diff --git a/fs_mgr/libdm/dm_target.cpp b/fs_mgr/libdm/dm_target.cpp
index 9152677..7c9804c 100644
--- a/fs_mgr/libdm/dm_target.cpp
+++ b/fs_mgr/libdm/dm_target.cpp
@@ -210,6 +210,18 @@
     return false;
 }
 
+bool DmTargetSnapshot::GetDevicesFromParams(const std::string& params, std::string* base_device,
+                                            std::string* cow_device) {
+    auto pieces = android::base::Split(params, " ");
+    if (pieces.size() < 2) {
+        LOG(ERROR) << "Parameter string is invalid: " << params;
+        return false;
+    }
+    *base_device = pieces[0];
+    *cow_device = pieces[1];
+    return true;
+}
+
 std::string DmTargetCrypt::GetParameterString() const {
     std::vector<std::string> argv = {
             cipher_,
diff --git a/fs_mgr/libdm/include/libdm/dm.h b/fs_mgr/libdm/include/libdm/dm.h
index c6b37cf..f5783cb 100644
--- a/fs_mgr/libdm/include/libdm/dm.h
+++ b/fs_mgr/libdm/include/libdm/dm.h
@@ -47,6 +47,8 @@
 
 enum class DmDeviceState { INVALID, SUSPENDED, ACTIVE };
 
+static constexpr uint64_t kSectorSize = 512;
+
 class DeviceMapper final {
   public:
     class DmBlockDevice final {
diff --git a/fs_mgr/libdm/include/libdm/dm_target.h b/fs_mgr/libdm/include/libdm/dm_target.h
index ab7c2db..a66ab7a 100644
--- a/fs_mgr/libdm/include/libdm/dm_target.h
+++ b/fs_mgr/libdm/include/libdm/dm_target.h
@@ -219,6 +219,8 @@
     static double MergePercent(const Status& status, uint64_t sectors_initial = 0);
     static bool ParseStatusText(const std::string& text, Status* status);
     static bool ReportsOverflow(const std::string& target_type);
+    static bool GetDevicesFromParams(const std::string& params, std::string* base_device,
+                                     std::string* cow_device);
 
   private:
     std::string base_device_;
diff --git a/fs_mgr/libsnapshot/Android.bp b/fs_mgr/libsnapshot/Android.bp
index 3a08049..52aad12 100644
--- a/fs_mgr/libsnapshot/Android.bp
+++ b/fs_mgr/libsnapshot/Android.bp
@@ -14,15 +14,13 @@
 // limitations under the License.
 //
 
-cc_library {
-    name: "libsnapshot",
-    recovery_available: true,
+cc_defaults {
+    name: "libsnapshot_defaults",
     defaults: ["fs_mgr_defaults"],
-    cppflags: [
+    cflags: [
         "-D_FILE_OFFSET_BITS=64",
-    ],
-    srcs: [
-        "snapshot.cpp",
+        "-Wall",
+        "-Werror",
     ],
     shared_libs: [
         "libbase",
@@ -30,7 +28,53 @@
     ],
     static_libs: [
         "libdm",
+    ],
+    whole_static_libs: [
         "libext2_uuid",
+        "libext4_utils",
+        "libfiemap",
     ],
     export_include_dirs: ["include"],
 }
+
+filegroup {
+    name: "libsnapshot_sources",
+    srcs: [
+        "snapshot.cpp",
+    ],
+}
+
+cc_library_static {
+    name: "libsnapshot",
+    defaults: ["libsnapshot_defaults"],
+    srcs: [":libsnapshot_sources"],
+    static_libs: [
+        "libfiemap_binder",
+    ],
+}
+
+cc_library_static {
+    name: "libsnapshot_nobinder",
+    defaults: ["libsnapshot_defaults"],
+    srcs: [":libsnapshot_sources"],
+    recovery_available: true,
+}
+
+cc_test {
+    name: "libsnapshot_test",
+    defaults: ["libsnapshot_defaults"],
+    srcs: [
+        "snapshot_test.cpp",
+    ],
+    shared_libs: [
+        "libbinder",
+        "libutils",
+    ],
+    static_libs: [
+        "libcutils",
+        "libcrypto",
+        "libfs_mgr",
+        "liblp",
+        "libsnapshot",
+    ],
+}
diff --git a/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h b/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
index 5cfd7fa..03e9eef 100644
--- a/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
+++ b/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
@@ -19,14 +19,28 @@
 #include <chrono>
 #include <memory>
 #include <string>
+#include <vector>
+
+#include <android-base/unique_fd.h>
+#include <libdm/dm_target.h>
+#include <libfiemap/image_manager.h>
+
+#ifndef FRIEND_TEST
+#define FRIEND_TEST(test_set_name, individual_test) \
+    friend class test_set_name##_##individual_test##_Test
+#define DEFINED_FRIEND_TEST
+#endif
 
 namespace android {
 namespace snapshot {
 
-enum class UpdateStatus {
+enum class UpdateState {
     // No update or merge is in progress.
     None,
 
+    // An update is applying; snapshots may already exist.
+    Initiated,
+
     // An update is pending, but has not been successfully booted yet.
     Unverified,
 
@@ -34,36 +48,39 @@
     Merging,
 
     // Merging is complete, and needs to be acknowledged.
-    MergeCompleted
+    MergeCompleted,
+
+    // Merging failed due to an unrecoverable error.
+    MergeFailed
 };
 
 class SnapshotManager final {
   public:
-    // Return a new SnapshotManager instance, or null on error.
-    static std::unique_ptr<SnapshotManager> New();
+    // Dependency injection for testing.
+    class IDeviceInfo {
+      public:
+        virtual ~IDeviceInfo() {}
+        virtual std::string GetGsidDir() const = 0;
+        virtual std::string GetMetadataDir() const = 0;
 
-    // Create a new snapshot device with the given name, base device, and COW device
-    // size. The new device path will be returned in |dev_path|. If timeout_ms is
-    // greater than zero, this function will wait the given amount of time for
-    // |dev_path| to become available, and fail otherwise. If timeout_ms is 0, then
-    // no wait will occur and |dev_path| may not yet exist on return.
-    bool CreateSnapshot(const std::string& name, const std::string& base_device, uint64_t cow_size,
-                        std::string* dev_path, const std::chrono::milliseconds& timeout_ms);
+        // Return true if the device is currently running off snapshot devices,
+        // indicating that we have booted after applying (but not merging) an
+        // OTA.
+        virtual bool IsRunningSnapshot() const = 0;
+    };
 
-    // Map a snapshot device that was previously created with CreateSnapshot.
-    // If a merge was previously initiated, the device-mapper table will have a
-    // snapshot-merge target instead of a snapshot target. The timeout parameter
-    // is the same as in CreateSnapshotDevice.
-    bool MapSnapshotDevice(const std::string& name, const std::string& base_device,
-                           const std::chrono::milliseconds& timeout_ms, std::string* dev_path);
+    // Return a new SnapshotManager instance, or null on error. The device
+    // pointer is owned for the lifetime of SnapshotManager. If null, a default
+    // instance will be created.
+    static std::unique_ptr<SnapshotManager> New(IDeviceInfo* device = nullptr);
 
-    // Unmap a snapshot device previously mapped with MapSnapshotDevice().
-    bool UnmapSnapshotDevice(const std::string& name);
+    // Begin an update. This must be called before creating any snapshots. It
+    // will fail if GetUpdateState() != None.
+    bool BeginUpdate();
 
-    // Remove the backing copy-on-write image for the named snapshot. If the
-    // device is still mapped, this will attempt an Unmap, and fail if the
-    // unmap fails.
-    bool DeleteSnapshot(const std::string& name);
+    // Cancel an update; any snapshots will be deleted. This will fail if the
+    // state != Initiated or None.
+    bool CancelUpdate();
 
     // Initiate a merge on all snapshot devices. This should only be used after an
     // update has been marked successful after booting.
@@ -77,12 +94,129 @@
     // Find the status of the current update, if any.
     //
     // |progress| depends on the returned status:
-    //   None: 0
-    //   Unverified: 0
-    //   Merging: Value in the range [0, 100)
+    //   Merging: Value in the range [0, 100]
     //   MergeCompleted: 100
-    UpdateStatus GetUpdateStatus(double* progress);
+    //   Other: 0
+    UpdateState GetUpdateState(double* progress = nullptr);
+
+  private:
+    FRIEND_TEST(SnapshotTest, CreateSnapshot);
+    FRIEND_TEST(SnapshotTest, MapSnapshot);
+    FRIEND_TEST(SnapshotTest, MapPartialSnapshot);
+    friend class SnapshotTest;
+
+    using IImageManager = android::fiemap::IImageManager;
+
+    explicit SnapshotManager(IDeviceInfo* info);
+
+    // This is created lazily since it connects via binder.
+    bool EnsureImageManager();
+
+    // Helper function for tests.
+    IImageManager* image_manager() const { return images_.get(); }
+
+    // Since libsnapshot is included into multiple processes, we flock() our
+    // files for simple synchronization. LockedFile is a helper to assist with
+    // this. It also serves as a proof-of-lock for some functions.
+    class LockedFile final {
+      public:
+        LockedFile(const std::string& path, android::base::unique_fd&& fd)
+            : path_(path), fd_(std::move(fd)) {}
+        ~LockedFile();
+
+        const std::string& path() const { return path_; }
+        int fd() const { return fd_; }
+
+      private:
+        std::string path_;
+        android::base::unique_fd fd_;
+    };
+    std::unique_ptr<LockedFile> OpenFile(const std::string& file, int open_flags, int lock_flags);
+    bool Truncate(LockedFile* file);
+
+    // Create a new snapshot record. This creates the backing COW store and
+    // persists information needed to map the device. The device can be mapped
+    // with MapSnapshot().
+    //
+    // |device_size| should be the size of the base_device that will be passed
+    // via MapDevice(). |snapshot_size| should be the number of bytes in the
+    // base device, starting from 0, that will be snapshotted. The cow_size
+    // should be the amount of space that will be allocated to store snapshot
+    // deltas.
+    //
+    // If |snapshot_size| < device_size, then the device will always
+    // be mapped with two table entries: a dm-snapshot range covering
+    // snapshot_size, and a dm-linear range covering the remainder.
+    //
+    // All sizes are specified in bytes, and the device and snapshot sizes
+    // must be a multiple of the sector size (512 bytes). |cow_size| will
+    // be rounded up to the nearest sector.
+    bool CreateSnapshot(LockedFile* lock, const std::string& name, uint64_t device_size,
+                        uint64_t snapshot_size, uint64_t cow_size);
+
+    // Map a snapshot device that was previously created with CreateSnapshot.
+    // If a merge was previously initiated, the device-mapper table will have a
+    // snapshot-merge target instead of a snapshot target. If the timeout
+    // parameter greater than zero, this function will wait the given amount
+    // of time for |dev_path| to become available, and fail otherwise. If
+    // timeout_ms is 0, then no wait will occur and |dev_path| may not yet
+    // exist on return.
+    bool MapSnapshot(LockedFile* lock, const std::string& name, const std::string& base_device,
+                     const std::chrono::milliseconds& timeout_ms, std::string* dev_path);
+
+    // Remove the backing copy-on-write image for the named snapshot. If the
+    // device is still mapped, this will attempt an Unmap, and fail if the
+    // unmap fails.
+    bool DeleteSnapshot(LockedFile* lock, const std::string& name);
+
+    // Unmap a snapshot device previously mapped with MapSnapshotDevice().
+    bool UnmapSnapshot(LockedFile* lock, const std::string& name);
+
+    // Unmap and remove all known snapshots.
+    bool RemoveAllSnapshots(LockedFile* lock);
+
+    // List the known snapshot names.
+    bool ListSnapshots(LockedFile* lock, std::vector<std::string>* snapshots);
+
+    // Interact with /metadata/ota/state.
+    std::unique_ptr<LockedFile> OpenStateFile(int open_flags, int lock_flags);
+    std::unique_ptr<LockedFile> LockShared();
+    std::unique_ptr<LockedFile> LockExclusive();
+    UpdateState ReadUpdateState(LockedFile* file);
+    bool WriteUpdateState(LockedFile* file, UpdateState state);
+
+    // This state is persisted per-snapshot in /metadata/ota/snapshots/.
+    struct SnapshotStatus {
+        std::string state;
+        uint64_t device_size;
+        uint64_t snapshot_size;
+        // These are non-zero when merging.
+        uint64_t sectors_allocated = 0;
+        uint64_t metadata_sectors = 0;
+    };
+
+    // Interact with status files under /metadata/ota/snapshots.
+    std::unique_ptr<LockedFile> OpenSnapshotStatusFile(const std::string& name, int open_flags,
+                                                       int lock_flags);
+    bool WriteSnapshotStatus(LockedFile* file, const SnapshotStatus& status);
+    bool ReadSnapshotStatus(LockedFile* file, SnapshotStatus* status);
+
+    // Return the name of the device holding the "snapshot" or "snapshot-merge"
+    // target. This may not be the final device presented via MapSnapshot(), if
+    // for example there is a linear segment.
+    std::string GetSnapshotDeviceName(const std::string& snapshot_name,
+                                      const SnapshotStatus& status);
+
+    std::string gsid_dir_;
+    std::string metadata_dir_;
+    std::unique_ptr<IDeviceInfo> device_;
+    std::unique_ptr<IImageManager> images_;
 };
 
