FindObsoleteFiles() to directly check whether candidate files are live (#10040)

Summary:
Right now, in FindObsoleteFiles() we build a list of all live SST files from all existing Versions. This is all done in DB mutex, and is O(m*n) where m is number of versions and n is number of files. In some extereme cases, it can take very long. The list is used to see whether a candidate file still shows up in a version. With this commit, every candidate file is directly check against all the versions for file existance. This operation would be O(m*k) where k is number of candidate files. Since is usually small (except perhaps full compaction in universal compaction), this is usually much faster than the previous solution.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10040

Test Plan: TBD

Reviewed By: riversand963

Differential Revision: D36613391

fbshipit-source-id: 3f13b090f755d9b3ae417faec62cd6e798bac1eb

HISTORY.md

@@ -49,6 +49,7 @@
 
 ### Performance Improvements
 * Avoid calling malloc_usable_size() in LRU Cache's mutex.
+* Reduce DB mutex holding time when finding obsolete files to delete. When a file is trivial moved to another level, the internal files will be referenced twice internally and sometimes opened twice too. If a deletion candidate file is not the last reference, we need to destroy the reference and close the file but not deleting the file. Right now we determine it by building a set of all live files. With the improvement, we check the file against all live LSM-tree versions instead.
 
 ## 7.2.0 (04/15/2022)
 ### Bug Fixes

db/db_impl/db_impl_files.cc

@@ -166,8 +166,8 @@ void DBImpl::FindObsoleteFiles(JobContext* job_context, bool force,
   job_context->log_number = MinLogNumberToKeep();
   job_context->prev_log_number = versions_->prev_log_number();
 
-  versions_->AddLiveFiles(&job_context->sst_live, &job_context->blob_live);
   if (doing_the_full_scan) {
+    versions_->AddLiveFiles(&job_context->sst_live, &job_context->blob_live);
     InfoLogPrefix info_log_prefix(!immutable_db_options_.db_log_dir.empty(),
                                   dbname_);
     std::set<std::string> paths;
@@ -242,6 +242,14 @@ void DBImpl::FindObsoleteFiles(JobContext* job_context, bool force,
             log_file, immutable_db_options_.db_log_dir);
       }
     }
+  } else {
+    // Instead of filling ob_context->sst_live and job_context->blob_live,
+    // directly remove files that show up in any Version. This is because
+    // candidate files tend to be a small percentage of all files, so it is
+    // usually cheaper to check them against every version, compared to
+    // building a map for all files.
+    versions_->RemoveLiveFiles(job_context->sst_delete_files,
+                               job_context->blob_delete_files);
   }
 
   // logs_ is empty when called during recovery, in which case there can't yet
@@ -395,8 +403,10 @@ void DBImpl::PurgeObsoleteFiles(JobContext& state, bool schedule_only) {
       state.manifest_delete_files.size());
   // We may ignore the dbname when generating the file names.
   for (auto& file : state.sst_delete_files) {
+    if (!file.only_delete_metadata) {
       candidate_files.emplace_back(
           MakeTableFileName(file.metadata->fd.GetNumber()), file.path);
+    }
     if (file.metadata->table_reader_handle) {
       table_cache_->Release(file.metadata->table_reader_handle);
     }

db/version_set.cc

@@ -3941,6 +3941,26 @@ void Version::AddLiveFiles(std::vector<uint64_t>* live_table_files,
   }
 }
 
+void Version::RemoveLiveFiles(
+    std::vector<ObsoleteFileInfo>& sst_delete_candidates,
+    std::vector<ObsoleteBlobFileInfo>& blob_delete_candidates) const {
+  for (ObsoleteFileInfo& fi : sst_delete_candidates) {
+    if (!fi.only_delete_metadata &&
+        storage_info()->GetFileLocation(fi.metadata->fd.GetNumber()) !=
+            VersionStorageInfo::FileLocation::Invalid()) {
+      fi.only_delete_metadata = true;
+    }
+  }
+
+  blob_delete_candidates.erase(
+      std::remove_if(
+          blob_delete_candidates.begin(), blob_delete_candidates.end(),
+          [this](ObsoleteBlobFileInfo& x) {
+            return storage_info()->GetBlobFileMetaData(x.GetBlobFileNumber());
+          }),
+      blob_delete_candidates.end());
+}
+
 std::string Version::DebugString(bool hex, bool print_stats) const {
   std::string r;
   for (int level = 0; level < storage_info_.num_levels_; level++) {
@@ -5745,6 +5765,38 @@ uint64_t VersionSet::ApproximateSize(Version* v, const FdWithKeyRange& f,
       v->GetMutableCFOptions().prefix_extractor);
 }
 
+void VersionSet::RemoveLiveFiles(
+    std::vector<ObsoleteFileInfo>& sst_delete_candidates,
+    std::vector<ObsoleteBlobFileInfo>& blob_delete_candidates) const {
+  assert(column_family_set_);
+  for (auto cfd : *column_family_set_) {
+    assert(cfd);
+    if (!cfd->initialized()) {
+      continue;
+    }
+
+    auto* current = cfd->current();
+    bool found_current = false;
+
+    Version* const dummy_versions = cfd->dummy_versions();
+    assert(dummy_versions);
+
+    for (Version* v = dummy_versions->next_; v != dummy_versions;
+         v = v->next_) {
+      v->RemoveLiveFiles(sst_delete_candidates, blob_delete_candidates);
+      if (v == current) {
+        found_current = true;
+      }
+    }
+
+    if (!found_current && current != nullptr) {
+      // Should never happen unless it is a bug.
+      assert(false);
+      current->RemoveLiveFiles(sst_delete_candidates, blob_delete_candidates);
+    }
+  }
+}
+
 void VersionSet::AddLiveFiles(std::vector<uint64_t>* live_table_files,
                               std::vector<uint64_t>* live_blob_files) const {
   assert(live_table_files);

db/version_set.h

@@ -688,6 +688,53 @@ class VersionStorageInfo {
   friend class VersionSet;
 };
 
