问题
I am taking the backup of SQLite DB using cp commmand after running wal_checkpoint(FULL). The DB is being used in WAL mode so there are other files like -shm and -wal in my folder. When I run wal_checkpoint(FULL), the changes in WAL file get committed to the database. I waqs wondering whether -wal and -shm files get deelted after running a checkpoint. If not, then what do they contain ?
I know my backup process is not good since I am not using SQLite backup APIs. This is a bug in my code.
Can anyone please suggest what content do -shm and -wal files contain after running checkpoint.
Any link provided would be helpful.
Thanks
回答1:
After searching through numerous sources, I believe the following to be true:
- The
-shmfile contains an index to the-walfile. The-shmfile improves performance when reading the-walfile. - If the
-shmfile gets deleted, it get created again during next database access. - If
checkpointis run, the-walfile can be deleted.
To perform safe backups:
- It is recommended that you use SQLite backup functions for making backups. SQLite library can even make backups of an online database.
- If you don't want to use (1), then the best way is to close the database handles. This ensures a clean and consistent state of the database file, and deletes the
-shmand-walfiles. A backup can then be made usingcp,scpetc. - If the SQLite database file is intended to be transmitted over a network, then the vacuum command should be run after
checkpoint. This removes the fragmentation in the database file thereby reducing its size, so you transfer less data through network.
回答2:
The -shm file does not contain any permanent data.
When the last connection is closed, the database is automatically checkpointed, and the -wal file is then deleted.
This implies that after a checkpoint, and if no other connections exist, the -wal file does not contain any important data.
If possible, you should close the connection before taking the backup.
来源：https://stackoverflow.com/questions/12928294/shm-and-wal-files-in-sqlite-db