 }  // namespace snapshot
 }  // namespace android
+
+#ifdef DEFINED_FRIEND_TEST
+#undef DEFINED_FRIEND_TEST
+#undef FRIEND_TEST
+#endif
diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp
index 3e80239..7d36b06 100644
--- a/fs_mgr/libsnapshot/snapshot.cpp
+++ b/fs_mgr/libsnapshot/snapshot.cpp
@@ -14,45 +14,309 @@
 
 #include <libsnapshot/snapshot.h>
 
+#include <dirent.h>
+#include <sys/file.h>
+#include <sys/types.h>
+#include <sys/unistd.h>
+
+#include <android-base/file.h>
+#include <android-base/logging.h>
+#include <android-base/parseint.h>
+#include <android-base/strings.h>
+#include <android-base/unique_fd.h>
+#include <ext4_utils/ext4_utils.h>
+#include <libdm/dm.h>
+
 namespace android {
 namespace snapshot {
 
-std::unique_ptr<SnapshotManager> SnapshotManager::New() {
-    return std::make_unique<SnapshotManager>();
+using android::base::unique_fd;
+using android::dm::DeviceMapper;
+using android::dm::DmDeviceState;
+using android::dm::DmTable;
+using android::dm::DmTargetLinear;
+using android::dm::DmTargetSnapshot;
+using android::dm::kSectorSize;
+using android::dm::SnapshotStorageMode;
+using android::fiemap::IImageManager;
+using namespace std::chrono_literals;
+using namespace std::string_literals;
+
+// Unit is sectors, this is a 4K chunk.
+static constexpr uint32_t kSnapshotChunkSize = 8;
+
+class DeviceInfo final : public SnapshotManager::IDeviceInfo {
+  public:
+    std::string GetGsidDir() const override { return "ota"s; }
+    std::string GetMetadataDir() const override { return "/metadata/ota/test"s; }
+    bool IsRunningSnapshot() const override;
+};
+
+bool DeviceInfo::IsRunningSnapshot() const {
+    // :TODO: implement this check.
+    return true;
 }
 
-bool SnapshotManager::CreateSnapshot(const std::string& name, const std::string& base_device,
-                                     uint64_t cow_size, std::string* dev_path,
-                                     const std::chrono::milliseconds& timeout_ms) {
-    // (1) Create COW device using libgsi_image.
-    // (2) Create snapshot device using libdm + DmTargetSnapshot.
-    // (3) Record partition in /metadata/ota.
-    (void)name;
-    (void)base_device;
-    (void)cow_size;
-    (void)dev_path;
-    (void)timeout_ms;
-    return false;
+std::unique_ptr<SnapshotManager> SnapshotManager::New(IDeviceInfo* info) {
+    if (!info) {
+        info = new DeviceInfo();
+    }
+    return std::unique_ptr<SnapshotManager>(new SnapshotManager(info));
 }
 
