journal 主要用途是，当事务要修改page时，先把未修改的page存入journal中，如果事务rollback时，就从journal中得到修改前的数据，覆盖已改的，达到事务的一致性。
journal是一个不同于数据库文件的另一个文件，它在事务开始时创建，当事务结束时就删除了
The format for the journal header is as follows:
- 8 bytes: Magic identifying journal format.
- 4 bytes: Number of records in journal, or -1 no-sync mode is on.
- 4 bytes: Random number used for page hash.
- 4 bytes: Initial database page count.
- 4 bytes: Sector size used by the process that wrote this journal.
- 4 bytes: Database page size.
** The journal file format is as follows:
**
** (1) 8 byte prefix. A copy of aJournalMagic[].
** (2) 4 byte big-endian integer which is the number of valid page records
** in the journal. If this value is 0xffffffff, then compute the
** number of page records from the journal size.
** (3) 4 byte big-endian integer which is the initial value for the
** sanity checksum.
** (4) 4 byte integer which is the number of pages to truncate the
** database to during a rollback.
** (5) 4 byte big-endian integer which is the sector size. The header
** is this many bytes in size.
** (6) 4 byte big-endian integer which is the page size.
** (7) zero padding out to the next sector size.
** (8) Zero or more pages instances, each as follows:
** + 4 byte page number.
** + pPager->pageSize bytes of data.
** + 4 byte checksum
**
** When we speak of the journal header, we mean the first 7 items above.
** Each entry in the journal is an instance of the 8th item.