Multithreaded backup and restore in BackupEngineImpl

Summary:
Add a new field: BackupableDBOptions.max_background_copies.
CreateNewBackup() and RestoreDBFromBackup() will use this number of threads to perform copies.
If there is a backup rate limit, then max_background_copies must be 1.
Update backupable_db_test.cc to test multi-threaded backup and restore.
Update backupable_db_test.cc to test backups when the backup environment is not the same as the database environment.

Test Plan:
Run ./backupable_db_test
Run valgrind ./backupable_db_test
Run with TSAN and ASAN

Reviewers: yhchiang, rven, anthony, sdong, igor

Reviewed By: igor

Subscribers: yhchiang, anthony, sdong, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D40725

HISTORY.md

@@ -22,6 +22,7 @@
 * Add Cache.GetPinnedUsage() to get the size of memory occupied by entries that are in use by the system.
 * DB:Open() will fail if the compression specified in Options is not linked with the binary. If you see this failure, recompile RocksDB with compression libraries present on your system. Also, previously our default compression was snappy. This behavior is now changed. Now, the default compression is snappy only if it's available on the system. If it isn't we change the default to kNoCompression.
 * We changed how we account for memory used in block cache. Previously, we only counted the sum of block sizes currently present in block cache. Now, we count the actual memory usage of the blocks. For example, a block of size 4.5KB will use 8KB memory with jemalloc. This might decrease your memory usage and possibly decrease performance. Increase block cache size if you see this happening after an upgrade.
+* Add BackupEngineImpl.options_.max_background_operations to specify the maximum number of operations that may be performed in parallel. Add support for parallelized backup and restore.
 
 ## 3.11.0 (5/19/2015)
 ### New Features

include/rocksdb/utilities/backupable_db.h

@@ -87,6 +87,11 @@ struct BackupableDBOptions {
   // *turn it on only if you know what you're doing*
   bool share_files_with_checksum;
 
+  // Up to this many background threads will copy files for CreateNewBackup()
+  // and RestoreDBFromBackup()
+  // Default: 1
+  int max_background_operations;
+
   void Dump(Logger* logger) const;
 
   explicit BackupableDBOptions(const std::string& _backup_dir,
@@ -96,7 +101,8 @@ struct BackupableDBOptions {
                                bool _destroy_old_data = false,
                                bool _backup_log_files = true,
                                uint64_t _backup_rate_limit = 0,
-                               uint64_t _restore_rate_limit = 0)
+                               uint64_t _restore_rate_limit = 0,
+                               int _max_background_operations = 1)
       : backup_dir(_backup_dir),
         backup_env(_backup_env),
         share_table_files(_share_table_files),
@@ -106,7 +112,8 @@ struct BackupableDBOptions {
         backup_log_files(_backup_log_files),
         backup_rate_limit(_backup_rate_limit),
         restore_rate_limit(_restore_rate_limit),
-        share_files_with_checksum(false) {
+        share_files_with_checksum(false),
+        max_background_operations(_max_background_operations) {
     assert(share_table_files || !share_files_with_checksum);
   }
 };

util/channel.h

@@ -0,0 +1,67 @@
+//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+
+#include <condition_variable>
+#include <mutex>
+#include <queue>
+#include <utility>
+
+#pragma once
+
+namespace rocksdb {
+
+template <class T>
+class channel {
+ public:
+  explicit channel() : eof_(false) {}
+
+  channel(const channel&) = delete;
+  void operator=(const channel&) = delete;
+
+  void sendEof() {
+    std::lock_guard<std::mutex> lk(lock_);
+    eof_ = true;
+    cv_.notify_all();
+  }
+
+  bool eof() {
+    std::lock_guard<std::mutex> lk(lock_);
+    return buffer_.empty() && eof_;
+  }
+
+  size_t size() const {
+    std::lock_guard<std::mutex> lk(lock_);
+    return buffer_.size();
+  }
+
+  // writes elem to the queue
+  void write(T&& elem) {
+    std::unique_lock<std::mutex> lk(lock_);
+    buffer_.emplace(std::forward<T>(elem));
+    cv_.notify_one();
+  }
+
+  /// Moves a dequeued element onto elem, blocking until an element
+  /// is available.
+  // returns false if EOF
+  bool read(T& elem) {
+    std::unique_lock<std::mutex> lk(lock_);
+    cv_.wait(lk, [&] { return eof_ || !buffer_.empty(); });
+    if (eof_ && buffer_.empty()) {
+      return false;
+    }
+    elem = std::move(buffer_.front());
+    buffer_.pop();
+    cv_.notify_one();
+    return true;
+  }
+
+ private:
+  std::condition_variable cv_;
+  std::mutex lock_;
+  std::queue<T> buffer_;
+  bool eof_;
+};
+}  // namespace rocksdb

