diff --git a/fs_mgr/libsnapshot/include_test/libsnapshot/test_helpers.h b/fs_mgr/libsnapshot/include_test/libsnapshot/test_helpers.h
index 24c91a8..f45d4ed 100644
--- a/fs_mgr/libsnapshot/include_test/libsnapshot/test_helpers.h
+++ b/fs_mgr/libsnapshot/include_test/libsnapshot/test_helpers.h
@@ -214,29 +214,6 @@
 // Get partition size from update package metadata.
 uint64_t GetSize(PartitionUpdate* partition_update);
 
-// Util class for test cases on low space scenario. These tests assumes image manager
-// uses /data as backup device.
-class LowSpaceUserdata {
-  public:
-    // Set the maximum free space allowed for this test. If /userdata has more space than the given
-    // number, a file is allocated to consume space.
-    AssertionResult Init(uint64_t max_free_space);
-
-    uint64_t free_space() const;
-    uint64_t available_space() const;
-    uint64_t bsize() const;
-
-  private:
-    AssertionResult ReadUserdataStats();
-
-    static constexpr const char* kUserDataDevice = "/data";
-    std::unique_ptr<TemporaryFile> big_file_;
-    bool initialized_ = false;
-    uint64_t free_space_ = 0;
-    uint64_t available_space_ = 0;
-    uint64_t bsize_ = 0;
-};
-
 bool IsVirtualAbEnabled();
 
 #define SKIP_IF_NON_VIRTUAL_AB()                                                        \
diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp
index 64637c2..2661482 100644
--- a/fs_mgr/libsnapshot/snapshot.cpp
+++ b/fs_mgr/libsnapshot/snapshot.cpp
@@ -3133,6 +3133,7 @@
     for (auto&& [name, status] : all_snapshot_status) {
         sum += status.cow_file_size();
     }
+    LOG(INFO) << "Calculated needed COW space: " << sum << " bytes";
     return Return::NoSpace(sum);
 }
 
@@ -3279,7 +3280,10 @@
 
     auto ret = CreateUpdateSnapshotsInternal(lock.get(), manifest, &cow_creator, &created_devices,
                                              &all_snapshot_status);
-    if (!ret.is_ok()) return ret;
+    if (!ret.is_ok()) {
+        LOG(ERROR) << "CreateUpdateSnapshotsInternal failed: " << ret.string();
+        return ret;
+    }
 
     auto exported_target_metadata = target_metadata->Export();
     if (exported_target_metadata == nullptr) {
@@ -3496,7 +3500,10 @@
         // Create the backing COW image if necessary.
         if (snapshot_status.cow_file_size() > 0) {
             auto ret = CreateCowImage(lock, name);
-            if (!ret.is_ok()) return AddRequiredSpace(ret, *all_snapshot_status);
+            if (!ret.is_ok()) {
+                LOG(ERROR) << "CreateCowImage failed: " << ret.string();
+                return AddRequiredSpace(ret, *all_snapshot_status);
+            }
         }
 
         LOG(INFO) << "Successfully created snapshot for " << name;
diff --git a/fs_mgr/libsnapshot/snapshot_test.cpp b/fs_mgr/libsnapshot/snapshot_test.cpp
index 1fbfaf7..22731e7 100644
--- a/fs_mgr/libsnapshot/snapshot_test.cpp
+++ b/fs_mgr/libsnapshot/snapshot_test.cpp
@@ -19,6 +19,7 @@
 #include <signal.h>
 #include <sys/file.h>
 #include <sys/stat.h>
+#include <sys/statvfs.h>
 #include <sys/types.h>
 
 #include <chrono>
@@ -2371,20 +2372,44 @@
             << "FinishedSnapshotWrites should detect overflow of CoW device.";
 }
 
-TEST_F(SnapshotUpdateTest, LowSpace) {
-    static constexpr auto kMaxFree = 10_MiB;
-    auto userdata = std::make_unique<LowSpaceUserdata>();
-    ASSERT_TRUE(userdata->Init(kMaxFree));
+// Get max file size and free space.
+std::pair<uint64_t, uint64_t> GetBigFileLimit() {
+    struct statvfs fs;
+    if (statvfs("/data", &fs) < 0) {
+        PLOG(ERROR) << "statfs failed";
+        return {0, 0};
+    }
 
-    // Grow all partitions to 10_MiB, total 30_MiB. This requires 30 MiB of CoW space. After
-    // using the empty space in super (< 1 MiB), it uses 30 MiB of /userdata space.
+    auto fs_limit = static_cast<uint64_t>(fs.f_blocks) * (fs.f_bsize - 1);
+    auto fs_free = static_cast<uint64_t>(fs.f_bfree) * fs.f_bsize;
+
+    LOG(INFO) << "Big file limit: " << fs_limit << ", free space: " << fs_free;
+
+    return {fs_limit, fs_free};
+}
+
+TEST_F(SnapshotUpdateTest, LowSpace) {
+    // To make the low space test more reliable, we force a large cow estimate.
+    // However legacy VAB ignores the COW estimate and uses InstallOperations
+    // to compute the exact size required for dm-snapshot. It's difficult to
+    // make this work reliably (we'd need to somehow fake an extremely large
+    // super partition, and we don't have that level of dependency injection).
+    //
+    // For now, just skip this test on legacy VAB.
+    if (!snapuserd_required_) {
+        GTEST_SKIP() << "Skipping test on legacy VAB";
+    }
+
+    auto fs = GetBigFileLimit();
+    ASSERT_NE(fs.first, 0);
+
     constexpr uint64_t partition_size = 10_MiB;
     SetSize(sys_, partition_size);
     SetSize(vnd_, partition_size);
     SetSize(prd_, partition_size);
-    sys_->set_estimate_cow_size(partition_size);
-    vnd_->set_estimate_cow_size(partition_size);
-    prd_->set_estimate_cow_size(partition_size);
+    sys_->set_estimate_cow_size(fs.first);
+    vnd_->set_estimate_cow_size(fs.first);
+    prd_->set_estimate_cow_size(fs.first);
 
