-shm and -wal files in SQLite DB
After searching through numerous sources, I believe the following to be true:
- The
-shm
file contains an index to the-wal
file. The-shm
file improves performance when reading the-wal
file. - If the
-shm
file gets deleted, it get created again during next database access. - If
checkpoint
is run, the-wal
file 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
-shm
and-wal
files. A backup can then be made usingcp
,scp
etc. - 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.
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.