utilities/backupable/backupable_db.cc

@@ -11,6 +11,7 @@
 
 #include "rocksdb/utilities/backupable_db.h"
 #include "db/filename.h"
+#include "util/channel.h"
 #include "util/coding.h"
 #include "util/crc32c.h"
 #include "util/logging.h"
@@ -30,7 +31,10 @@
 #include <string>
 #include <limits>
 #include <atomic>
+#include <future>
+#include <thread>
 #include <unordered_map>
+#include <unordered_set>
 
 namespace rocksdb {
 
@@ -99,18 +103,21 @@ std::string BackupStatistics::ToString() const {
 }
 
 void BackupableDBOptions::Dump(Logger* logger) const {
-  Log(logger, "        Options.backup_dir: %s", backup_dir.c_str());
-  Log(logger, "        Options.backup_env: %p", backup_env);
-  Log(logger, " Options.share_table_files: %d",
+  Log(logger, "               Options.backup_dir: %s", backup_dir.c_str());
+  Log(logger, "               Options.backup_env: %p", backup_env);
+  Log(logger, "        Options.share_table_files: %d",
       static_cast<int>(share_table_files));
-  Log(logger, "          Options.info_log: %p", info_log);
-  Log(logger, "              Options.sync: %d", static_cast<int>(sync));
-  Log(logger, "  Options.destroy_old_data: %d",
+  Log(logger, "                 Options.info_log: %p", info_log);
+  Log(logger, "                     Options.sync: %d", static_cast<int>(sync));
+  Log(logger, "         Options.destroy_old_data: %d",
       static_cast<int>(destroy_old_data));
-  Log(logger, "  Options.backup_log_files: %d",
+  Log(logger, "         Options.backup_log_files: %d",
       static_cast<int>(backup_log_files));
-  Log(logger, " Options.backup_rate_limit: %" PRIu64, backup_rate_limit);
-  Log(logger, "Options.restore_rate_limit: %" PRIu64, restore_rate_limit);
+  Log(logger, "        Options.backup_rate_limit: %" PRIu64, backup_rate_limit);
+  Log(logger, "       Options.restore_rate_limit: %" PRIu64,
+      restore_rate_limit);
+  Log(logger, "Options.max_background_operations: %d",
+      max_background_operations);
 }
 
 // -------- BackupEngineImpl class ---------
@@ -303,21 +310,92 @@ class BackupEngineImpl : public BackupEngine {
                   uint64_t* size = nullptr,
                   uint32_t* checksum_value = nullptr,
                   uint64_t size_limit = 0);
-  // if size_limit == 0, there is no size limit, copy everything
-  Status BackupFile(BackupID backup_id,
-                    BackupMeta* backup,
-                    bool shared,
-                    const std::string& src_dir,
-                    const std::string& src_fname,  // starts with "/"
-                    BackupRateLimiter* rate_limiter,
-                    uint64_t size_limit = 0,
-                    bool shared_checksum = false);
 
   Status CalculateChecksum(const std::string& src,
                            Env* src_env,
                            uint64_t size_limit,
                            uint32_t* checksum_value);
 
