diff --git a/db/compaction.cc b/db/compaction.cc
index 768306ed4..5c382d163 100644
--- a/db/compaction.cc
+++ b/db/compaction.cc
@@ -415,14 +415,15 @@ void Compaction::Summary(char* output, int len) {
 uint64_t Compaction::OutputFilePreallocationSize() const {
   uint64_t preallocation_size = 0;
 
-  if (cfd_->ioptions()->compaction_style == kCompactionStyleLevel ||
-      output_level() > 0) {
+  if (max_output_file_size_ != port::kMaxUint64 &&
+      (cfd_->ioptions()->compaction_style == kCompactionStyleLevel ||
+       output_level() > 0)) {
     preallocation_size = max_output_file_size_;
   } else {
-    // output_level() == 0
-    assert(num_input_levels() > 0);
-    for (const auto& f : inputs_[0].files) {
-      preallocation_size += f->fd.GetFileSize();
+    for (const auto& level_files : inputs_) {
+      for (const auto& file : level_files.files) {
+        preallocation_size += file->fd.GetFileSize();
+      }
     }
   }
   // Over-estimate slightly so we don't end up just barely crossing
diff --git a/db/db_compaction_test.cc b/db/db_compaction_test.cc
index cc0b01e01..e6d820ad4 100644
--- a/db/db_compaction_test.cc
+++ b/db/db_compaction_test.cc
@@ -780,6 +780,41 @@ TEST_F(DBCompactionTest, ZeroSeqIdCompaction) {
   ASSERT_OK(Put("", ""));
 }
 
+TEST_F(DBCompactionTest, ManualCompactionUnknownOutputSize) {
+  // github issue #2249
+  Options options = CurrentOptions();
+  options.compaction_style = kCompactionStyleLevel;
+  options.level0_file_num_compaction_trigger = 3;
+  DestroyAndReopen(options);
+
+  // create two files in l1 that we can compact
+  for (int i = 0; i < 2; ++i) {
+    for (int j = 0; j < options.level0_file_num_compaction_trigger; j++) {
+      // make l0 files' ranges overlap to avoid trivial move
+      Put(std::to_string(2 * i), std::string(1, 'A'));
+      Put(std::to_string(2 * i + 1), std::string(1, 'A'));
+      Flush();
+      dbfull()->TEST_WaitForFlushMemTable();
+    }
+    dbfull()->TEST_WaitForCompact();
+    ASSERT_EQ(NumTableFilesAtLevel(0, 0), 0);
+    ASSERT_EQ(NumTableFilesAtLevel(1, 0), i + 1);
+  }
+
+  ColumnFamilyMetaData cf_meta;
+  dbfull()->GetColumnFamilyMetaData(dbfull()->DefaultColumnFamily(), &cf_meta);
+  ASSERT_EQ(2, cf_meta.levels[1].files.size());
+  std::vector<std::string> input_filenames;
+  for (const auto& sst_file : cf_meta.levels[1].files) {
+    input_filenames.push_back(sst_file.name);
+  }
+
+  // note CompactionOptions::output_file_size_limit is unset.
+  CompactionOptions compact_opt;
+  compact_opt.compression = kNoCompression;
+  dbfull()->CompactFiles(compact_opt, input_filenames, 1);
+}
+
 // Check that writes done during a memtable compaction are recovered
 // if the database is shutdown during the memtable compaction.
 TEST_F(DBCompactionTest, RecoverDuringMemtableCompaction) {
diff --git a/db/db_test_util.h b/db/db_test_util.h
index 65e983a50..03cb4f245 100644
--- a/db/db_test_util.h
+++ b/db/db_test_util.h
@@ -277,6 +277,9 @@ class SpecialEnv : public EnvWrapper {
       bool use_direct_io() const override {
         return base_->use_direct_io();
       }
+      Status Allocate(uint64_t offset, uint64_t len) override {
+        return base_->Allocate(offset, len);
+      }
     };
     class ManifestFile : public WritableFile {
      public:
diff --git a/include/rocksdb/env.h b/include/rocksdb/env.h
index a2fde4733..9af0261fa 100644
--- a/include/rocksdb/env.h
+++ b/include/rocksdb/env.h
@@ -703,14 +703,12 @@ class WritableFile {
     }
   }
 
- protected:
-  /*
-   * Pre-allocate space for a file.
-   */
+  // Pre-allocates space for a file.
   virtual Status Allocate(uint64_t offset, uint64_t len) {
     return Status::OK();
   }
 
+ protected:
   size_t preallocation_block_size() { return preallocation_block_size_; }
 
  private: