diff --git a/CMakeLists.txt b/CMakeLists.txt
index 79d81b6b..d7315e87 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -598,6 +598,10 @@ if(WITH_TESTS)
   target_link_libraries(dwarfs_utils_test gtest gtest_main)
   list(APPEND TEST_TARGETS dwarfs_utils_test)
 
+  add_executable(block_merger_test test/block_merger_test.cpp)
+  target_link_libraries(block_merger_test gtest gtest_main)
+  list(APPEND TEST_TARGETS block_merger_test)
+
   add_executable(dwarfs_pcm_sample_transformer_test test/pcm_sample_transformer_test.cpp)
   target_link_libraries(dwarfs_pcm_sample_transformer_test gtest gtest_main)
   list(APPEND TEST_TARGETS dwarfs_pcm_sample_transformer_test)
diff --git a/include/dwarfs/block_merger.h b/include/dwarfs/block_merger.h
new file mode 100644
index 00000000..27c92b0c
--- /dev/null
+++ b/include/dwarfs/block_merger.h
@@ -0,0 +1,37 @@
+/* vim:set ts=2 sw=2 sts=2 et: */
+/**
+ * \author     Marcus Holland-Moritz (github@mhxnet.de)
+ * \copyright  Copyright (c) Marcus Holland-Moritz
+ *
+ * This file is part of dwarfs.
+ *
+ * dwarfs is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * dwarfs is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with dwarfs.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+namespace dwarfs {
+
+template <typename SourceT, typename BlockT>
+class block_merger {
+ public:
+  using source_type = SourceT;
+  using block_type = BlockT;
+
+  virtual ~block_merger() = default;
+
+  virtual void add(source_type src, block_type blk, bool is_last) = 0;
+};
+
+} // namespace dwarfs
diff --git a/include/dwarfs/multi_queue_block_merger.h b/include/dwarfs/multi_queue_block_merger.h
new file mode 100644
index 00000000..0f8e7ab6
--- /dev/null
+++ b/include/dwarfs/multi_queue_block_merger.h
@@ -0,0 +1,143 @@
+/* vim:set ts=2 sw=2 sts=2 et: */
+/**
+ * \author     Marcus Holland-Moritz (github@mhxnet.de)
+ * \copyright  Copyright (c) Marcus Holland-Moritz
+ *
+ * This file is part of dwarfs.
+ *
+ * dwarfs is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * dwarfs is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with dwarfs.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <algorithm>
+#include <cassert>
+#include <condition_variable>
+#include <functional>
+#include <mutex>
+#include <optional>
+#include <queue>
+#include <unordered_map>
+#include <vector>
+
+#include "dwarfs/block_merger.h"
+
+namespace dwarfs {
+
+template <typename SourceT, typename BlockT>
+class multi_queue_block_merger : public block_merger<SourceT, BlockT> {
+ public:
+  using source_type = SourceT;
+  using block_type = BlockT;
+
+  multi_queue_block_merger(size_t num_active_slots, size_t max_queued_blocks,
+                           std::vector<source_type> const& sources,
+                           std::function<void(block_type)> on_block_merged)
+      : num_queueable_{max_queued_blocks}
+      , sources_{sources.begin(), sources.end()}
+      , active_(num_active_slots)
+      , on_block_merged_{on_block_merged} {
+    for (size_t i = 0; i < active_.size() && !sources_.empty(); ++i) {
+      active_[i] = sources_.front();
+      sources_.pop_front();
+    }
+  }
+
+  void add(source_type src, block_type blk, bool is_last) override {
+    std::unique_lock lock{mx_};
+
+    cv_.wait(lock,
+             [this, &src] { return source_distance(src) < num_queueable_; });
+
+    --num_queueable_;
+
+    queues_[src].emplace(std::move(blk), is_last);
+
+    while (try_merge_block()) {
+    }
+
+    cv_.notify_all();
+  }
+
+ private:
+  size_t source_distance(source_type src) const {
+    auto ix = active_index_;
+    size_t distance{0};
+
+    while (active_[ix] && active_[ix].value() != src) {
+      ++distance;
+      ix = (ix + 1) % active_.size();
+
+      if (ix == active_index_) {
+        auto it = std::find(begin(sources_), end(sources_), src);
+        distance += std::distance(begin(sources_), it);
+        break;
+      }
+    }
+
+    return distance;
+  }
+
+  bool try_merge_block() {
+    auto const ix = active_index_;
+
+    assert(active_[ix]);
+
+    auto src = active_[ix].value();
+    auto it = queues_.find(src);
+
+    if (it == queues_.end() || it->second.empty()) {
+      return false;
+    }
+
+    auto [blk, is_last] = std::move(it->second.front());
+    it->second.pop();
+
+    on_block_merged_(std::move(blk));
+
+    ++num_queueable_;
+
+    if (is_last) {
+      queues_.erase(it);
+      update_active(ix);
+    }
+
+    do {
+      active_index_ = (active_index_ + 1) % active_.size();
+    } while (active_index_ != ix && !active_[active_index_]);
+
+    return active_index_ != ix || active_[active_index_];
+  }
+
+  void update_active(size_t ix) {
+    if (!sources_.empty()) {
+      active_[ix] = sources_.front();
+      sources_.pop_front();
+    } else {
+      active_[ix].reset();
+    }
+  }
+
+  std::mutex mx_;
+  std::condition_variable cv_;
+  size_t active_index_{0};
+  size_t num_queueable_;
+  std::unordered_map<source_type, std::queue<std::pair<block_type, bool>>>
+      queues_;
+  std::deque<source_type> sources_;
+  std::vector<std::optional<source_type>> active_;
+  std::function<void(block_type)> on_block_merged_;
+};
+
+} // namespace dwarfs
diff --git a/test/block_merger_test.cpp b/test/block_merger_test.cpp
new file mode 100644
index 00000000..20c32c56
--- /dev/null
+++ b/test/block_merger_test.cpp
@@ -0,0 +1,272 @@
+/* vim:set ts=2 sw=2 sts=2 et: */
+/**
+ * \author     Marcus Holland-Moritz (github@mhxnet.de)
+ * \copyright  Copyright (c) Marcus Holland-Moritz
+ *
+ * This file is part of dwarfs.
