RocksDB源码分析 VersionSet

MANIFEST

存储引擎状态的元数据持久化的文件

CURRENT:指向最新的MANIFEST文件

MANIFEST-<seq-no>

在RocksDB中任意时间存储引擎的状态都会保存为一个Version(也就是SST的集合)，而每次对Version的修改都是一个VersionEdit,而最终这些VersionEdit就是组成manifest-log文件的内容.

下面就是MANIFEST的log文件的基本构成:

version-edit      = Any RocksDB state change
version           = { version-edit* }
manifest-log-file = { version, version-edit* }
                  = { version-edit* }

VersionSet实现

VersionSet::LogAndApply

->VersionSet::ProcessManifestWrites

descriptor_log_ 表示了当前manifest-log文件的写入句柄

manifest_writers__表示需要写入到manifest-log文件中的writes的数组.

ManifestWriter的结构，包含了VersionEdit的数组，这个数组就是即将要写入到manifest文件中version_edit.

// this is used to batch writes to the manifest file
struct VersionSet::ManifestWriter {
  Status status;
  bool done;
  InstrumentedCondVar cv;
  ColumnFamilyData* cfd;
  const autovector<VersionEdit*>& edit_list;

  explicit ManifestWriter(InstrumentedMutex* mu, ColumnFamilyData* _cfd,
                          const autovector<VersionEdit*>& e)
      : done(false), cv(mu), cfd(_cfd), edit_list(e) {}
};

VersionSet更改（LogAndApply）

创建新的MANIFEST

//如果manifest的大小触发阈值则创建新的manifest

...
  assert(pending_manifest_file_number_ == 0);
  //如果manifest的大小触发阈值则创建新的manifest
  if (!descriptor_log_ ||
      manifest_file_size_ > db_options_->max_manifest_file_size) {
    TEST_SYNC_POINT("VersionSet::ProcessManifestWrites:BeforeNewManifest");
    pending_manifest_file_number_ = NewFileNumber();
    batch_edits.back()->SetNextFile(next_file_number_.load());
    new_descriptor_log = true;
  } else {
    pending_manifest_file_number_ = manifest_file_number_;
  }

// This is fine because everything inside of this block is serialized --
// only one thread can be here at the same time
if (new_descriptor_log) {
  // create new manifest file
  ROCKS_LOG_INFO(db_options_->info_log, "Creating manifest %" PRIu64 "\n",
                 pending_manifest_file_number_);
  std::string descriptor_fname =
      DescriptorFileName(dbname_, pending_manifest_file_number_);
  std::unique_ptr<WritableFile> descriptor_file;
  s = NewWritableFile(env_, descriptor_fname, &descriptor_file,
                      opt_env_opts);
  if (s.ok()) {
    descriptor_file->SetPreallocationBlockSize(
        db_options_->manifest_preallocation_size);

    std::unique_ptr<WritableFileWriter> file_writer(new WritableFileWriter(
        std::move(descriptor_file), descriptor_fname, opt_env_opts, env_,
        nullptr, db_options_->listeners));
    descriptor_log_.reset(
        new log::Writer(std::move(file_writer), 0, false));
        
    //创建完新的file writer后就将现在的状态写入Manifest
    //依次写入db状态，cf信息，数据文件信息，lognumber等
    s = WriteCurrentStateToManifest(descriptor_log_.get());
  }
}

如果新创建了manifest，将其写入写的CURRENT 文件中（通过rename保证原子性）

// If we just created a new descriptor file, install it by writing a
// new CURRENT file that points to it.
if (s.ok() && new_descriptor_log) {
  s = SetCurrentFile(env_, dbname_, pending_manifest_file_number_,
                     db_directory);
  TEST_SYNC_POINT("VersionSet::ProcessManifestWrites:AfterNewManifest");
}

Install new version

Version Edit

Version Edit Layout

Version edit是每次元数据变化时的增量（添加/删除文件，添加/删除 column family）

Data Types

Simple data types

VarX   - Variable character encoding of intX
FixedX - Fixed character encoding of intX

Complex data types

String - Length prefixed string data
+-----------+--------------------+
| size (n)  | content of string  |
+-----------+--------------------+
|<- Var32 ->|<-- n            -->|

Version Edit Record Types and Layout

不同的状态改变的数据记录，大致格式

+-------------+------ ......... ----------+
| Record ID   | Variable size record data |
+-------------+------ .......... ---------+
<-- Var32 --->|<-- varies by type       -->

Comparator edit record
Log number edit record
Previous File Number edit record:
Next File Number edit record:
Last Sequence Number edit record
Max Column Family edit record

Deleted File edit record

Mark a file as deleted from database.

+-----------------+-------------+--------------+
| kDeletedFile    | level       | file number  |
+-----------------+-------------+--------------+
<-- Var32     --->|<-- Var32 -->|<-- Var64  -->|

New File edit record
Mark a file as newly added to the database and provide RocksDB meta information.
- File edit record with compaction information（compaction信息的）
  
  kNewFile Level file number file size Smallest_key Largest_key Smallest_seqno Largest_seqno ...
  
  var32 var32 var64 var64 String String var64 var64 ...
- File edit record backward compatible
- File edit record with path information
Column family status edit record

kNewFile	Level	file number	file size	Smallest_key	Largest_key	Smallest_seqno	Largest_seqno	...
var32	var32	var64	var64	String	String	var64	var64	...

Column family add edit record

Add a column family

+---------------------+----------------+
| kColumnFamilyAdd    | cf name        |
+---------------------+----------------+
<-- Var32         --->|<-- String   -->|

Column family drop edit record

Drop all column family

+---------------------+
| kColumnFamilyDrop   |
+---------------------+
<-- Var32         --->|