-bool SnapshotManager::MapSnapshotDevice(const std::string& name, const std::string& base_device,
-                                        const std::chrono::milliseconds& timeout_ms,
-                                        std::string* dev_path) {
-    (void)name;
-    (void)base_device;
-    (void)dev_path;
-    (void)timeout_ms;
-    return false;
+SnapshotManager::SnapshotManager(IDeviceInfo* device) : device_(device) {
+    gsid_dir_ = device_->GetGsidDir();
+    metadata_dir_ = device_->GetMetadataDir();
 }
 
-bool SnapshotManager::UnmapSnapshotDevice(const std::string& name) {
-    (void)name;
-    return false;
+static std::string GetCowName(const std::string& snapshot_name) {
+    return snapshot_name + "-cow";
 }
 
-bool SnapshotManager::DeleteSnapshot(const std::string& name) {
-    (void)name;
-    return false;
+bool SnapshotManager::BeginUpdate() {
+    auto file = LockExclusive();
+    if (!file) return false;
+
+    auto state = ReadUpdateState(file.get());
+    if (state != UpdateState::None) {
+        LOG(ERROR) << "An update is already in progress, cannot begin a new update";
+        return false;
+    }
+    return WriteUpdateState(file.get(), UpdateState::Initiated);
+}
+
+bool SnapshotManager::CancelUpdate() {
+    auto file = LockExclusive();
+    if (!file) return false;
+
+    UpdateState state = ReadUpdateState(file.get());
+    if (state == UpdateState::None) return true;
+    if (state != UpdateState::Initiated) {
+        LOG(ERROR) << "Cannot cancel update after it has completed or started merging";
+        return false;
+    }
+
+    if (!RemoveAllSnapshots(file.get())) {
+        LOG(ERROR) << "Could not remove all snapshots";
+        return false;
+    }
+
+    if (!WriteUpdateState(file.get(), UpdateState::None)) {
+        LOG(ERROR) << "Could not write new update state";
+        return false;
+    }
+    return true;
+}
+
+bool SnapshotManager::CreateSnapshot(LockedFile* lock, const std::string& name,
+                                     uint64_t device_size, uint64_t snapshot_size,
+                                     uint64_t cow_size) {
+    CHECK(lock);
+    if (!EnsureImageManager()) return false;
+
+    // Sanity check these sizes. Like liblp, we guarantee the partition size
+    // is respected, which means it has to be sector-aligned. (This guarantee
+    // is useful for locating avb footers correctly). The COW size, however,
+    // can be arbitrarily larger than specified, so we can safely round it up.
+    if (device_size % kSectorSize != 0) {
+        LOG(ERROR) << "Snapshot " << name
+                   << " device size is not a multiple of the sector size: " << device_size;
+        return false;
+    }
+    if (snapshot_size % kSectorSize != 0) {
+        LOG(ERROR) << "Snapshot " << name
+                   << " snapshot size is not a multiple of the sector size: " << snapshot_size;
+        return false;
+    }
+
+    // Round the COW size up to the nearest sector.
+    cow_size += kSectorSize - 1;
+    cow_size &= ~(kSectorSize - 1);
+
+    LOG(INFO) << "Snapshot " << name << " will have COW size " << cow_size;
+
+    auto status_file = OpenSnapshotStatusFile(name, O_RDWR | O_CREAT, LOCK_EX);
+    if (!status_file) return false;
+
+    // Note, we leave the status file hanging around if we fail to create the
+    // actual backing image. This is harmless, since it'll get removed when
+    // CancelUpdate is called.
+    SnapshotStatus status = {
+            .state = "created",
+            .device_size = device_size,
+            .snapshot_size = snapshot_size,
+    };
+    if (!WriteSnapshotStatus(status_file.get(), status)) {
+        PLOG(ERROR) << "Could not write snapshot status: " << name;
+        return false;
+    }
+
+    auto cow_name = GetCowName(name);
+    int cow_flags = IImageManager::CREATE_IMAGE_ZERO_FILL;
+    return images_->CreateBackingImage(cow_name, cow_size, cow_flags);
+}
+
+bool SnapshotManager::MapSnapshot(LockedFile* lock, const std::string& name,
+                                  const std::string& base_device,
+                                  const std::chrono::milliseconds& timeout_ms,
+                                  std::string* dev_path) {
+    CHECK(lock);
+    if (!EnsureImageManager()) return false;
+
+    auto status_file = OpenSnapshotStatusFile(name, O_RDWR, LOCK_EX);
+    if (!status_file) return false;
+
+    SnapshotStatus status;
+    if (!ReadSnapshotStatus(status_file.get(), &status)) {
+        return false;
+    }
+
+    // Validate the block device size, as well as the requested snapshot size.
+    // During this we also compute the linear sector region if any.
+    {
+        unique_fd fd(open(base_device.c_str(), O_RDONLY | O_CLOEXEC));
+        if (fd < 0) {
+            PLOG(ERROR) << "open failed: " << base_device;
+            return false;
+        }
+        auto dev_size = get_block_device_size(fd);
+        if (!dev_size) {
+            PLOG(ERROR) << "Could not determine block device size: " << base_device;
+            return false;
+        }
+        if (status.device_size != dev_size) {
+            LOG(ERROR) << "Block device size for " << base_device << " does not match"