+ *
+ * dwarfs is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * dwarfs is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with dwarfs.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include <gtest/gtest.h>
+
+#include <atomic>
+#include <mutex>
+#include <numeric>
+#include <queue>
+#include <random>
+#include <shared_mutex>
+#include <thread>
+#include <tuple>
+#include <vector>
+
+#include <fmt/format.h>
+#include <folly/String.h>
+#include <folly/Synchronized.h>
+
+#include "dwarfs/multi_queue_block_merger.h"
+
+using namespace dwarfs;
+
+namespace {
+
+constexpr int const debuglevel{0};
+
+constexpr size_t const max_runs{250};
+constexpr size_t const num_runner_threads{16};
+constexpr size_t const num_repetitions{4};
+
+using block = std::pair<size_t, size_t>;
+
+// Use std::shared_mutex because folly::SharedMutex might trigger TSAN
+template <typename T>
+using synchronized = folly::Synchronized<T, std::shared_mutex>;
+
+template <typename T>
+using sync_queue = synchronized<std::queue<T>>;
+
+class source {
+ public:
+  source(size_t id, std::mt19937& delay_rng, std::mt19937& rng,
+         size_t max_blocks = 20, double ips = 5000.0)
+      : id_{id}
+      , blocks_{init_blocks(delay_rng, rng, max_blocks, ips)} {}
+
+  std::tuple<block, bool, double> next() {
+    auto idx = idx_++;
+    return {std::make_pair(id_, idx), idx_ >= blocks_.size(), blocks_[idx]};
+  }
+
+  size_t id() const { return id_; }
+
+  size_t num_blocks() const { return blocks_.size(); }
+
+  std::chrono::nanoseconds total_time() const {
+    auto seconds = std::accumulate(begin(blocks_), end(blocks_), 0.0);
+    return std::chrono::duration_cast<std::chrono::nanoseconds>(
+        std::chrono::duration<double>(seconds));
+  }
+
+ private:
+  static std::vector<double>
+  init_blocks(std::mt19937& delay_rng, std::mt19937& rng, size_t max_blocks,
+              double ips) {
+    std::uniform_int_distribution<> idist(1, max_blocks);
+    std::exponential_distribution<> edist(ips);
+    std::vector<double> blocks;
+    blocks.resize(idist(rng));
+    std::generate(begin(blocks), end(blocks), [&] { return edist(delay_rng); });
+    return blocks;
+  }
+
+  size_t idx_{0};
+  size_t id_;
+  std::vector<double> blocks_;
+};
+
+void emitter(sync_queue<source>& sources,
+             dwarfs::block_merger<size_t, block>& merger) {
+  for (;;) {
+    auto src = sources.withWLock([](auto&& q) {
+      std::optional<source> src;
+
+      if (!q.empty()) {
+        src = std::move(q.front());
+        q.pop();
+      }
+
+      return src;
+    });
+
+    if (!src) {
+      break;
+    }
+
+    auto t = std::chrono::steady_clock::now();
+
+    for (;;) {
+      auto [blk, is_last, wait] = src->next();
+
+      t += std::chrono::duration_cast<std::chrono::nanoseconds>(
+          std::chrono::duration<double>(wait));
+
+      std::this_thread::sleep_until(t);
+
+      merger.add(blk.first, blk, is_last);
+
+      if (is_last) {
+        break;
+      }
+    }
+  }
+}
+
+std::vector<block>
+do_run(std::mutex& out_mx, size_t run, std::mt19937& delay_rng) {
+  std::mt19937 rng(run);
+  std::exponential_distribution<> sources_dist(0.1);
+  std::exponential_distribution<> threads_dist(0.1);
+  std::exponential_distribution<> slots_dist(0.1);
+  std::exponential_distribution<> inflight_dist(0.1);
+  std::uniform_real_distribution<> speed_dist(0.1, 10.0);
+  auto const num_sources{std::max<size_t>(1, sources_dist(rng))};
+  auto const num_slots{std::max<size_t>(1, slots_dist(rng))};
+  auto const num_threads{std::max<size_t>(num_slots, threads_dist(delay_rng))};
+  auto const max_in_flight{std::max<size_t>(1, inflight_dist(delay_rng))};
+
+  std::vector<size_t> source_ids;
+  sync_queue<source> sources;
+  std::chrono::nanoseconds total_time{};
+
+  for (size_t i = 0; i < num_sources; ++i) {
+    auto src = source(i, delay_rng, rng, 30, 10000.0 * speed_dist(delay_rng));
+    total_time += src.total_time();
+    source_ids.emplace_back(src.id());
+    sources.wlock()->emplace(std::move(src));
+  }
+
+  auto config =
+      fmt::format("sources: {}, slots: {}, threads: {}, max in flight: {}",
+                  num_sources, num_slots, num_threads, max_in_flight);
+
+  if constexpr (debuglevel > 0) {
+    std::lock_guard lock{out_mx};
+    std::cout << config << "\n";
+  }
+
+  std::vector<block> merged;
+
+  dwarfs::multi_queue_block_merger<size_t, block> merger(
+      num_slots, max_in_flight, source_ids,
+      [&merged](block blk) { merged.emplace_back(std::move(blk)); });
+
+  std::vector<std::thread> thr;
+
+  auto t0 = std::chrono::steady_clock::now();
+
+  for (size_t i = 0; i < num_threads; ++i) {
+    thr.emplace_back([&] { emitter(sources, merger); });
+  }
+
+  for (auto& t : thr) {
+    t.join();
+  }
+
+  auto t1 = std::chrono::steady_clock::now();
+
+  auto elapsed = num_threads * (t1 - t0);
+  auto efficiency =
+      std::chrono::duration_cast<std::chrono::duration<double>>(total_time)
+          .count() /
+      std::chrono::duration_cast<std::chrono::duration<double>>(elapsed)
+          .count();
+
+  if constexpr (debuglevel > 0) {
+    std::lock_guard lock{out_mx};
+    std::cout << config
+              << fmt::format(" => efficiency: {:.2f}%\n", 100.0 * efficiency);
+  }
+
+  return merged;
+}
+
+[[maybe_unused]] void
+dump(std::mutex& out_mx, std::vector<block> const& blocks) {
+  if constexpr (debuglevel > 1) {
+    std::lock_guard lock{out_mx};
+    for (size_t i = 0; i < blocks.size(); ++i) {
+      if (i > 0) {
+        std::cout << ", ";
+      }
+      auto const& b = blocks[i];
+      std::cout << b.first << "." << b.second;
+    }
+    std::cout << "\n";
+  }
+}
+
+void runner_thread(size_t tid, std::mutex& out_mx, std::atomic<size_t>& runs,
+                   size_t const max_runs, std::atomic<size_t>& passes,
+                   synchronized<std::vector<size_t>>& fails) {
+  std::mt19937 delay_rng(tid);
+
+  for (;;) {
+    auto run = runs++;
+    if (run >= max_runs) {
+      break;
+    }
+    if constexpr (debuglevel > 0) {
+      std::lock_guard lock{out_mx};
+      std::cout << "[" << run << "/" << tid << "] ref\n";
+    }
+    auto ref = do_run(out_mx, run, delay_rng);
+    dump(out_mx, ref);
+    for (size_t rep = 0; rep < num_repetitions; ++rep) {
+      if constexpr (debuglevel > 0) {
+        std::lock_guard lock{out_mx};
+        std::cout << "[" << run << "/" << tid << "] test\n";
+      }
+      auto test = do_run(out_mx, run, delay_rng);
+      dump(out_mx, test);
+      if (test == ref) {
+        ++passes;
+      } else {
+        fails.wlock()->emplace_back(run);
+      }
+    }
+  }
+}
+
+} // namespace
+
+TEST(block_merger, random) {
+  std::mutex out_mx;
+  std::atomic<size_t> runs{0};
+  std::atomic<size_t> passes{0};
+  synchronized<std::vector<size_t>> fails;
+
+  std::vector<std::thread> thr;
+
+  for (size_t i = 0; i < num_runner_threads; ++i) {
+    thr.emplace_back(
+        [&, i] { runner_thread(i, out_mx, runs, max_runs, passes, fails); });
+  }
+
+  for (auto& t : thr) {
+    t.join();
+  }
+
+  EXPECT_EQ(max_runs * num_repetitions, passes);
+  EXPECT_TRUE(fails.rlock()->empty()) << folly::join(", ", *fails.rlock());
+}