Is it possible to backup a live comdb2 database?

[AnDongLi]

I read something about backup general database: the database back-end file is not guaranteed to be in a consistent state at any time while the database is running: the database front-end caches LOTS of things that it hasn't necessarily written back to disk. The back-end file is, in busy databases, frequently in an inconsistent state.

I wonder how comdb2 handle this issue.

[markhannum]

Yes- the comdb2ar utility can be used to perform stable backups against a running database, and this backup is restore-able to a stable database image. The trick is to prevent the deletion of logfiles while the backup is running, and to perform recovery on the restored database. comdb2ar queries a running database for the precise LSN to begin recovery against a restored copy. Loosely speaking, this will be a point prior to the oldest LSN of all outstanding dirty pages. See the handling for cmd_logdelete3 in db/appsock_handler.c -> bdb_get_recovery_start_lsn for the details.

[AnDongLi]

So for the recovery to succeed there must always be a database running, right?
Is there a test I can run to see the backup and restore?

[markhannum]

No- if the database isn't running comdb2ar will default to running fullrecovery against the restored copy. The incremental_backup_load test verifies that comdb2ar is able to perform a stable full-backup and several subsequent incremental backups against a running database loaded with several active writers. See the documentation on our incremental backup system at the bottom of https://bloomberg.github.io/comdb2/backups.html for more information on incremental backups.

[akshatsikarwar]

No, that's just an optimization. You can always run recovery starting at the oldest log file. So you can copy while db is down as long as log files are not removed.

[AnDongLi]

I see. Thanks.
I'll try the test.