看这个：
* backupStartPoint is the redo pointer of the backup start checkpoint, if
* we are recovering from an online backup and haven't reached the end of
* backup yet. It is reset to zero when the end of backup is reached, and
* we mustn't start up before that. A boolean would suffice otherwise, but
* we use the redo pointer as a cross-check when we see an end-of-backup
* record, to make sure the end-of-backup record corresponds the base
* backup we're recovering from.
*/
...
XLogRecPtr backupStartPoint;
是说如果处在online backup中时会使用它。只有下面一个地方进行了赋值：
/*
* This must be called ONCE during postmaster or standalone-backend startup
*/
void
StartupXLOG(void)
{
… /* REDO */
if (InRecovery)
{
…
/*
* Set backupStartPoint if we're starting recovery from a base backup.
* However, if there was no recovery.conf, and the backup was taken
* with pg_start_backup(), we don't know if the server crashed before
* the backup was finished and we're doing crash recovery on the
* original server, or if we're restoring from the base backup. We
* have to assume we're doing crash recovery in that case, or the
* database would refuse to start up after a crash.
*/
if ((InArchiveRecovery && haveBackupLabel) || backupEndRequired)
ControlFile->backupStartPoint = checkPoint.redo; ControlFile->time = (pg_time_t) time(NULL);
/* No need to hold ControlFileLock yet, we aren't up far enough */
UpdateControlFile();
…
} …
}