+                       << "(expected " << status.device_size << ", got " << dev_size << ")";
+            return false;
+        }
+    }
+    if (status.device_size % kSectorSize != 0) {
+        LOG(ERROR) << "invalid blockdev size for " << base_device << ": " << status.device_size;
+        return false;
+    }
+    if (status.snapshot_size % kSectorSize != 0 || status.snapshot_size > status.device_size) {
+        LOG(ERROR) << "Invalid snapshot size for " << base_device << ": " << status.snapshot_size;
+        return false;
+    }
+    uint64_t snapshot_sectors = status.snapshot_size / kSectorSize;
+    uint64_t linear_sectors = (status.device_size - status.snapshot_size) / kSectorSize;
+
+    auto cow_name = GetCowName(name);
+
+    std::string cow_dev;
+    if (!images_->MapImageDevice(cow_name, timeout_ms, &cow_dev)) {
+        return false;
+    }
+
+    auto& dm = DeviceMapper::Instance();
+
+    // Merging is a global state, not per-snapshot. We do however track the
+    // progress of individual snapshots' merges.
+    SnapshotStorageMode mode;
+    UpdateState update_state = ReadUpdateState(lock);
+    if (update_state == UpdateState::Merging || update_state == UpdateState::MergeCompleted) {
+        mode = SnapshotStorageMode::Merge;
+    } else {
+        mode = SnapshotStorageMode::Persistent;
+    }
+
+    // The kernel (tested on 4.19) crashes horribly if a device has both a snapshot
+    // and a linear target in the same table. Instead, we stack them, and give the
+    // snapshot device a different name. It is not exposed to the caller in this
+    // case.
+    auto snap_name = (linear_sectors > 0) ? name + "-inner" : name;
+
+    DmTable table;
+    table.Emplace<DmTargetSnapshot>(0, snapshot_sectors, base_device, cow_dev, mode,
+                                    kSnapshotChunkSize);
+    if (!dm.CreateDevice(snap_name, table, dev_path, timeout_ms)) {
+        LOG(ERROR) << "Could not create snapshot device: " << snap_name;
+        images_->UnmapImageDevice(cow_name);
+        return false;
+    }
+
+    if (linear_sectors) {
+        // Our stacking will looks like this:
+        //     [linear, linear] ; to snapshot, and non-snapshot region of base device
+        //     [snapshot-inner]
+        //     [base device]   [cow]
+        DmTable table;
+        table.Emplace<DmTargetLinear>(0, snapshot_sectors, *dev_path, 0);
+        table.Emplace<DmTargetLinear>(snapshot_sectors, linear_sectors, base_device,
+                                      snapshot_sectors);
+        if (!dm.CreateDevice(name, table, dev_path, timeout_ms)) {
+            LOG(ERROR) << "Could not create outer snapshot device: " << name;
+            dm.DeleteDevice(snap_name);
+            images_->UnmapImageDevice(cow_name);
+            return false;
+        }
+    }
+
+    // :TODO: when merging is implemented, we need to add an argument to the
+    // status indicating how much progress is left to merge. (device-mapper
+    // does not retain the initial values, so we can't derive them.)
+    return true;
+}
+
+bool SnapshotManager::UnmapSnapshot(LockedFile* lock, const std::string& name) {
+    CHECK(lock);
+    if (!EnsureImageManager()) return false;
+
+    auto status_file = OpenSnapshotStatusFile(name, O_RDWR, LOCK_EX);
+    if (!status_file) return false;
+
+    SnapshotStatus status;
+    if (!ReadSnapshotStatus(status_file.get(), &status)) {
+        return false;
+    }
+
+    auto& dm = DeviceMapper::Instance();
+    if (dm.GetState(name) != DmDeviceState::INVALID && !dm.DeleteDevice(name)) {
+        LOG(ERROR) << "Could not delete snapshot device: " << name;
+        return false;
+    }
+
+    // There may be an extra device, since the kernel doesn't let us have a
+    // snapshot and linear target in the same table.
+    auto dm_name = GetSnapshotDeviceName(name, status);
+    if (name != dm_name && !dm.DeleteDevice(dm_name)) {
+        LOG(ERROR) << "Could not delete inner snapshot device: " << dm_name;
+        return false;
+    }
+
+    auto cow_name = GetCowName(name);
+    if (images_->IsImageMapped(cow_name) && !images_->UnmapImageDevice(cow_name)) {
+        return false;
+    }
+    return true;
+}
+
+bool SnapshotManager::DeleteSnapshot(LockedFile* lock, const std::string& name) {
+    CHECK(lock);
+    if (!EnsureImageManager()) return false;
+
+    if (!UnmapSnapshot(lock, name)) {
+        LOG(ERROR) << "Snapshot could not be unmapped for deletion: " << name;
+        return false;
+    }
+
+    // Take the snapshot's lock after Unmap, since it will also try to lock.
+    auto status_file = OpenSnapshotStatusFile(name, O_RDONLY, LOCK_EX);
+    if (!status_file) return false;
+
+    auto cow_name = GetCowName(name);
+    if (!images_->BackingImageExists(cow_name)) {
+        return true;
+    }
+    if (!images_->DeleteBackingImage(cow_name)) {
+        return false;
+    }
+
+    if (!android::base::RemoveFileIfExists(status_file->path())) {
+        LOG(ERROR) << "Failed to remove status file: " << status_file->path();
+        return false;
+    }
+    return true;
 }
 
 bool SnapshotManager::InitiateMerge() {
@@ -63,9 +327,242 @@
     return false;
 }
 
