASSERT_NE(system_a, nullptr);
+    ASSERT_TRUE(builder->ResizePartition(system_a, 3_GiB));
+
+    EXPECT_EQ(system_a->extents().size(), static_cast<size_t>(1));
+    EXPECT_EQ(system_b->extents().size(), static_cast<size_t>(1));
+    ASSERT_TRUE(builder->ResizePartition(system_b, 6_GiB));
+    EXPECT_EQ(system_b->extents().size(), static_cast<size_t>(3));
+
+    unique_ptr<LpMetadata> exported = builder->Export();
+    ASSERT_NE(exported, nullptr);
+    ASSERT_EQ(exported->extents.size(), static_cast<size_t>(4));
+    EXPECT_EQ(exported->extents[0].target_data, 10487808);
+    EXPECT_EQ(exported->extents[0].num_sectors, 4194304);
+    EXPECT_EQ(exported->extents[1].target_data, 14682624);
+    EXPECT_EQ(exported->extents[1].num_sectors, 6288896);
+    EXPECT_EQ(exported->extents[2].target_data, 6292992);
+    EXPECT_EQ(exported->extents[2].num_sectors, 2099712);
+    EXPECT_EQ(exported->extents[3].target_data, 1536);
+    EXPECT_EQ(exported->extents[3].num_sectors, 6291456);
+}
diff --git a/fs_mgr/liblp/include/liblp/builder.h b/fs_mgr/liblp/include/liblp/builder.h
index 4bb38d6..f477b4b 100644
--- a/fs_mgr/liblp/include/liblp/builder.h
+++ b/fs_mgr/liblp/include/liblp/builder.h
@@ -275,6 +275,7 @@
                        const LpMetadataPartition& source);
     bool ImportPartition(const LpMetadata& metadata, const LpMetadataPartition& source);
     bool IsABDevice() const;
+    bool IsRetrofitDevice() const;
 
     struct Interval {
         uint32_t device_index;
@@ -294,6 +295,7 @@
     std::vector<Interval> GetFreeRegions() const;
     void ExtentsToFreeList(const std::vector<Interval>& extents,
                            std::vector<Interval>* free_regions) const;
+    std::vector<Interval> PrioritizeSecondHalfOfSuper(const std::vector<Interval>& free_list);
 
     static bool sABOverrideValue;
     static bool sABOverrideSet;