+struct ObsoleteFileInfo {
+  FileMetaData* metadata;
+  std::string path;
+  // If true, the FileMataData should be destroyed but the file should
+  // not be deleted. This is because another FileMetaData still references
+  // the file, usually because the file is trivial moved so two FileMetadata
+  // is managing the file.
+  bool only_delete_metadata = false;
+
+  ObsoleteFileInfo() noexcept
+      : metadata(nullptr), only_delete_metadata(false) {}
+  ObsoleteFileInfo(FileMetaData* f, const std::string& file_path)
+      : metadata(f), path(file_path), only_delete_metadata(false) {}
+
+  ObsoleteFileInfo(const ObsoleteFileInfo&) = delete;
+  ObsoleteFileInfo& operator=(const ObsoleteFileInfo&) = delete;
+
+  ObsoleteFileInfo(ObsoleteFileInfo&& rhs) noexcept : ObsoleteFileInfo() {
+    *this = std::move(rhs);
+  }
+
+  ObsoleteFileInfo& operator=(ObsoleteFileInfo&& rhs) noexcept {
+    path = std::move(rhs.path);
+    metadata = rhs.metadata;
+    rhs.metadata = nullptr;
+
+    return *this;
+  }
+  void DeleteMetadata() {
+    delete metadata;
+    metadata = nullptr;
+  }
+};
+
+class ObsoleteBlobFileInfo {
+ public:
+  ObsoleteBlobFileInfo(uint64_t blob_file_number, std::string path)
+      : blob_file_number_(blob_file_number), path_(std::move(path)) {}
+
+  uint64_t GetBlobFileNumber() const { return blob_file_number_; }
+  const std::string& GetPath() const { return path_; }
+
+ private:
+  uint64_t blob_file_number_;
+  std::string path_;
+};
+
 using MultiGetRange = MultiGetContext::Range;
 // A column family's version consists of the table and blob files owned by
 // the column family at a certain point in time.
@@ -789,6 +836,11 @@ class Version {
   void AddLiveFiles(std::vector<uint64_t>* live_table_files,
                     std::vector<uint64_t>* live_blob_files) const;
 
+  // Remove live files that are in the delete candidate lists.
+  void RemoveLiveFiles(
+      std::vector<ObsoleteFileInfo>& sst_delete_candidates,
+      std::vector<ObsoleteBlobFileInfo>& blob_delete_candidates) const;
+
   // Return a human readable string that describes this version's contents.
   std::string DebugString(bool hex = false, bool print_stats = false) const;
 
@@ -927,49 +979,6 @@ class Version {
   void operator=(const Version&) = delete;
 };
 
-struct ObsoleteFileInfo {
-  FileMetaData* metadata;
-  std::string   path;
-
-  ObsoleteFileInfo() noexcept : metadata(nullptr) {}
-  ObsoleteFileInfo(FileMetaData* f, const std::string& file_path)
-      : metadata(f), path(file_path) {}
-
-  ObsoleteFileInfo(const ObsoleteFileInfo&) = delete;
-  ObsoleteFileInfo& operator=(const ObsoleteFileInfo&) = delete;
-
-  ObsoleteFileInfo(ObsoleteFileInfo&& rhs) noexcept :
-    ObsoleteFileInfo() {
-      *this = std::move(rhs);
-  }
-
-  ObsoleteFileInfo& operator=(ObsoleteFileInfo&& rhs) noexcept {
-    path = std::move(rhs.path);
-    metadata = rhs.metadata;
-    rhs.metadata = nullptr;
-
-    return *this;
-  }
-
-  void DeleteMetadata() {
-    delete metadata;
-    metadata = nullptr;
-  }
-};
-
-class ObsoleteBlobFileInfo {
- public:
-  ObsoleteBlobFileInfo(uint64_t blob_file_number, std::string path)
-      : blob_file_number_(blob_file_number), path_(std::move(path)) {}
-
-  uint64_t GetBlobFileNumber() const { return blob_file_number_; }
-  const std::string& GetPath() const { return path_; }
-
- private:
-  uint64_t blob_file_number_;
-  std::string path_;
-};
-
 class BaseReferencedVersionBuilder;
 
 class AtomicGroupReadBuffer {
@@ -1287,6 +1296,11 @@ class VersionSet {
   void AddLiveFiles(std::vector<uint64_t>* live_table_files,
                     std::vector<uint64_t>* live_blob_files) const;
 
+  // Remove live files that are in the delete candidate lists.
+  void RemoveLiveFiles(
+      std::vector<ObsoleteFileInfo>& sst_delete_candidates,
+      std::vector<ObsoleteBlobFileInfo>& blob_delete_candidates) const;
+
   // Return the approximate size of data to be scanned for range [start, end)
   // in levels [start_level, end_level). If end_level == -1 it will search
   // through all non-empty levels