-UpdateStatus SnapshotManager::GetUpdateStatus(double* progress) {
-    *progress = 0.0f;
-    return UpdateStatus::None;
+bool SnapshotManager::RemoveAllSnapshots(LockedFile* lock) {
+    std::vector<std::string> snapshots;
+    if (!ListSnapshots(lock, &snapshots)) {
+        LOG(ERROR) << "Could not list snapshots";
+        return false;
+    }
+
+    bool ok = true;
+    for (const auto& name : snapshots) {
+        ok &= DeleteSnapshot(lock, name);
+    }
+    return ok;
+}
+
+UpdateState SnapshotManager::GetUpdateState(double* progress) {
+    auto file = LockShared();
+    if (!file) {
+        return UpdateState::None;
+    }
+
+    auto state = ReadUpdateState(file.get());
+    if (progress) {
+        *progress = 0.0;
+        if (state == UpdateState::Merging) {
+            // :TODO: When merging is implemented, set progress_val.
+        } else if (state == UpdateState::MergeCompleted) {
+            *progress = 100.0;
+        }
+    }
+    return state;
+}
+
+bool SnapshotManager::ListSnapshots(LockedFile* lock, std::vector<std::string>* snapshots) {
+    CHECK(lock);
+
+    auto dir_path = metadata_dir_ + "/snapshots"s;
+    std::unique_ptr<DIR, decltype(&closedir)> dir(opendir(dir_path.c_str()), closedir);
+    if (!dir) {
+        PLOG(ERROR) << "opendir failed: " << dir_path;
+        return false;
+    }
+
+    struct dirent* dp;
+    while ((dp = readdir(dir.get())) != nullptr) {
+        if (dp->d_type != DT_REG) continue;
+        snapshots->emplace_back(dp->d_name);
+    }
+    return true;
+}
+
+auto SnapshotManager::OpenFile(const std::string& file, int open_flags, int lock_flags)
+        -> std::unique_ptr<LockedFile> {
+    unique_fd fd(open(file.c_str(), open_flags | O_CLOEXEC | O_NOFOLLOW | O_SYNC, 0660));
+    if (fd < 0) {
+        PLOG(ERROR) << "Open failed: " << file;
+        return nullptr;
+    }
+    if (flock(fd, lock_flags) < 0) {
+        PLOG(ERROR) << "Acquire flock failed: " << file;
+        return nullptr;
+    }
+    return std::make_unique<LockedFile>(file, std::move(fd));
+}
+
+SnapshotManager::LockedFile::~LockedFile() {
+    if (flock(fd_, LOCK_UN) < 0) {
+        PLOG(ERROR) << "Failed to unlock file: " << path_;
+    }
+}
+
+std::unique_ptr<SnapshotManager::LockedFile> SnapshotManager::OpenStateFile(int open_flags,
+                                                                            int lock_flags) {
+    auto state_file = metadata_dir_ + "/state"s;
+    return OpenFile(state_file, open_flags, lock_flags);
+}
+
+std::unique_ptr<SnapshotManager::LockedFile> SnapshotManager::LockShared() {
+    return OpenStateFile(O_RDONLY, LOCK_SH);
+}
+
+std::unique_ptr<SnapshotManager::LockedFile> SnapshotManager::LockExclusive() {
+    return OpenStateFile(O_RDWR | O_CREAT, LOCK_EX);
+}
+
+UpdateState SnapshotManager::ReadUpdateState(LockedFile* file) {
+    // Reset position since some calls read+write.
+    if (lseek(file->fd(), 0, SEEK_SET) < 0) {
+        PLOG(ERROR) << "lseek state file failed";
+        return UpdateState::None;
+    }
+
+    std::string contents;
+    if (!android::base::ReadFdToString(file->fd(), &contents)) {
+        PLOG(ERROR) << "Read state file failed";
+        return UpdateState::None;
+    }
+
+    if (contents.empty() || contents == "none") {
+        return UpdateState::None;
+    } else if (contents == "initiated") {
+        return UpdateState::Initiated;
+    } else if (contents == "unverified") {
+        return UpdateState::Unverified;
+    } else if (contents == "merging") {
+        return UpdateState::Merging;
+    } else if (contents == "merge-completed") {
+        return UpdateState::MergeCompleted;
+    } else {
+        LOG(ERROR) << "Unknown merge state in update state file";
+        return UpdateState::None;
+    }
+}
+
+bool SnapshotManager::WriteUpdateState(LockedFile* file, UpdateState state) {
+    std::string contents;
+    switch (state) {
+        case UpdateState::None:
+            contents = "none";
+            break;
+        case UpdateState::Initiated:
