Do not full scan obsolete files on compaction busy (#7739)

Summary:
When ConcurrentTaskLimiter is enabled and there are too many outstanding compactions, BackgroundCompaction returns Status::Busy(), which shouldn't be treat as compaction failure.
This caused performance issue when outstanding compactions reached the limit.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7739

Reviewed By: cheng-chang

Differential Revision: D25508319

Pulled By: ltamasi

fbshipit-source-id: 3b181b16ada0ca3393cfa3a7412985764e79c719

HISTORY.md

@@ -6,6 +6,7 @@
 ### Bug Fixes
 * Truncated WALs ending in incomplete records can no longer produce gaps in the recovered data when `WALRecoveryMode::kPointInTimeRecovery` is used. Gaps are still possible when WALs are truncated exactly on record boundaries; for complete protection, users should enable `track_and_verify_wals_in_manifest`.
 * Fix a bug where compressed blocks read by MultiGet are not inserted into the compressed block cache when use_direct_reads = true.
+* Fixed the issue of full scanning on obsolete files when there are too many outstanding compactions with ConcurrentTaskLimiter enabled.
 
 ### Bug Fixes
 * Fixed the logic of populating native data structure for `read_amp_bytes_per_bit` during OPTIONS file parsing on big-endian architecture. Without this fix, original code introduced in PR7659, when running on big-endian machine, can mistakenly store read_amp_bytes_per_bit (an uint32) in little endian format. Future access to `read_amp_bytes_per_bit` will give wrong values. Little endian architecture is not affected.

db/db_impl/db_impl_compaction_flush.cc

@@ -2626,7 +2626,8 @@ void DBImpl::BackgroundCallCompaction(PrepickedCompaction* prepicked_compaction,
     // failure). Thus, we force full scan in FindObsoleteFiles()
     FindObsoleteFiles(&job_context, !s.ok() && !s.IsShutdownInProgress() &&
                                         !s.IsManualCompactionPaused() &&
-                                        !s.IsColumnFamilyDropped());
+                                        !s.IsColumnFamilyDropped() &&
+                                        !s.IsBusy());
     TEST_SYNC_POINT("DBImpl::BackgroundCallCompaction:FoundObsoleteFiles");
 
     // delete unnecessary files if any, this is done outside the mutex