diff --git a/fs_mgr/libsnapshot/android/snapshot/snapshot.proto b/fs_mgr/libsnapshot/android/snapshot/snapshot.proto
index 38c6bf8..36e1169 100644
--- a/fs_mgr/libsnapshot/android/snapshot/snapshot.proto
+++ b/fs_mgr/libsnapshot/android/snapshot/snapshot.proto
@@ -34,7 +34,19 @@
     MERGE_COMPLETED = 3;
 }
 
-// Next: 10
+// Next: 3
+enum MergePhase {
+    // No merge is in progress.
+    NO_MERGE = 0;
+
+    // Shrunk partitions can merge.
+    FIRST_PHASE = 1;
+
+    // Grown partitions can merge.
+    SECOND_PHASE = 2;
+}
+
+// Next: 11
 message SnapshotStatus {
     // Name of the snapshot. This is usually the name of the snapshotted
     // logical partition; for example, "system_b".
@@ -87,6 +99,9 @@
 
     // True if compression is enabled, false otherwise.
     bool compression_enabled = 9;
+
+    // The old partition size (if none existed, this will be zero).
+    uint64 old_partition_size = 10;
 }
 
 // Next: 8
@@ -118,7 +133,7 @@
     Cancelled = 7;
 };
 
-// Next: 6
+// Next: 7
 message SnapshotUpdateStatus {
     UpdateState state = 1;
 
@@ -136,6 +151,9 @@
 
     // Whether compression/dm-user was used for any snapshots.
     bool compression_enabled = 5;
+
+    // Merge phase (if state == MERGING).
+    MergePhase merge_phase = 6;
 }
 
 // Next: 4
diff --git a/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h b/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
index b8a4fad..0a8567f 100644
--- a/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
+++ b/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
@@ -525,11 +525,13 @@
     std::string GetMergeStateFilePath() const;
 
     // Helpers for merging.
+    bool MergeSecondPhaseSnapshots(LockedFile* lock);
     bool SwitchSnapshotToMerge(LockedFile* lock, const std::string& name);
     bool RewriteSnapshotDeviceTable(const std::string& dm_name);
     bool MarkSnapshotMergeCompleted(LockedFile* snapshot_lock, const std::string& snapshot_name);
     void AcknowledgeMergeSuccess(LockedFile* lock);
     void AcknowledgeMergeFailure();
+    MergePhase DecideMergePhase(const SnapshotStatus& status);
     std::unique_ptr<LpMetadata> ReadCurrentMetadata();
 
     enum class MetadataPartitionState {
@@ -562,7 +564,8 @@
     //   UpdateState::MergeNeedsReboot
     UpdateState CheckMergeState(const std::function<bool()>& before_cancel);
     UpdateState CheckMergeState(LockedFile* lock, const std::function<bool()>& before_cancel);
-    UpdateState CheckTargetMergeState(LockedFile* lock, const std::string& name);
+    UpdateState CheckTargetMergeState(LockedFile* lock, const std::string& name,
+                                      const SnapshotUpdateStatus& update_status);
 
     // Interact with status files under /metadata/ota/snapshots.
     bool WriteSnapshotStatus(LockedFile* lock, const SnapshotStatus& status);
diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp
index a3d3668..ebda430 100644
--- a/fs_mgr/libsnapshot/snapshot.cpp
+++ b/fs_mgr/libsnapshot/snapshot.cpp
@@ -157,6 +157,9 @@
         images_->RemoveAllImages();
     }
 
+    // Clear any cached metadata (this allows re-using one manager across tests).
+    old_partition_metadata_ = nullptr;
+
     auto state = ReadUpdateState(file.get());
     if (state != UpdateState::None) {
         LOG(ERROR) << "An update is already in progress, cannot begin a new update";
@@ -480,7 +483,8 @@
     // have completed merging, but the start of the merge process is considered
     // atomic.
     SnapshotStorageMode mode;
-    switch (ReadUpdateState(lock)) {
+    SnapshotUpdateStatus update_status = ReadSnapshotUpdateStatus(lock);
+    switch (update_status.state()) {
         case UpdateState::MergeCompleted:
         case UpdateState::MergeNeedsReboot:
             LOG(ERROR) << "Should not create a snapshot device for " << name
@@ -490,7 +494,11 @@
         case UpdateState::MergeFailed:
             // Note: MergeFailed indicates that a merge is in progress, but
             // is possibly stalled. We still have to honor the merge.
-            mode = SnapshotStorageMode::Merge;
+            if (DecideMergePhase(status) == update_status.merge_phase()) {
+                mode = SnapshotStorageMode::Merge;
+            } else {
+                mode = SnapshotStorageMode::Persistent;
+            }
             break;
         default:
             mode = SnapshotStorageMode::Persistent;
@@ -675,6 +683,8 @@
 
     bool compression_enabled = false;
 
+    std::vector<std::string> first_merge_group;
+
     uint64_t total_cow_file_size = 0;
     DmTargetSnapshot::Status initial_target_values = {};
     for (const auto& snapshot : snapshots) {
@@ -693,6 +703,9 @@
         total_cow_file_size += snapshot_status.cow_file_size();
 
         compression_enabled |= snapshot_status.compression_enabled();
+        if (DecideMergePhase(snapshot_status) == MergePhase::FIRST_PHASE) {
+            first_merge_group.emplace_back(snapshot);
+        }
     }
 
     if (cow_file_size) {
@@ -706,14 +719,26 @@
     initial_status.set_metadata_sectors(initial_target_values.metadata_sectors);
     initial_status.set_compression_enabled(compression_enabled);
 
+    // If any partitions shrunk, we need to merge them before we merge any other
+    // partitions (see b/177935716). Otherwise, a merge from another partition
+    // may overwrite the source block of a copy operation.
+    const std::vector<std::string>* merge_group;
+    if (first_merge_group.empty()) {
+        merge_group = &snapshots;
+        initial_status.set_merge_phase(MergePhase::SECOND_PHASE);
+    } else {
+        merge_group = &first_merge_group;
+        initial_status.set_merge_phase(MergePhase::FIRST_PHASE);
+    }
+
     // Point of no return - mark that we're starting a merge. From now on every
-    // snapshot must be a merge target.
+    // eligible snapshot must be a merge target.
     if (!WriteSnapshotUpdateStatus(lock.get(), initial_status)) {
         return false;
     }
 
     bool rewrote_all = true;
-    for (const auto& snapshot : snapshots) {
+    for (const auto& snapshot : *merge_group) {
         // If this fails, we have no choice but to continue. Everything must
         // be merged. This is not an ideal state to be in, but it is safe,
         // because we the next boot will try again.
@@ -904,13 +929,13 @@
 
 UpdateState SnapshotManager::CheckMergeState(LockedFile* lock,
                                              const std::function<bool()>& before_cancel) {
-    UpdateState state = ReadUpdateState(lock);
-    switch (state) {
+    SnapshotUpdateStatus update_status = ReadSnapshotUpdateStatus(lock);
+    switch (update_status.state()) {
         case UpdateState::None:
         case UpdateState::MergeCompleted:
             // Harmless races are allowed between two callers of WaitForMerge,
             // so in both of these cases we just propagate the state.
-            return state;
+            return update_status.state();
 
         case UpdateState::Merging:
         case UpdateState::MergeNeedsReboot:
@@ -927,10 +952,10 @@
             if (HandleCancelledUpdate(lock, before_cancel)) {
                 return UpdateState::Cancelled;
             }
-            return state;
+            return update_status.state();
 
         default:
-            return state;
+            return update_status.state();
     }
 
     std::vector<std::string> snapshots;
@@ -942,8 +967,9 @@
     bool failed = false;
     bool merging = false;
     bool needs_reboot = false;
+    bool wrong_phase = false;
     for (const auto& snapshot : snapshots) {
-        UpdateState snapshot_state = CheckTargetMergeState(lock, snapshot);
+        UpdateState snapshot_state = CheckTargetMergeState(lock, snapshot, update_status);
         switch (snapshot_state) {
             case UpdateState::MergeFailed:
                 failed = true;
@@ -959,6 +985,9 @@
             case UpdateState::Cancelled:
                 cancelled = true;
                 break;
+            case UpdateState::None:
+                wrong_phase = true;
+                break;
             default:
                 LOG(ERROR) << "Unknown merge status for \"" << snapshot << "\": "
                            << "\"" << snapshot_state << "\"";
@@ -978,6 +1007,14 @@
         // it in WaitForMerge rather than here and elsewhere.
         return UpdateState::MergeFailed;
     }
+    if (wrong_phase) {
+        // If we got here, no other partitions are being merged, and nothing
+        // failed to merge. It's safe to move to the next merge phase.
+        if (!MergeSecondPhaseSnapshots(lock)) {
+            return UpdateState::MergeFailed;
+        }
+        return UpdateState::Merging;
+    }
     if (needs_reboot) {
         WriteUpdateState(lock, UpdateState::MergeNeedsReboot);
         return UpdateState::MergeNeedsReboot;
@@ -993,7 +1030,8 @@
     return UpdateState::MergeCompleted;
 }
 
-UpdateState SnapshotManager::CheckTargetMergeState(LockedFile* lock, const std::string& name) {
+UpdateState SnapshotManager::CheckTargetMergeState(LockedFile* lock, const std::string& name,
+                                                   const SnapshotUpdateStatus& update_status) {
     SnapshotStatus snapshot_status;
     if (!ReadSnapshotStatus(lock, name, &snapshot_status)) {
         return UpdateState::MergeFailed;
@@ -1015,7 +1053,7 @@
         // During a check, we decided the merge was complete, but we were unable to
         // collapse the device-mapper stack and perform COW cleanup. If we haven't
         // rebooted after this check, the device will still be a snapshot-merge
-        // target. If the have rebooted, the device will now be a linear target,
+        // target. If we have rebooted, the device will now be a linear target,
         // and we can try cleanup again.
         if (snapshot_status.state() == SnapshotState::MERGE_COMPLETED) {
             // NB: It's okay if this fails now, we gave cleanup our best effort.
@@ -1036,6 +1074,12 @@
     if (!QuerySnapshotStatus(name, &target_type, &status)) {
         return UpdateState::MergeFailed;
     }
+    if (target_type == "snapshot" &&
+        DecideMergePhase(snapshot_status) == MergePhase::SECOND_PHASE &&
+        update_status.merge_phase() == MergePhase::FIRST_PHASE) {
+        // The snapshot is not being merged because it's in the wrong phase.
+        return UpdateState::None;
+    }
     if (target_type != "snapshot-merge") {
         // We can get here if we failed to rewrite the target type in
         // InitiateMerge(). If we failed to create the target in first-stage
@@ -1071,6 +1115,38 @@
     return UpdateState::MergeCompleted;
 }
 
+bool SnapshotManager::MergeSecondPhaseSnapshots(LockedFile* lock) {
+    std::vector<std::string> snapshots;
+    if (!ListSnapshots(lock, &snapshots)) {
+        return UpdateState::MergeFailed;
+    }
+
+    SnapshotUpdateStatus update_status = ReadSnapshotUpdateStatus(lock);
+    CHECK(update_status.state() == UpdateState::Merging);
+    CHECK(update_status.merge_phase() == MergePhase::FIRST_PHASE);
+
+    update_status.set_merge_phase(MergePhase::SECOND_PHASE);
+    if (!WriteSnapshotUpdateStatus(lock, update_status)) {
+        return false;
+    }
+
+    bool rewrote_all = true;
+    for (const auto& snapshot : snapshots) {
+        SnapshotStatus snapshot_status;
+        if (!ReadSnapshotStatus(lock, snapshot, &snapshot_status)) {
+            return UpdateState::MergeFailed;
+        }
+        if (DecideMergePhase(snapshot_status) != MergePhase::SECOND_PHASE) {
+            continue;
+        }
+        if (!SwitchSnapshotToMerge(lock, snapshot)) {
+            LOG(ERROR) << "Failed to switch snapshot to a second-phase merge target: " << snapshot;
+            rewrote_all = false;
+        }
+    }
+    return rewrote_all;
+}
+
 std::string SnapshotManager::GetSnapshotBootIndicatorPath() {
     return metadata_dir_ + "/" + android::base::Basename(kBootIndicatorPath);
 }
@@ -1211,6 +1287,10 @@
     if (!dm.DeleteDeviceIfExists(base_name)) {
         LOG(ERROR) << "Unable to delete base device for snapshot: " << base_name;
     }
+    auto source_name = GetSourceDeviceName(name);
+    if (!dm.DeleteDeviceIfExists(source_name)) {
+        LOG(ERROR) << "Unable to delete source device for snapshot: " << source_name;
+    }
     return true;
 }
 
@@ -2654,6 +2734,15 @@
             continue;
         }
 
+        // Find the original partition size.
+        auto name = target_partition->name();
+        auto old_partition_name =
+                name.substr(0, name.size() - target_suffix.size()) + cow_creator->current_suffix;
+        auto old_partition = cow_creator->current_metadata->FindPartition(old_partition_name);
+        if (old_partition) {
+            cow_creator_ret->snapshot_status.set_old_partition_size(old_partition->size());
+        }
+
         // Store these device sizes to snapshot status file.
         if (!CreateSnapshot(lock, &cow_creator_ret->snapshot_status)) {
             return Return::Error();
@@ -3355,5 +3444,12 @@
     return old_partition_metadata_.get();
 }
 
+MergePhase SnapshotManager::DecideMergePhase(const SnapshotStatus& status) {
+    if (status.compression_enabled() && status.device_size() < status.old_partition_size()) {
+        return MergePhase::FIRST_PHASE;
+    }
+    return MergePhase::SECOND_PHASE;
+}
+
 }  // namespace snapshot
 }  // namespace android