+            contents = "initiated";
+            break;
+        case UpdateState::Unverified:
+            contents = "unverified";
+            break;
+        case UpdateState::Merging:
+            contents = "merging";
+            break;
+        case UpdateState::MergeCompleted:
+            contents = "merge-completed";
+            break;
+        default:
+            LOG(ERROR) << "Unknown update state";
+            return false;
+    }
+
+    if (!Truncate(file)) return false;
+    if (!android::base::WriteStringToFd(contents, file->fd())) {
+        PLOG(ERROR) << "Could not write to state file";
+        return false;
+    }
+    return true;
+}
+
+auto SnapshotManager::OpenSnapshotStatusFile(const std::string& name, int open_flags,
+                                             int lock_flags) -> std::unique_ptr<LockedFile> {
+    auto file = metadata_dir_ + "/snapshots/"s + name;
+    return OpenFile(file, open_flags, lock_flags);
+}
+
+bool SnapshotManager::ReadSnapshotStatus(LockedFile* file, SnapshotStatus* status) {
+    // Reset position since some calls read+write.
+    if (lseek(file->fd(), 0, SEEK_SET) < 0) {
+        PLOG(ERROR) << "lseek status file failed";
+        return false;
+    }
+
+    std::string contents;
+    if (!android::base::ReadFdToString(file->fd(), &contents)) {
+        PLOG(ERROR) << "read status file failed";
+        return false;
+    }
+    auto pieces = android::base::Split(contents, " ");
+    if (pieces.size() != 5) {
+        LOG(ERROR) << "Invalid status line for snapshot: " << file->path();
+        return false;
+    }
+
+    status->state = pieces[0];
+    if (!android::base::ParseUint(pieces[1], &status->device_size)) {
+        LOG(ERROR) << "Invalid device size in status line for: " << file->path();
+        return false;
+    }
+    if (!android::base::ParseUint(pieces[2], &status->snapshot_size)) {
+        LOG(ERROR) << "Invalid snapshot size in status line for: " << file->path();
+        return false;
+    }
+    if (!android::base::ParseUint(pieces[3], &status->sectors_allocated)) {
+        LOG(ERROR) << "Invalid snapshot size in status line for: " << file->path();
+        return false;
+    }
+    if (!android::base::ParseUint(pieces[4], &status->metadata_sectors)) {
+        LOG(ERROR) << "Invalid snapshot size in status line for: " << file->path();
+        return false;
+    }
+    return true;
+}
+
+bool SnapshotManager::WriteSnapshotStatus(LockedFile* file, const SnapshotStatus& status) {
+    std::vector<std::string> pieces = {
+            status.state,
+            std::to_string(status.device_size),
+            std::to_string(status.snapshot_size),
+            std::to_string(status.sectors_allocated),
+            std::to_string(status.metadata_sectors),
+    };
+    auto contents = android::base::Join(pieces, " ");
+
+    if (!Truncate(file)) return false;
+    if (!android::base::WriteStringToFd(contents, file->fd())) {
+        PLOG(ERROR) << "write to status file failed: " << file->path();
+        return false;
+    }
+    return true;
+}
+
+bool SnapshotManager::Truncate(LockedFile* file) {
+    if (lseek(file->fd(), 0, SEEK_SET) < 0) {
+        PLOG(ERROR) << "lseek file failed: " << file->path();
+        return false;
+    }
+    if (ftruncate(file->fd(), 0) < 0) {
+        PLOG(ERROR) << "truncate failed: " << file->path();
+        return false;
+    }
+    return true;
+}
+
+std::string SnapshotManager::GetSnapshotDeviceName(const std::string& snapshot_name,
+                                                   const SnapshotStatus& status) {
+    if (status.device_size != status.snapshot_size) {
+        return snapshot_name + "-inner";
+    }
+    return snapshot_name;
+}
+
+bool SnapshotManager::EnsureImageManager() {
+    if (images_) return true;
+
+    // For now, use a preset timeout.
+    images_ = android::fiemap::IImageManager::Open(gsid_dir_, 15000ms);
+    if (!images_) {
+        LOG(ERROR) << "Could not open ImageManager";
+        return false;
+    }
+    return true;
 }
 
