If the segment size is 16MB it shouldn't take much time but higher segment values this can be a problem. But again, the current segment has to be filled 75% to precreate new one. I am not sure how much we gain. Do you have some numbers with different segment sizes?
The function PreallocXlogFiles doesn't get called during end-of-recovery checkpoint in CreateCheckPoint, see [1]. The server becomes operational after the end-of-recovery checkpoint and may need WAL files. However, I'm not sure how beneficial it is going to be if the WAL is pre-allocated (as PreallocXlogFiles just allocates only 1 extra WAL file).
Thoughts?
[1] /* * An end-of-recovery checkpoint is really a shutdown checkpoint, just * issued at a different time. */ if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY)) shutdown = true; else shutdown = false;
/* * Make more log segments if needed. (Do this after recycling old log * segments, since that may supply some of the needed files.) */ if (!shutdown) PreallocXlogFiles(recptr, checkPoint.ThisTimeLineID);