Use scanned_pages to decide when to failsafe check.

petergeoghegan · petergeoghegan · commit 07eef53955ea · 2022-12-22T10:41:40.000-08:00
Perform a failsafe check every time VACUUM's first heap scan scans a further FAILSAFE_EVERY_PAGES pages, rather than using an approach based on the number of physical blocks that our current blkno is from the blkno at the time of the previous failsafe check. That way VACUUM will perform a failsafe check every time it has scanned a uniform number of pages, without it mattering when or how VACUUM skipped pages using the visibility map. Sami Imseih, with changes to FAILSAFE_EVERY_PAGES comments added by me. Author: Sami Imseih <simseih@amazon.com> Reviewed-By: Peter Geoghegan <pg@bowt.ie> Discussion: https://postgr.es/m/401CE010-4049-4B94-9961-0B610A5D254D%40amazon.com
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
@@ -94,7 +94,8 @@
 #define BYPASS_THRESHOLD_PAGES	0.02	/* i.e. 2% of rel_pages */
 
 /*
- * Perform a failsafe check every 4GB during the heap scan, approximately
+ * Perform a failsafe check each time we scan another 4GB of pages.
+ * (Note that this is deliberately kept to a power-of-two, usually 2^19.)
  */
 #define FAILSAFE_EVERY_PAGES \
 	((BlockNumber) (((uint64) 4 * 1024 * 1024 * 1024) / BLCKSZ))
@@ -821,7 +822,6 @@ lazy_scan_heap(LVRelState *vacrel)
 	BlockNumber rel_pages = vacrel->rel_pages,
 				blkno,
 				next_unskippable_block,
-				next_failsafe_block = 0,
 				next_fsm_block_to_vacuum = 0;
 	VacDeadItems *dead_items = vacrel->dead_items;
 	Buffer		vmbuffer = InvalidBuffer;
@@ -895,11 +895,8 @@ lazy_scan_heap(LVRelState *vacrel)
 		 * one-pass strategy, and the two-pass strategy with the index_cleanup
 		 * param set to 'off'.
 		 */
-		if (blkno - next_failsafe_block >= FAILSAFE_EVERY_PAGES)
-		{
+		if (vacrel->scanned_pages % FAILSAFE_EVERY_PAGES == 0)
 			lazy_check_wraparound_failsafe(vacrel);
-			next_failsafe_block = blkno;
-		}
 
 		/*
 		 * Consider if we definitely have enough space to process TIDs on page
