From 2a8e1ac598c864ac2775f33da21a117c363c6c7f Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Thu, 17 Apr 2014 17:47:50 +0300
Subject: [PATCH] Set the all-visible flag on heap page before writing WAL
record, not after.
If we set the all-visible flag after writing WAL record, and XLogInsert
takes a full-page image of the page, the image would not include the flag.
We will then proceed to set the VM bit, which would then be set without the
corresponding all-visible flag on the heap page.
Found by comparing page images on master and standby, after writing/replaying
each WAL record. (There is still a discrepancy: the all-visible flag won't
be set after replaying the HEAP_CLEAN record, even though it is set in the
master. However, it will be set when replaying the HEAP2_VISIBLE record and
setting the VM bit, so the all-visible flag and VM bit are always consistent
on the standby, even though they are momentarily out-of-sync with master)
Backpatch to 9.3 where this code was introduced.
---
src/backend/commands/vacuumlazy.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index d5db917d97f..3870df606b7 100644
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -1213,6 +1213,13 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
PageRepairFragmentation(page);
+ /*
+ * Now that we have removed the dead tuples from the page, once again
+ * check if the page has become all-visible.
+ */
+ if (heap_page_is_all_visible(onerel, buffer, &visibility_cutoff_xid))
+ PageSetAllVisible(page);
+
/*
* Mark buffer dirty before we write WAL.
*/
@@ -1231,14 +1238,13 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
}
/*
- * Now that we have removed the dead tuples from the page, once again
- * check if the page has become all-visible.
+ * All the changes to the heap page have been done. If the all-visible
+ * flag is now set, also set the VM bit.
*/
- if (!visibilitymap_test(onerel, blkno, vmbuffer) &&
- heap_page_is_all_visible(onerel, buffer, &visibility_cutoff_xid))
+ if (PageIsAllVisible(page) &&
+ !visibilitymap_test(onerel, blkno, vmbuffer))
{
Assert(BufferIsValid(*vmbuffer));
- PageSetAllVisible(page);
visibilitymap_set(onerel, blkno, buffer, InvalidXLogRecPtr, *vmbuffer,
visibility_cutoff_xid);
}
--
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