Hi,
In standby mode, the state machine in WaitForWALToBecomeAvailable()
reads WAL from pg_wal after failing to read from the archive. This is
currently implemented in XLogFileReadAnyTLI() by calling
XLogFileRead() with source XLOG_FROM_PG_WAL after it fails with source
XLOG_FROM_PG_ARCHIVE and the current source isn't changed at all.
Also, passing the source to XLogFileReadAnyTLI() in
WaitForWALToBecomeAvailable() isn't straight i.e. it's not necessary
to pass in XLOG_FROM_ANY at all. These things make the state machine a
bit complicated and hard to understand.
The attached patch attempts to simplify the code a bit by changing the
current source to XLOG_FROM_PG_WAL after failing in
XLOG_FROM_PG_ARCHIVE so that the state machine can move smoothly to
read from pg_wal. And we can just pass the current source to
XLogFileReadAnyTLI(). It also enables us to reduce a bit of extra
XLogFileRead() code in XLogFileReadAnyTLI().
Thoughts?
--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
From e20354b60f4f7040e034cbd26142d8e69dd01be5 Mon Sep 17 00:00:00 2001
From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Date: Tue, 18 Oct 2022 05:52:35 +0000
Subject: [PATCH v1] Simplify standby state machine in
WaitForWALToBecomeAvailable()
In standby mode, the state machine in
WaitForWALToBecomeAvailable() reads WAL from pg_wal after failing
to read from the archive. This is currently implemented in
XLogFileReadAnyTLI() by calling XLogFileRead() with source
XLOG_FROM_PG_WAL after it fails with source XLOG_FROM_PG_ARCHIVE
and the current source isn't changed at all. Also, passing the
source to XLogFileReadAnyTLI() in WaitForWALToBecomeAvailable()
isn't straight i.e. it's not necessary to pass in XLOG_FROM_ANY at
all. These things make the state machine a bit complicated and
hard to understand.
This patch attempts to simplify the code a bit by changing the
current source to XLOG_FROM_PG_WAL after failing in
XLOG_FROM_PG_ARCHIVE so that the state machine can move smoothly
to read from pg_wal. And we can just pass the current source to
XLogFileReadAnyTLI(). It also enables us to reduce a bit of extra
XLogFileRead() code in XLogFileReadAnyTLI().
---
src/backend/access/transam/xlogrecovery.c | 40 ++++++++---------------
1 file changed, 14 insertions(+), 26 deletions(-)
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index cb07694aea..6ac961f32b 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3478,6 +3478,14 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
switch (currentSource)
{
case XLOG_FROM_ARCHIVE:
+
+ /*
+ * After failing to read from archive, we try to read from
+ * pg_wal.
+ */
+ currentSource = XLOG_FROM_PG_WAL;
+ break;
+
case XLOG_FROM_PG_WAL:
/*
@@ -3632,12 +3640,7 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
if (randAccess)
curFileTLI = 0;
- /*
- * Try to restore the file from archive, or read an existing
- * file from pg_wal.
- */
readFile = XLogFileReadAnyTLI(readSegNo, DEBUG2,
- currentSource == XLOG_FROM_ARCHIVE ? XLOG_FROM_ANY :
currentSource);
if (readFile >= 0)
return XLREAD_SUCCESS; /* success! */
@@ -4199,29 +4202,14 @@ XLogFileReadAnyTLI(XLogSegNo segno, int emode, XLogSource source)
continue;
}
- if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE)
+ fd = XLogFileRead(segno, emode, tli, source, true);
+ if (fd != -1)
{
- fd = XLogFileRead(segno, emode, tli,
- XLOG_FROM_ARCHIVE, true);
- if (fd != -1)
- {
+ if (source == XLOG_FROM_ARCHIVE)
elog(DEBUG1, "got WAL segment from archive");
- if (!expectedTLEs)
- expectedTLEs = tles;
- return fd;
- }
- }
-
- if (source == XLOG_FROM_ANY || source == XLOG_FROM_PG_WAL)
- {
- fd = XLogFileRead(segno, emode, tli,
- XLOG_FROM_PG_WAL, true);
- if (fd != -1)
- {
- if (!expectedTLEs)
- expectedTLEs = tles;
- return fd;
- }
+ if (!expectedTLEs)
+ expectedTLEs = tles;
+ return fd;
}
}
--
2.34.1
