Regarding the the patch, pg_rewind starts reading segments from the
divergence point back to the nearest checkpoint, then moves foward
during rewinding. So, the fact that SimpleXLogPageRead have read a
segment suggests that the segment is required during the next startup.
So I don't think we need to move around the keepWalSeg flag. All
files that are wanted while rewinding should be preserved
unconditionally.
I am probably not getting this right but as far as I see SimpleXLogPageRead is called at most 3 times during pg_rewind run:
1. From readOneRecord to determine the end-of-WAL on the target by reading the last shutdown checkpoint record/minRecoveryPoint on it
2. From findLastCheckpoint to find last common checkpoint (here it indeed reads all the segments that are required during the startup, hence the keepWalSeg flag set to true)
3. From extractPageMap to extract all the pages modified after the fork (here we also read all the segments that should be kept but also the ones further, until the target's end record. Doesn't seem we should unconditionally preserve them all).
Am I missing something?
+ /*
+ * Some entries (WAL segments) already have an action assigned
+ * (see SimpleXLogPageRead()).
+ */
+ if (entry->action == FILE_ACTION_NONE)
+ continue;
entry->action = decide_file_action(entry);
It might be more reasonable to call decide_file_action() when action
is UNDECIDED.
Agree, will change this part.
Regards,
Polina Bungina