Hi,
Thanks for the explanation.
On Wed, 21 Jun 2023 at 18:03, Matthias van de Meent <
boekewurm+postg...@gmail.com> wrote:
>
> On Wed, 21 Jun 2023 at 13:04, Nazir Bilal Yavuz <byavu...@gmail.com>
wrote:
> > I was trying to add WAL stats to pg_stat_io. While doing that I was
comparing pg_stat_wal and pg_stat_io's WAL stats and there was some
inequality between the total number of WALs. I found that the difference
comes from bgwriter's WALs. bgwriter generates WAL but it doesn't flush
them because the pgstat_report_wal() function isn't called in bgwriter. I
attached a small patch for calling the pgstat_report_wal() function in
bgwriter.
> >
> > bgwriter generates WAL by calling functions in this order:
> > bgwriter.c -> BackgroundWriterMain() -> BgBufferSync() ->
SyncOneBuffer() -> FlushBuffer() -> XLogFlush() -> XLogWrite()
>
> I was quite confused here, as XLogWrite() does not generate any WAL;
> it only writes existing WAL from buffers to disk.
> In a running PostgreSQL instance, WAL is only generated through
> XLogInsert(xloginsert.c) and serialized / written to buffers in its
> call to XLogInsertRecord(xlog.c); XLogFlush and XLogWrite are only
> responsible for writing those buffers to disk.
Yes, you are right. Correct explanation should be "bgwriter writes existing
WAL from buffers to disk but pg_stat_wal doesn't count them because
bgwriter doesn't call pgstat_report_wal() to update WAL statistics".
> I also got confused with your included views; they're not included in
> the patch and the current master branch doesn't emit object=wal, so I
> can't really check that the patch works as intended.
I attached a WIP patch for showing WAL stats in pg_stat_io.
After applying patch, I used these queries for the getting views I shared
in the first mail;
Query for the first view:
SELECT
'pg_stat_wal' AS view_name,
SUM(wal_write) AS total_wal_write
FROM
pg_stat_wal
UNION ALL
SELECT
'pg_stat_io' AS view_name,
SUM(writes) AS total_wal_write
FROM
pg_stat_io
WHERE
object = 'wal';
Query for the second view:
SELECT backend_type, object, writes FROM pg_stat_io where object = 'wal';
I also changed the description on the patch file and attached it.
Regards,
Nazir Bilal Yavuz
Microsoft
From 674b54adc3a92093f0cc25cd9117bf32956c6f2c Mon Sep 17 00:00:00 2001
From: Nazir Bilal Yavuz <byavu...@gmail.com>
Date: Wed, 21 Jun 2023 13:39:13 +0300
Subject: [PATCH v2 2/2] Flush WAL in bgwriter
bgwriter writes existing WAL from buffers to disk but pg_stat_wal
doesn't count them because bgwriter doesn't call pgstat_report_wal()
to update WAL statistics. Call pgstat_report_wal() to flush WAL in
bgwriter.
---
src/backend/postmaster/bgwriter.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/src/backend/postmaster/bgwriter.c b/src/backend/postmaster/bgwriter.c
index caad642ec9..f2e4f23d9f 100644
--- a/src/backend/postmaster/bgwriter.c
+++ b/src/backend/postmaster/bgwriter.c
@@ -241,6 +241,7 @@ BackgroundWriterMain(void)
/* Report pending statistics to the cumulative stats system */
pgstat_report_bgwriter();
+ pgstat_report_wal(true);
if (FirstCallSinceLastCheckpoint())
{
--
2.40.1
From 45f0dc4ee660baceb91a96f1e5aafab3adc9bf13 Mon Sep 17 00:00:00 2001
From: Nazir Bilal Yavuz <byavu...@gmail.com>
Date: Wed, 21 Jun 2023 18:36:25 +0300
Subject: [PATCH v2 1/2] [WIP] Show WAL stats on pg_stat_io
---
src/backend/access/transam/xlog.c | 11 +++++++++--
src/backend/utils/activity/pgstat_io.c | 9 ++++++++-
src/include/pgstat.h | 3 ++-
3 files changed, 19 insertions(+), 4 deletions(-)
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8b0710abe6..37028429c7 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2177,7 +2177,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
Size nbytes;
Size nleft;
int written;
- instr_time start;
+ instr_time start, io_start;
/* OK to write the page(s) */
from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
@@ -2185,6 +2185,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
nleft = nbytes;
do
{
+ io_start = pgstat_prepare_io_time();
errno = 0;
/* Measure I/O timing to write WAL data */
@@ -2209,6 +2210,8 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, duration, start);
}
+ pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL, IOOP_WRITE, io_start, 1);
+
PendingWalStats.wal_write++;
if (written <= 0)
@@ -8163,7 +8166,7 @@ void
issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
{
char *msg = NULL;
- instr_time start;
+ instr_time start, io_start;
Assert(tli != 0);
@@ -8176,6 +8179,8 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
sync_method == SYNC_METHOD_OPEN_DSYNC)
return;
+ io_start = pgstat_prepare_io_time();
+
/* Measure I/O timing to sync the WAL file */
if (track_wal_io_timing)
INSTR_TIME_SET_CURRENT(start);
@@ -8235,6 +8240,8 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, duration, start);
}
+ pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL, IOOP_FSYNC, io_start, 1);
+
PendingWalStats.wal_sync++;
}
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index eb7d35d422..f05c62973a 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -231,6 +231,8 @@ pgstat_get_io_object_name(IOObject io_object)
return "relation";
case IOOBJECT_TEMP_RELATION:
return "temp relation";
+ case IOOBJECT_WAL:
+ return "wal";
}
elog(ERROR, "unrecognized IOObject value: %d", io_object);
@@ -313,8 +315,8 @@ pgstat_tracks_io_bktype(BackendType bktype)
case B_ARCHIVER:
case B_LOGGER:
case B_WAL_RECEIVER:
- case B_WAL_WRITER:
return false;
+ case B_WAL_WRITER:
case B_AUTOVAC_LAUNCHER:
case B_AUTOVAC_WORKER:
@@ -350,6 +352,11 @@ pgstat_tracks_io_object(BackendType bktype, IOObject io_object,
if (!pgstat_tracks_io_bktype(bktype))
return false;
+
+ if (io_context != IOCONTEXT_NORMAL &&
+ io_object == IOOBJECT_WAL)
+ return false;
+
/*
* Currently, IO on temporary relations can only occur in the
* IOCONTEXT_NORMAL IOContext.
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 57a2c0866a..fb7d3abf22 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -277,9 +277,10 @@ typedef enum IOObject
{
IOOBJECT_RELATION,
IOOBJECT_TEMP_RELATION,
+ IOOBJECT_WAL,
} IOObject;
-#define IOOBJECT_NUM_TYPES (IOOBJECT_TEMP_RELATION + 1)
+#define IOOBJECT_NUM_TYPES (IOOBJECT_WAL + 1)
typedef enum IOContext
{
--
2.40.1