     AddOperationForPartitions();
 
@@ -2393,8 +2418,12 @@
     auto res = sm->CreateUpdateSnapshots(manifest_);
     ASSERT_FALSE(res);
     ASSERT_EQ(Return::ErrorCode::NO_SPACE, res.error_code());
-    ASSERT_GE(res.required_size(), 14_MiB);
-    ASSERT_LT(res.required_size(), 40_MiB);
+
+    // It's hard to predict exactly how much free space is needed, since /data
+    // is writable and the test is not the only process running. Divide by two
+    // as a rough lower bound, and adjust this in the future as necessary.
+    auto expected_delta = fs.first - fs.second;
+    ASSERT_GE(res.required_size(), expected_delta / 2);
 }
 
 TEST_F(SnapshotUpdateTest, AddPartition) {
@@ -2776,6 +2805,7 @@
     void TearDown() override {
         RETURN_IF_NON_VIRTUAL_AB();
         CleanUp();
+        SnapshotTest::TearDown();
     }
     void CleanUp() {
         if (!image_manager_) {
@@ -2789,26 +2819,13 @@
 };
 
 TEST_F(ImageManagerTest, CreateImageNoSpace) {
-    bool at_least_one_failure = false;
-    for (uint64_t size = 1_MiB; size <= 512_MiB; size *= 2) {
-        auto userdata = std::make_unique<LowSpaceUserdata>();
-        ASSERT_TRUE(userdata->Init(size));
+    auto fs = GetBigFileLimit();
+    ASSERT_NE(fs.first, 0);
 
-        uint64_t to_allocate = userdata->free_space() + userdata->bsize();
-
-        auto res = image_manager_->CreateBackingImage(kImageName, to_allocate,
-                                                      IImageManager::CREATE_IMAGE_DEFAULT);
-        if (!res) {
-            at_least_one_failure = true;
-        } else {
-            ASSERT_EQ(res.error_code(), FiemapStatus::ErrorCode::NO_SPACE) << res.string();
-        }
-
-        CleanUp();
-    }
-
-    ASSERT_TRUE(at_least_one_failure)
-            << "We should have failed to allocate at least one over-sized image";
+    auto res = image_manager_->CreateBackingImage(kImageName, fs.first,
+                                                  IImageManager::CREATE_IMAGE_DEFAULT);
+    ASSERT_FALSE(res);
+    ASSERT_EQ(res.error_code(), FiemapStatus::ErrorCode::NO_SPACE) << res.string();
 }
 
 bool Mkdir(const std::string& path) {
diff --git a/fs_mgr/libsnapshot/test_helpers.cpp b/fs_mgr/libsnapshot/test_helpers.cpp
index 9f1d676..a224f6b 100644
--- a/fs_mgr/libsnapshot/test_helpers.cpp
+++ b/fs_mgr/libsnapshot/test_helpers.cpp
@@ -214,68 +214,6 @@
     return partition_update->mutable_new_partition_info()->size();
 }
 
-AssertionResult LowSpaceUserdata::Init(uint64_t max_free_space) {
-    auto res = ReadUserdataStats();
-    if (!res) return res;
-
-    // Try to fill up the disk as much as possible until free_space_ <= max_free_space.
-    big_file_ = std::make_unique<TemporaryFile>();
-    if (big_file_->fd == -1) {
-        return AssertionFailure() << strerror(errno);
-    }
-    if (!android::base::StartsWith(big_file_->path, kUserDataDevice)) {
-        return AssertionFailure() << "Temp file allocated to " << big_file_->path << ", not in "
-                                  << kUserDataDevice;
-    }
-    uint64_t next_consume = std::min(std::max(available_space_, max_free_space) - max_free_space,
-                                     (uint64_t)std::numeric_limits<off_t>::max());
-    off_t allocated = 0;
-    while (next_consume > 0 && free_space_ > max_free_space) {
-        int status = fallocate(big_file_->fd, 0, allocated, next_consume);
-        if (status == -1 && errno == ENOSPC) {
-            next_consume /= 2;
-            continue;
-        }
-        if (status == -1) {
-            return AssertionFailure() << strerror(errno);
-        }
-        allocated += next_consume;
-
-        res = ReadUserdataStats();
-        if (!res) return res;
-    }
-
-    LOG(INFO) << allocated << " bytes allocated to " << big_file_->path;
-    initialized_ = true;
-    return AssertionSuccess();
-}
-
-AssertionResult LowSpaceUserdata::ReadUserdataStats() {
-    struct statvfs buf;
-    if (statvfs(kUserDataDevice, &buf) == -1) {
-        return AssertionFailure() << strerror(errno);
-    }
-    bsize_ = buf.f_bsize;
-    free_space_ = bsize_ * buf.f_bfree;
-    available_space_ = bsize_ * buf.f_bavail;
-    return AssertionSuccess();
-}
-
-uint64_t LowSpaceUserdata::free_space() const {
-    CHECK(initialized_);
-    return free_space_;
-}
-
-uint64_t LowSpaceUserdata::available_space() const {
-    CHECK(initialized_);
-    return available_space_;
-}
-
-uint64_t LowSpaceUserdata::bsize() const {
-    CHECK(initialized_);
-    return bsize_;
-}
-
 bool IsVirtualAbEnabled() {
     return android::base::GetBoolProperty("ro.virtual_ab.enabled", false);
 }