diff --git a/HISTORY.md b/HISTORY.md
index 5b0ae1bc2..725013df8 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -8,6 +8,9 @@
 * Fixed a bug in CompactionIterator when write-prepared transaction is used. A released earliest write conflict snapshot may cause assertion failure in dbg mode and unexpected key in opt mode.
 * Fix ticker WRITE_WITH_WAL("rocksdb.write.wal"), this bug is caused by a bad extra `RecordTick(stats_, WRITE_WITH_WAL)` (at 2 place), this fix remove the extra `RecordTick`s and fix the corresponding test case.
 
+### Behavior Changes
+* `NUM_FILES_IN_SINGLE_COMPACTION` was only counting the first input level files, now it's including all input files.
+
 ## 6.26.0 (2021-10-20)
 ### Bug Fixes
 * Fixes a bug in directed IO mode when calling MultiGet() for blobs in the same blob file. The bug is caused by not sorting the blob read requests by file offsets.
diff --git a/db/db_impl/db_impl_compaction_flush.cc b/db/db_impl/db_impl_compaction_flush.cc
index 6b23f926f..6d7b0478a 100644
--- a/db/db_impl/db_impl_compaction_flush.cc
+++ b/db/db_impl/db_impl_compaction_flush.cc
@@ -3076,8 +3076,12 @@ Status DBImpl::BackgroundCompaction(bool* made_progress,
           status = Status::CompactionTooLarge();
         } else {
           // update statistics
-          RecordInHistogram(stats_, NUM_FILES_IN_SINGLE_COMPACTION,
-                            c->inputs(0)->size());
+          size_t num_files = 0;
+          for (auto& each_level : *c->inputs()) {
+            num_files += each_level.files.size();
+          }
+          RecordInHistogram(stats_, NUM_FILES_IN_SINGLE_COMPACTION, num_files);
+
           // There are three things that can change compaction score:
           // 1) When flush or compaction finish. This case is covered by
           // InstallSuperVersionAndScheduleWork