+  struct CopyResult {
+    uint64_t size;
+    uint32_t checksum_value;
+    Status status;
+  };
+  struct CopyWorkItem {
+    std::string src_path;
+    std::string dst_path;
+    Env* src_env;
+    Env* dst_env;
+    bool sync;
+    BackupRateLimiter* rate_limiter;
+    uint64_t size_limit;
+    std::promise<CopyResult> result;
+    CopyWorkItem() {}
+    CopyWorkItem(std::string _src_path,
+                 std::string _dst_path,
+                 Env* _src_env,
+                 Env* _dst_env,
+                 bool _sync,
+                 BackupRateLimiter* _rate_limiter,
+                 uint64_t _size_limit)
+        : src_path(std::move(_src_path)),
+          dst_path(std::move(_dst_path)),
+          src_env(_src_env),
+          dst_env(_dst_env),
+          sync(_sync),
+          rate_limiter(_rate_limiter),
+          size_limit(_size_limit) {}
+  };
+
+  struct BackupAfterCopyWorkItem {
+    std::future<CopyResult> result;
+    bool shared;
+    bool needed_to_copy;
+    Env* backup_env;
+    std::string dst_path_tmp;
+    std::string dst_path;
+    std::string dst_relative;
+    BackupAfterCopyWorkItem() {}
+    BackupAfterCopyWorkItem(std::future<CopyResult> _result,
+                            bool _shared,
+                            bool _needed_to_copy,
+                            Env* _backup_env,
+                            std::string _dst_path_tmp,
+                            std::string _dst_path,
+                            std::string _dst_relative)
+        : result(std::move(_result)),
+          shared(_shared),
+          needed_to_copy(_needed_to_copy),
+          backup_env(_backup_env),
+          dst_path_tmp(std::move(_dst_path_tmp)),
+          dst_path(std::move(_dst_path)),
+          dst_relative(std::move(_dst_relative)) {}
+  };
+
+  struct RestoreAfterCopyWorkItem {
+    std::future<CopyResult> result;
+    uint32_t checksum_value;
+    RestoreAfterCopyWorkItem() {}
+    RestoreAfterCopyWorkItem(std::future<CopyResult> _result,
+                             uint32_t _checksum_value)
+        : result(std::move(_result)),
+          checksum_value(_checksum_value) {}
+  };
+
+  channel<CopyWorkItem> files_to_copy_;
+  std::vector<std::thread> threads_;
+
+  Status AddBackupFileWorkItem(
+          std::unordered_set<std::string>& live_dst_paths,
+          std::vector<BackupAfterCopyWorkItem>& backup_items_to_finish,
+          BackupID backup_id,
+          bool shared,
+          const std::string& src_dir,
+          const std::string& src_fname,  // starts with "/"
+          BackupRateLimiter* rate_limiter,
+          uint64_t size_limit = 0,
+          bool shared_checksum = false);
+
   // backup state data
   BackupID latest_backup_id_;
   std::map<BackupID, unique_ptr<BackupMeta>> backups_;
@@ -365,6 +443,27 @@ BackupEngineImpl::BackupEngineImpl(Env* db_env,
       backup_env_(options.backup_env != nullptr ? options.backup_env : db_env_),
       copy_file_buffer_size_(kDefaultCopyFileBufferSize),
       read_only_(read_only) {
+
+  // set up threads perform copies from files_to_copy_ in the background
+  for (int t = 0; t < options_.max_background_operations; t++) {
+    threads_.emplace_back([&]() {
+      CopyWorkItem work_item;
+      while (files_to_copy_.read(work_item)) {
+        CopyResult result;
+        result.status = CopyFile(work_item.src_path,
+                                 work_item.dst_path,
+                                 work_item.src_env,
+                                 work_item.dst_env,
+                                 work_item.sync,
+                                 work_item.rate_limiter,
+                                 &result.size,
+                                 &result.checksum_value,
+                                 work_item.size_limit);
+        work_item.result.set_value(std::move(result));
+      }
+    });
+  }
+
   if (read_only_) {
     Log(options_.info_log, "Starting read_only backup engine");
   }
@@ -487,9 +586,20 @@ BackupEngineImpl::BackupEngineImpl(Env* db_env,
   Log(options_.info_log, "Initialized BackupEngine");
 }
 
-BackupEngineImpl::~BackupEngineImpl() { LogFlush(options_.info_log); }
+BackupEngineImpl::~BackupEngineImpl() {
+  files_to_copy_.sendEof();
+  for (int i = 0; i < options_.max_background_operations; i++) {
+    threads_[i].join();
+  }
+  LogFlush(options_.info_log);
+}
 
 Status BackupEngineImpl::CreateNewBackup(DB* db, bool flush_before_backup) {
+  if (options_.max_background_operations > 1 &&
+          options_.backup_rate_limit != 0) {
+    return Status::InvalidArgument(
+        "Multi-threaded backups cannot use a backup_rate_limit");
+  }
   assert(!read_only_);
   Status s;
   std::vector<std::string> live_files;
@@ -539,7 +649,15 @@ Status BackupEngineImpl::CreateNewBackup(DB* db, bool flush_before_backup) {
           options_.backup_rate_limit, copy_file_buffer_size_));
   }
 
-  // copy live_files
+  // A set into which we will insert the dst_paths that are calculated for live
+  // files and live WAL files.
+  // This is used to check whether a live files shares a dst_path with another
+  // live file.
+  std::unordered_set<std::string> live_dst_paths;
+  live_dst_paths.reserve(live_files.size() + live_wal_files.size());
+
+  std::vector<BackupAfterCopyWorkItem> backup_items_to_finish;
+  // Add a CopyWorkItem to the channel for each live file
   for (size_t i = 0; s.ok() && i < live_files.size(); ++i) {
     uint64_t number;
     FileType type;
@@ -555,27 +673,49 @@ Status BackupEngineImpl::CreateNewBackup(DB* db, bool flush_before_backup) {
     // rules:
     // * if it's kTableFile, then it's shared
     // * if it's kDescriptorFile, limit the size to manifest_file_size
-    s = BackupFile(new_backup_id,
-                   new_backup.get(),
-                   options_.share_table_files && type == kTableFile,
-                   db->GetName(),            /* src_dir */
-                   live_files[i],            /* src_fname */
-                   rate_limiter.get(),
-                   (type == kDescriptorFile) ? manifest_file_size : 0,
-                   options_.share_files_with_checksum && type == kTableFile);
-  }
-
-  // copy WAL files
+    s = AddBackupFileWorkItem(
+            live_dst_paths,
+            backup_items_to_finish,
+            new_backup_id,
+            options_.share_table_files && type == kTableFile,
+            db->GetName(),
+            live_files[i],
+            rate_limiter.get(),
+            (type == kDescriptorFile) ? manifest_file_size : 0,
+            options_.share_files_with_checksum && type == kTableFile);
+  }
+  // Add a CopyWorkItem to the channel for each WAL file
   for (size_t i = 0; s.ok() && i < live_wal_files.size(); ++i) {
     if (live_wal_files[i]->Type() == kAliveLogFile) {
       // we only care about live log files
       // copy the file into backup_dir/files/<new backup>/
-      s = BackupFile(new_backup_id,
-                     new_backup.get(),
-                     false, /* not shared */
-                     db->GetOptions().wal_dir,
-                     live_wal_files[i]->PathName(),
-                     rate_limiter.get());
+      s = AddBackupFileWorkItem(live_dst_paths,
+                                backup_items_to_finish,
+                                new_backup_id,
+                                false, /* not shared */
+                                db->GetOptions().wal_dir,
+                                live_wal_files[i]->PathName(),
+                                rate_limiter.get());
+    }
+  }
+
+  Status item_status;
+  for (auto& item : backup_items_to_finish) {
+    item.result.wait();
+    auto result = item.result.get();
+    item_status = result.status;
+    if (item_status.ok() && item.shared && item.needed_to_copy) {
+      item_status = item.backup_env->RenameFile(item.dst_path_tmp,
+                                                item.dst_path);
+    }
+    if (item_status.ok()) {
+      item_status = new_backup.get()->AddFile(
+              std::make_shared<FileInfo>(item.dst_relative,
+                                         result.size,
+                                         result.checksum_value));
+    }
+    if (!item_status.ok()) {
+      s = item_status;
     }
   }
 
@@ -737,6 +877,11 @@ BackupEngineImpl::GetCorruptedBackups(
 Status BackupEngineImpl::RestoreDBFromBackup(
     BackupID backup_id, const std::string& db_dir, const std::string& wal_dir,
     const RestoreOptions& restore_options) {
+  if (options_.max_background_operations > 1 &&
+          options_.restore_rate_limit != 0) {
+    return Status::InvalidArgument(
+        "Multi-threaded restores cannot use a restore_rate_limit");
+  }
   auto corrupt_itr = corrupt_backups_.find(backup_id);
   if (corrupt_itr != corrupt_backups_.end()) {
     return corrupt_itr->second.first;
@@ -794,6 +939,7 @@ Status BackupEngineImpl::RestoreDBFromBackup(
           options_.restore_rate_limit, copy_file_buffer_size_));
   }
   Status s;
+  std::vector<RestoreAfterCopyWorkItem> restore_items_to_finish;
   for (const auto& file_info : backup->GetFiles()) {
     const std::string &file = file_info->filename;
     std::string dst;
@@ -823,14 +969,30 @@ Status BackupEngineImpl::RestoreDBFromBackup(
       "/" + dst;
 
