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help / color / mirror / Atom feedFrom: Kirill Reshke <[email protected]>
To: Melanie Plageman <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Andrey Borodin <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Heikki Linnakangas <[email protected]>
Cc: Chao Li <[email protected]>
Subject: Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)
Date: Thu, 18 Dec 2025 23:07:20 +0500
Message-ID: <CALdSSPgKH7TdW0zcDPjBxx+MX18bjc65K12KgoGtX6ZP8Qq63w@mail.gmail.com> (raw)
In-Reply-To: <CAAKRu_baqEkdJ0_1vokvgbaN52ycR65HM2-DR-VsRmYVVQLV8w@mail.gmail.com>
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On Thu, 18 Dec 2025 at 20:18, Melanie Plageman
<[email protected]> wrote:
> But you are right, I don't see any non-error code path where a heap
> page would become empty (all line pointers set unused) and then not be
> set all-visible. Only vacuum sets line pointers unused and if all the
> line pointers are unused it will always set the page all-visible.
>
> I think, though, that if we error out in lazy_scan_prune() after
> returning from heap_page_prune_and_freeze() such that we don't set the
> empty page all-visible, we can end up with an empty page without
> PD_ALL_VISIBLE set. You can see how this might work by patching the VM
> set code in lazy_scan_prune() to skip empty pages.
>
Thank you for your explanation! I completely forgot that PD_ALL_VIS
is a non-persistent change (hint bit). so its update can be trivially
lost.
The simplest real-life example is being killed just after returning
from heap_page_prune_and_freeze, yes.
PFA tap test covering lazy_scan_new_or_empty code path for
empty-but-not-all-visible page
--
Best regards,
Kirill Reshke
Attachments:
[application/octet-stream] v1-0001-Add-TAP-test-for-empty-page-vacuum.patch (4.3K, 2-v1-0001-Add-TAP-test-for-empty-page-vacuum.patch)
download | inline diff:
From ac838953de9c4ab0cb5f13d1e1b8ad0a18e73e39 Mon Sep 17 00:00:00 2001
From: reshke <[email protected]>
Date: Thu, 18 Dec 2025 18:00:22 +0000
Subject: [PATCH v1] Add TAP test for empty page vacuum.
VACUUM can be run for empty pages with DISABLE_PAGE_SKIPPING option.
In this case, VACUUM wil set up page-level visibility bit
(PD_ALL_VISIBLE) if not previously set.
To end up with empty page which is missing visibility hint bit, we need
to forcefuly cancel (kill -9) backend, executing page freezing, jsut
after it did page pruning. Use injeciton point for this purpose and
add TAP test to cover "recovery" after error code path.
---
src/backend/access/heap/vacuumlazy.c | 6 ++
.../test_misc/t/010_vacuum_empty_page.pl | 75 +++++++++++++++++++
2 files changed, 81 insertions(+)
create mode 100644 src/test/modules/test_misc/t/010_vacuum_empty_page.pl
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 30778a15639..9b8cbb67f11 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -153,6 +153,7 @@
#include "storage/freespace.h"
#include "storage/lmgr.h"
#include "storage/read_stream.h"
+#include "utils/injection_point.h"
#include "utils/lsyscache.h"
#include "utils/pg_rusage.h"
#include "utils/timestamp.h"
@@ -1899,6 +1900,8 @@ lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf, BlockNumber blkno,
*/
if (!PageIsAllVisible(page))
{
+ INJECTION_POINT("vacuum-empty-page-non-all-vis", NULL);
+
START_CRIT_SECTION();
/* mark buffer dirty before writing a WAL record */
@@ -2012,6 +2015,9 @@ lazy_scan_prune(LVRelState *vacrel,
&vacrel->offnum,
&vacrel->NewRelfrozenXid, &vacrel->NewRelminMxid);
+
+ INJECTION_POINT("vacuum-heap-prune-and-freeze-after", NULL);
+
Assert(MultiXactIdIsValid(vacrel->NewRelminMxid));
Assert(TransactionIdIsValid(vacrel->NewRelfrozenXid));
diff --git a/src/test/modules/test_misc/t/010_vacuum_empty_page.pl b/src/test/modules/test_misc/t/010_vacuum_empty_page.pl
new file mode 100644
index 00000000000..af5c39d2435
--- /dev/null
+++ b/src/test/modules/test_misc/t/010_vacuum_empty_page.pl
@@ -0,0 +1,75 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+# Check how temporary file removals and statement queries are associated
+# in the server logs for various query sequences with the simple and
+# extended query protocols.
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+if ($ENV{enable_injection_points} ne 'yes')
+{
+ plan skip_all => 'Injection points not supported by this build';
+}
+
+# Initialize a new PostgreSQL test cluster
+my $node = PostgreSQL::Test::Cluster->new('primary');
+$node->init();
+$node->append_conf(
+ 'postgresql.conf', qq(
+log_min_messages = 'notice'
+));
+$node->start;
+
+# Check if the extension injection_points is available, as it may be
+# possible that this script is run with installcheck, where the module
+# would not be installed by default.
+if (!$node->check_extension('injection_points'))
+{
+ plan skip_all => 'Extension injection_points not installed';
+}
+
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+
+# Setup table and populate with data
+$node->safe_psql(
+ "postgres", qq{
+CREATE TABLE vac_empty_test(a int);
+BEGIN;
+INSERT INTO vac_empty_test DEFAULT VALUES;
+ROLLBACK;
+});
+
+# From this point, autovacuum worker will wait at startup.
+$node->safe_psql('postgres',
+ "SELECT injection_points_attach('vacuum-heap-prune-and-freeze-after', 'error');");
+$node->safe_psql('postgres',
+ "SELECT injection_points_attach('vacuum-empty-page-non-all-vis', 'notice');");
+
+$node->psql('postgres', "VACUUM (FREEZE) vac_empty_test;", on_error_stop => 1);
+
+my $offset = -s $node->logfile;
+
+# Run vacuum, force it on empty page.
+$node->safe_psql(
+ "postgres", qq{
+VACUUM (DISABLE_PAGE_SKIPPING) vac_empty_test;
+});
+
+ok( $node->log_contains(
+ qr/NOTICE: notice triggered for injection point vacuum-empty-page-non-all-vis/,
+ $offset),
+ "vacuum sets all-visible page bit for empty page");
+
+
+$node->safe_psql('postgres',
+ "SELECT injection_points_detach('vacuum-heap-prune-and-freeze-after');");
+$node->safe_psql('postgres',
+ "SELECT injection_points_detach('vacuum-empty-page-non-all-vis');");
+
+$node->stop('fast');
+done_testing();
--
2.43.0
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Subject: Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)
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