??RocksDB對(duì)文件系統(tǒng)和存儲(chǔ)介質(zhì)是不可知的禁悠。文件系統(tǒng)的操作不是原子的刮便，所以很有可能在系統(tǒng)故障時(shí)導(dǎo)致不一致讹蘑。即使打開了日志記錄功能，文件系統(tǒng)也不能在unclean restart時(shí)保證一致性功偿。POSIX文件系統(tǒng)也不支持批量操作的原子性盆佣。所以，在RocksDB重啟時(shí)械荷，不能依靠存儲(chǔ)在RocksDB data store file中的元信息來重建啟動(dòng)前的一致性狀態(tài)共耍。
??RocksD有一個(gè)內(nèi)建的機(jī)制來克服POSIX文件系統(tǒng)的各種限制，這種機(jī)制就是通過一個(gè)MANIFEST文件記錄RocksDB狀態(tài)改變的所有事物日志吨瞎。所以痹兜，MANIFEST文件可以在DB重啟時(shí)恢復(fù)到最近一次的一致性狀態(tài)。

• MANIFEST
  記錄RocksDB狀態(tài)改變的所有transction log的一種機(jī)制
• Manifest log
  一個(gè)單獨(dú)的文件关拒，記錄了RocksDB狀態(tài)的snapshot和edit log
• CURRENT
  最新最近的manifest log

how does it work?

??MANIFEST是RocksDB狀態(tài)變更的transction log 記錄佃蚜。MANIFEST包含 manifest log文件和最新的manifest 文件指針庸娱。Manifest logs是滾動(dòng)的日子文件，命名為MANIFEST-(序列號(hào)谐算，遞增)熟尉。CURRENT是一個(gè)特殊的文件用于指定latest manifest log 文件。
??在系統(tǒng)啟動(dòng)或者重啟時(shí)洲脂，latest manifest log文件包含了RocksDB的一致性狀態(tài)斤儿。RocksDB后續(xù)的所有變更記錄都寫入這個(gè)manifest log 文件。當(dāng)一個(gè)manifest log 文件超過了固定大小時(shí)恐锦，一個(gè)新的manifest文件會(huì)被創(chuàng)建往果，這個(gè)新的文件是基于當(dāng)前DB的snapshot。CURRENT中的最新manifest 文件指針會(huì)更新一铅，同時(shí)sync到文件系統(tǒng)中陕贮。一旦成功寫入CURRENT file，冗余的manifest log文件會(huì)被清除潘飘。

MANIFEST = { CURRENT, MANIFEST-<seq-no>* } 
CURRENT = File pointer to the latest manifest log
MANIFEST-<seq no> = Contains snapshot of RocksDB state and subsequent modifications

Version Edit

??RocksDB在特定時(shí)間的特定狀態(tài)會(huì)關(guān)聯(lián)一個(gè)version肮之。針對(duì)version的任何改動(dòng)都被視為一個(gè)version edit。version（or RocksDB state snapshot）是通過join 一連串的version-edit構(gòu)建起來卜录「昵埽總之，一個(gè)manifest log文件是一連串的version edits艰毒。

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

Version Edit Layout

?? Manifest log是一串version edit記錄筐高。version edit 記錄的類型是通過 edit number指定。

1丑瞧、Data Types

簡(jiǎn)單數(shù)據(jù)類型

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

復(fù)雜數(shù)據(jù)類型

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

2柑土、Version Edit Record Format

version edit record 使用下面的格式，解碼器通過Record ID來識(shí)別記錄類型嗦篱。

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

3冰单、Version Edit Record Types and Layout

根據(jù)對(duì)RocksDB的狀態(tài)變更的不同幌缝，有不同類型的edit record灸促。

• Comparator edit record

Captures the comparator name

+-------------+----------------+
| kComparator | data           |
+-------------+----------------+
<-- Var32 --->|<-- String   -->|

• Log number edit record

Latest WAL log file number

+-------------+----------------+
| kLogNumber  | log number     |
+-------------+----------------+
<-- Var32 --->|<-- Var64    -->|

• Previous File Number edit record

Previous manifest file number

+------------------+----------------+
| kPrevFileNumber  | log number     |
+------------------+----------------+
<-- Var32      --->|<-- Var64    -->|

• Next File Number edit record

Next manifest file number

+------------------+----------------+
| kNextFileNumber  | log number     |
+------------------+----------------+
<-- Var32      --->|<-- Var64    -->|

• Last Sequence Number edit record

Last sequence number of RocksDB

+------------------+----------------+
| kLastSequence    | log number     |
+------------------+----------------+
<-- Var32      --->|<-- Var64    -->|

• Max Column Family edit record

Adjust the maximum number of family columns allowed.

+---------------------+----------------+
| kMaxColumnFamily    | log number     |
+---------------------+----------------+
<-- Var32         --->|<-- Var32    -->|

• Deleted File edit record

Mark a file as deleted from database.

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

• 新文件的edit record
  表明這個(gè)文件是新添加到數(shù)據(jù)庫中的，同時(shí)提供RocksDB元信息涵卵。
  ?a浴栽、File edit record with compaction information

+--------------+-------------+--------------+------------+----------------+--------------+----------------+----------------+
| kNewFile4    | level       | file number  | file size  | smallest_key   | largest_key  | smallest_seqno | largest_seq_no |
+--------------+-------------+--------------+------------+----------------+--------------+----------------+----------------+
|<-- var32  -->|<-- var32 -->|<-- var64  -->|<-  var64 ->|<-- String   -->|<-- String -->|<-- var64    -->|<-- var64    -->|

+-----------+---------------+-------+------------------+-------+--------------+
|kPathID ---| Path size(n)  | path  | kNeedCompaction  | 1     | value (0/1)  |
+-----------+---------------+-------+------------------+-------+--------------+
<- var32  ->|<-- var32   -->|<- n ->|<-- var32      -->|<- 1 ->|<-- 1      -->|

??b、File edit record backward compatible

+--------------+-------------+--------------+------------+----------------+--------------+----------------+----------------+
| kNewFile2    | level       | file number  | file size  | smallest_key   | largest_key  | smallest_seqno | largest_seq_no |
+--------------+-------------+--------------+------------+----------------+--------------+----------------+----------------+
<-- var32   -->|<-- var32 -->|<-- var64  -->|<-  var64 ->|<-- String   -->|<-- String -->|<-- var64    -->|<-- var64    -->|

??c轿偎、File edit record with path information

+--------------+-------------+--------------+-------------+-------------+----------------+--------------+
| kNewFile3    | level       | file number  | Path ID     | file size   | smallest_key   | largest_key  |
+--------------+-------------+--------------+-------------+-------------+----------------+--------------+
|<-- var32  -->|<-- var32 -->|<-- var64  -->|<-- var32 -->|<-- var64 -->|<-- String   -->|<-- String -->|
+----------------+----------------+
| smallest_seqno | largest_seq_no |
+----------------+----------------+
<-- var64     -->|<-- var64    -->|

• Column family status edit record

Note the status of column family feature (enabled/disabled)

+------------------+----------------+
| kColumnFamily    | 0/1            |
+------------------+----------------+
<-- Var32      --->|<-- Var32    -->|

• 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         --->|