To pause/resume the backup workers, the fdbbackup command will write to the
backupPausedKey. Then backup workers noticed the value of the key has been
changed and stops/resumes pulling from TLog.
For old submitBackup(), where partitionedLog is false, do not set the
backupStartedKey in BackupConfig, which signals backup workers to skip these
backups.
If backup worker is enabled, the current epoch's worker of tag (-2,0) will be
responsible for monitoring the backup progress of all workers and update the
BackupConfig with the latest saved log version, which is the minimum version
of all tags.
This change has been incorporated in the getLatestRestorableVersion() so that
it is transparent to clients.
Right now, the default is to keep the old backup behavior, i.e., do NOT use
backup workers. Specifically, if BackupType is not set (or is set to default),
the master will not recruit backup workers and will not add pseudo locality for
backup workers.
The StartFullBackupTaskFunc is updated to check if backup worker is enabled.
Only when it is not enabled, starting a backup will wait on all backup workers
to be started.
The backup task may be restarted multiple times so the started key for the
backup task may already be set. In this case, the wait on watch should be
skipped.
TaskBucket::keepRunning() needs to be called in backup transactions to be sure
that the task has not been cancelled. If so, the task is cancelled. Otherwise,
the task can continue run, causing multiple runs of the same task.
Another subtle issue is that the beginVersion is persisted on backupStartedKey.
So while reading it back from that key, we should set task's beginVersion with
the value persisted earlier.
This wait is to make sure that backup workers are already saving mutations so
that no mutations are missed. The idea is that the CLI sets a "backupStartedKey"
in the database and waits for allWorkerStarted() key of the backup to be set.
Backup workers monitor the changes to the "backupStartedKey" and start logging
mutations. Additionally, backup worker for Tag(-2,0) monitors all other workers
have started (checking their saved progress version is larger than the backup's
start version), and then sets the allWorkerStarted() key for the backup.
Jingyu Zhou