 }  // namespace snapshot
diff --git a/fs_mgr/libsnapshot/snapshot_test.cpp b/fs_mgr/libsnapshot/snapshot_test.cpp
new file mode 100644
index 0000000..38ba364
--- /dev/null
+++ b/fs_mgr/libsnapshot/snapshot_test.cpp
@@ -0,0 +1,189 @@
+// Copyright (C) 2018 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <libsnapshot/snapshot.h>
+
+#include <fcntl.h>
+#include <sys/file.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#include <chrono>
+#include <iostream>
+
+#include <android-base/strings.h>
+#include <gtest/gtest.h>
+
+namespace android {
+namespace snapshot {
+
+using namespace std::chrono_literals;
+using namespace std::string_literals;
+
+class TestDeviceInfo : public SnapshotManager::IDeviceInfo {
+  public:
+    std::string GetGsidDir() const override { return "ota/test"s; }
+    std::string GetMetadataDir() const override { return "/metadata/ota/test"s; }
+    bool IsRunningSnapshot() const override { return is_running_snapshot_; }
+
+    void set_is_running_snapshot(bool value) { is_running_snapshot_ = value; }
+
+  private:
+    bool is_running_snapshot_;
+};
+
+std::unique_ptr<SnapshotManager> sm;
+TestDeviceInfo* test_device = nullptr;
+
+class SnapshotTest : public ::testing::Test {
+  protected:
+    void SetUp() override {
+        test_device->set_is_running_snapshot(false);
+
+        if (sm->GetUpdateState() != UpdateState::None) {
+            ASSERT_TRUE(sm->CancelUpdate());
+        }
+        ASSERT_TRUE(sm->BeginUpdate());
+        ASSERT_TRUE(sm->EnsureImageManager());
+
+        image_manager_ = sm->image_manager();
+        ASSERT_NE(image_manager_, nullptr);
+    }
+
+    void TearDown() override {
+        lock_ = nullptr;
+
+        if (sm->GetUpdateState() != UpdateState::None) {
+            ASSERT_TRUE(sm->CancelUpdate());
+        }
+        for (const auto& temp_image : temp_images_) {
+            image_manager_->UnmapImageDevice(temp_image);
+            image_manager_->DeleteBackingImage(temp_image);
+        }
+    }
+
+    bool AcquireLock() {
+        lock_ = sm->OpenStateFile(O_RDWR, LOCK_EX);
+        return !!lock_;
+    }
+
+    bool CreateTempDevice(const std::string& name, uint64_t size, std::string* path) {
+        if (!image_manager_->CreateBackingImage(name, size, false)) {
+            return false;
+        }
+        temp_images_.emplace_back(name);
+        return image_manager_->MapImageDevice(name, 10s, path);
+    }
+
+    std::unique_ptr<SnapshotManager::LockedFile> lock_;
+    std::vector<std::string> temp_images_;
+    android::fiemap::IImageManager* image_manager_ = nullptr;
+};
+
+TEST_F(SnapshotTest, CreateSnapshot) {
+    ASSERT_TRUE(AcquireLock());
+
+    static const uint64_t kDeviceSize = 1024 * 1024;
+    ASSERT_TRUE(sm->CreateSnapshot(lock_.get(), "test-snapshot", kDeviceSize, kDeviceSize,
+                                   kDeviceSize));
+
+    std::vector<std::string> snapshots;
+    ASSERT_TRUE(sm->ListSnapshots(lock_.get(), &snapshots));
+    ASSERT_EQ(snapshots.size(), 1);
+    ASSERT_EQ(snapshots[0], "test-snapshot");
+
+    // Scope so delete can re-acquire the status file lock.
+    {
+        auto file = sm->OpenSnapshotStatusFile("test-snapshot", O_RDONLY, LOCK_SH);
+        ASSERT_NE(file, nullptr);
+
+        SnapshotManager::SnapshotStatus status;
+        ASSERT_TRUE(sm->ReadSnapshotStatus(file.get(), &status));
+        ASSERT_EQ(status.state, "created");
+        ASSERT_EQ(status.device_size, kDeviceSize);
+        ASSERT_EQ(status.snapshot_size, kDeviceSize);
+    }
+
+    ASSERT_TRUE(sm->DeleteSnapshot(lock_.get(), "test-snapshot"));
+}
+
+TEST_F(SnapshotTest, MapSnapshot) {
+    ASSERT_TRUE(AcquireLock());
+
+    static const uint64_t kDeviceSize = 1024 * 1024;
+    ASSERT_TRUE(sm->CreateSnapshot(lock_.get(), "test-snapshot", kDeviceSize, kDeviceSize,
+                                   kDeviceSize));
+
+    std::string base_device;
+    ASSERT_TRUE(CreateTempDevice("base-device", kDeviceSize, &base_device));
+
+    std::string snap_device;
+    ASSERT_TRUE(sm->MapSnapshot(lock_.get(), "test-snapshot", base_device, 10s, &snap_device));
+    ASSERT_TRUE(android::base::StartsWith(snap_device, "/dev/block/dm-"));
+}
+
+TEST_F(SnapshotTest, MapPartialSnapshot) {
+    ASSERT_TRUE(AcquireLock());
+
+    static const uint64_t kSnapshotSize = 1024 * 1024;
+    static const uint64_t kDeviceSize = 1024 * 1024 * 2;
+    ASSERT_TRUE(sm->CreateSnapshot(lock_.get(), "test-snapshot", kDeviceSize, kSnapshotSize,
+                                   kSnapshotSize));
+
+    std::string base_device;
+    ASSERT_TRUE(CreateTempDevice("base-device", kDeviceSize, &base_device));
+
+    std::string snap_device;
+    ASSERT_TRUE(sm->MapSnapshot(lock_.get(), "test-snapshot", base_device, 10s, &snap_device));
+    ASSERT_TRUE(android::base::StartsWith(snap_device, "/dev/block/dm-"));
+}
+
+}  // namespace snapshot
+}  // namespace android
+
+using namespace android::snapshot;
+
+bool Mkdir(const std::string& path) {
+    if (mkdir(path.c_str(), 0700) && errno != EEXIST) {
+        std::cerr << "Could not mkdir " << path << ": " << strerror(errno) << std::endl;
+        return false;
+    }
+    return true;
+}
+
+int main(int argc, char** argv) {
+    ::testing::InitGoogleTest(&argc, argv);
+
+    std::vector<std::string> paths = {
+            "/data/gsi/ota/test",
+            "/metadata/gsi/ota/test",
+            "/metadata/ota/test",
+            "/metadata/ota/test/snapshots",
+    };
+    for (const auto& path : paths) {
+        if (!Mkdir(path)) {
+            return 1;
+        }
+    }
+
+    // Create this once, otherwise, gsid will start/stop between each test.
+    test_device = new TestDeviceInfo();
+    sm = SnapshotManager::New(test_device);
+    if (!sm) {
+        std::cerr << "Could not create snapshot manager";
+        return 1;
+    }
+
+    return RUN_ALL_TESTS();
+}
diff --git a/rootdir/init.rc b/rootdir/init.rc
index d22e9a7..86d8042 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -415,6 +415,7 @@
     chmod 0700 /metadata/vold
     mkdir /metadata/password_slots 0771 root system
     mkdir /metadata/ota 0700 root system
+    mkdir /metadata/ota/snapshots 0700 root system
 
     mkdir /metadata/apex 0700 root system
     mkdir /metadata/apex/sessions 0700 root system