3. WalB デバイス
3
WalB device
Data device Log device
Read Write
Any application (File system, DBMS, etc.)
Linear mapped Ring buffer
4. WAL (Write-Ahead Logging)
• 書き込み履歴 (wlog) を先に書く
• データの一貫性を守るため
• WalB では wlog をバックアップのために利用
Data storageLog storage
Write at 0
Write at 2
Read at 2
Write at 2
0
0 2
0 2
0 2 2
Time
4
5. Log Device Layout
5
Checksum
2nd
written data
Ring buffer
Log pack
Logpack
header
block
1st
written data …
The oldest logpackThe latest logpack
1st log record
IO address
IO size
...
Logpack lsid
Log pack
header
block
Num of records
Total IO size
2nd log record
IO address
IO size
...
...