     Log(options_.info_log, "Restoring %s to %s\n", file.c_str(), dst.c_str());
-    uint32_t checksum_value;
-    s = CopyFile(GetAbsolutePath(file), dst, backup_env_, db_env_, false,
-                 rate_limiter.get(), nullptr /* size */, &checksum_value);
-    if (!s.ok()) {
+    CopyWorkItem copy_work_item(GetAbsolutePath(file),
+                                dst,
+                                backup_env_,
+                                db_env_,
+                                false,
+                                rate_limiter.get(),
+                                0 /* size_limit */);
+    RestoreAfterCopyWorkItem after_copy_work_item(
+            copy_work_item.result.get_future(),
+            file_info->checksum_value);
+    files_to_copy_.write(std::move(copy_work_item));
+    restore_items_to_finish.push_back(std::move(after_copy_work_item));
+  }
+  Status item_status;
+  for (auto& item : restore_items_to_finish) {
+    item.result.wait();
+    auto result = item.result.get();
+    item_status = result.status;
+    // Note: It is possible that both of the following bad-status cases occur
+    // during copying. But, we only return one status.
+    if (!item_status.ok()) {
+      s = item_status;
       break;
-    }
-
-    if (file_info->checksum_value != checksum_value) {
+    } else if (item.checksum_value != result.checksum_value) {
       s = Status::Corruption("Checksum check failed");
       break;
     }
@@ -972,13 +1134,16 @@ Status BackupEngineImpl::CopyFile(
 }
 
 // src_fname will always start with "/"
-Status BackupEngineImpl::BackupFile(BackupID backup_id, BackupMeta* backup,
-                                    bool shared, const std::string& src_dir,
-                                    const std::string& src_fname,
-                                    BackupRateLimiter* rate_limiter,
-                                    uint64_t size_limit,
-                                    bool shared_checksum) {
-
+Status BackupEngineImpl::AddBackupFileWorkItem(
+        std::unordered_set<std::string>& live_dst_paths,
+        std::vector<BackupAfterCopyWorkItem>& backup_items_to_finish,
+        BackupID backup_id,
+        bool shared,
+        const std::string& src_dir,
+        const std::string& src_fname,
+        BackupRateLimiter* rate_limiter,
+        uint64_t size_limit,
+        bool shared_checksum) {
   assert(src_fname.size() > 0 && src_fname[0] == '/');
   std::string dst_relative = src_fname.substr(1);
   std::string dst_relative_tmp;
@@ -1012,17 +1177,20 @@ Status BackupEngineImpl::BackupFile(BackupID backup_id, BackupMeta* backup,
   std::string dst_path = GetAbsolutePath(dst_relative);
   std::string dst_path_tmp = GetAbsolutePath(dst_relative_tmp);
 
-  // if it's shared, we also need to check if it exists -- if it does,
-  // no need to copy it again
+  // if it's shared, we also need to check if it exists -- if it does, no need
+  // to copy it again.
   bool need_to_copy = true;
-  if (shared && backup_env_->FileExists(dst_path)) {
+  // true if dst_path is the same path as another live file
+  const bool same_path =
+      live_dst_paths.find(dst_path) != live_dst_paths.end();
+  if (shared && (backup_env_->FileExists(dst_path) || same_path)) {
     need_to_copy = false;
     if (shared_checksum) {
       Log(options_.info_log,
           "%s already present, with checksum %u and size %" PRIu64,
           src_fname.c_str(), checksum_value, size);
     } else if (backuped_file_infos_.find(dst_relative) ==
-               backuped_file_infos_.end()) {
+               backuped_file_infos_.end() && !same_path) {
       // file already exists, but it's not referenced by any backup. overwrite
       // the file
       Log(options_.info_log,
@@ -1040,25 +1208,44 @@ Status BackupEngineImpl::BackupFile(BackupID backup_id, BackupMeta* backup,
                             &checksum_value);
     }
   }
+  live_dst_paths.insert(dst_path);
+
   if (need_to_copy) {
     Log(options_.info_log, "Copying %s to %s", src_fname.c_str(),
-        dst_path_tmp.c_str());
-    s = CopyFile(src_dir + src_fname,
-                 dst_path_tmp,
-                 db_env_,
-                 backup_env_,
-                 options_.sync,
-                 rate_limiter,
-                 &size,
-                 &checksum_value,
-                 size_limit);
-    if (s.ok() && shared) {
-      s = backup_env_->RenameFile(dst_path_tmp, dst_path);
-    }
-  }
-  if (s.ok()) {
-    s = backup->AddFile(std::make_shared<FileInfo>(
-          dst_relative, size, checksum_value));
+            dst_path_tmp.c_str());
+    CopyWorkItem copy_work_item(src_dir + src_fname,
+                                dst_path_tmp,
+                                db_env_,
+                                backup_env_,
+                                options_.sync,
+                                rate_limiter,
+                                size_limit);
+    BackupAfterCopyWorkItem after_copy_work_item(
+            copy_work_item.result.get_future(),
+            shared,
+            need_to_copy,
+            backup_env_,
+            dst_path_tmp,
+            dst_path,
+            dst_relative);
+    files_to_copy_.write(std::move(copy_work_item));
+    backup_items_to_finish.push_back(std::move(after_copy_work_item));
+  } else {
+    std::promise<CopyResult> promise_result;
+    BackupAfterCopyWorkItem after_copy_work_item(
+            promise_result.get_future(),
+            shared,
+            need_to_copy,
+            backup_env_,
+            dst_path_tmp,
+            dst_path,
+            dst_relative);
+    backup_items_to_finish.push_back(std::move(after_copy_work_item));
+    CopyResult result;
+    result.status = s;
+    result.size = size;
+    result.checksum_value = checksum_value;
+    promise_result.set_value(std::move(result));
   }
   return s;
 }

utilities/backupable/backupable_db_test.cc

@@ -21,6 +21,7 @@
 #include "util/string_util.h"
 #include "util/testutil.h"
 #include "util/auto_roll_logger.h"
+#include "util/mock_env.h"
 
 namespace rocksdb {
 
@@ -361,6 +362,7 @@ class BackupableDBTest : public testing::Test {
 
     // set up envs
     env_ = Env::Default();
+    mock_env_.reset(new MockEnv(env_));
     test_db_env_.reset(new TestEnv(env_));
     test_backup_env_.reset(new TestEnv(env_));
     file_manager_.reset(new FileManager(env_));
@@ -377,6 +379,9 @@ class BackupableDBTest : public testing::Test {
     backupable_options_.reset(new BackupableDBOptions(
         backupdir_, test_backup_env_.get(), true, logger_.get(), true));
 
+    // most tests will use multi-threaded backups
+    backupable_options_->max_background_operations = 7;
+
     // delete old files in db
     DestroyDB(dbname_, Options());
   }
@@ -474,6 +479,7 @@ class BackupableDBTest : public testing::Test {
 
   // envs
   Env* env_;
+  unique_ptr<MockEnv> mock_env_;
   unique_ptr<TestEnv> test_db_env_;
   unique_ptr<TestEnv> test_backup_env_;
   unique_ptr<FileManager> file_manager_;
@@ -555,6 +561,30 @@ TEST_F(BackupableDBTest, NoDoubleCopy) {
   CloseBackupableDB();
 }
 
+// Verify that backup works when the database environment is not the same as
+// the backup environment
+// TODO(agf): Make all/most tests use different db and backup environments.
+//            This will probably require more implementation of MockEnv.
+//            For example, MockEnv::RenameFile() must be able to rename
+//            directories.
+TEST_F(BackupableDBTest, DifferentEnvs) {
+  test_db_env_.reset(new TestEnv(mock_env_.get()));
+  options_.env = test_db_env_.get();
+
+  OpenBackupableDB(true, true);
+
+  // should write 5 DB files + LATEST_BACKUP + one meta file
+  test_backup_env_->SetLimitWrittenFiles(7);
+  test_backup_env_->ClearWrittenFiles();
+  test_db_env_->SetLimitWrittenFiles(0);
+  dummy_db_->live_files_ = { "/00010.sst", "/00011.sst",
+                             "/CURRENT",   "/MANIFEST-01" };
+  dummy_db_->wal_files_ = {{"/00011.log", true}, {"/00012.log", false}};
+  ASSERT_OK(db_->CreateNewBackup(false));
+
+  CloseBackupableDB();
+}
+
 // test various kind of corruptions that may happen:
 // 1. Not able to write a file for backup - that backup should fail,
 //      everything else should work
@@ -966,6 +996,8 @@ TEST_F(BackupableDBTest, RateLimiting) {
 
     backupable_options_->backup_rate_limit = limit.first;
     backupable_options_->restore_rate_limit = limit.second;
+    // rate-limiting backups must be single-threaded
+    backupable_options_->max_background_operations = 1;
     options_.compression = kNoCompression;
     OpenBackupableDB(true);
     size_t bytes_written = FillDB(db_.get(), 0, 100000);