Subject: [PATCH v2] pg_wal on COW fs

From: Alvaro Herrera <[email-masked]>
Date: Wed, 27 Feb 2019 19:41:05 -0300

---
 doc/src/sgml/config.sgml                      |  20 ++++
 src/backend/access/transam/xlog.c             | 101 ++++++++++++------
 src/backend/utils/misc/guc.c                  |  13 +++
 src/backend/utils/misc/postgresql.conf.sample |   1 +
 src/include/access/xlog.h                     |   1 +
 5 files changed, 102 insertions(+), 34 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 8bd57f376b2..60a873273aa 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2959,6 +2959,26 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-wal-cow-filesystem" xreflabel="wal_cow_filesystem">
+      <term><varname>wal_cow_filesystem</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>wal_cow_filesystem</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        This parameter should only be set to <literal>on</literal> when the WAL
+        resides on a <firstterm>Copy-On-Write</firstterm> (<acronym>COW</acronym>)
+        filesystem.
+        Enabling this option adjusts some behavior to take advantage of the
+        filesystem characteristics (for example, recycling WAL files and
+        zero-filling new WAL files are disabled).
+        This setting is only appropriate for filesystems which
+        allocate new disk blocks on every write.
+       </para>
+      </listitem>
+     </varlistentry>
+
      </variablelist>
      </sect2>
      <sect2 id="runtime-config-wal-archiving">
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index ecd12fc53ae..1acce1c70d6 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -94,6 +94,7 @@ bool		wal_log_hints = false;
 bool		wal_compression = false;
 char	   *wal_consistency_checking_string = NULL;
 bool	   *wal_consistency_checking = NULL;
+bool		wal_cow_filesystem = false;
 bool		log_checkpoints = false;
 int			sync_method = DEFAULT_SYNC_METHOD;
 int			wal_level = WAL_LEVEL_MINIMAL;
@@ -3216,6 +3217,7 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 	XLogSegNo	max_segno;
 	int			fd;
 	int			nbytes;
+	bool		fail;
 
 	XLogFilePath(path, ThisTimeLineID, logsegno, wal_segment_size);
 
@@ -3255,41 +3257,65 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 				(errcode_for_file_access(),
 				 errmsg("could not create file \"%s\": %m", tmppath)));
 
-	/*
-	 * Zero-fill the file.  We have to do this the hard way to ensure that all
-	 * the file space has really been allocated --- on platforms that allow
-	 * "holes" in files, just seeking to the end doesn't allocate intermediate
-	 * space.  This way, we know that we have all the space and (after the
-	 * fsync below) that all the indirect blocks are down on disk.  Therefore,
-	 * fdatasync(2) or O_DSYNC will be sufficient to sync future writes to the
-	 * log file.
-	 */
 	memset(zbuffer.data, 0, XLOG_BLCKSZ);
-	for (nbytes = 0; nbytes < wal_segment_size; nbytes += XLOG_BLCKSZ)
+
+	/*
+	 * Ensure the filesystem has physically allocated disk space for the data.
+	 */
+	if (!wal_cow_filesystem)
 	{
+		/*
+		 * In non-CoW filesystems, zero-fill the file.  We have to do this the
+		 * hard way to ensure that all the file space has really been
+		 * allocated --- on platforms that allow "holes" in files, just seeking
+		 * to the end doesn't allocate intermediate space.  This way, we know
+		 * that we have all the space and (after the fsync below) that all the
+		 * indirect blocks are down on disk.  Therefore, fdatasync(2) or
+		 * O_DSYNC will be sufficient to sync future writes to the log file.
+		 */
+		fail = false;	/* keep compiler quiet */
+		for (nbytes = 0; nbytes < wal_segment_size; nbytes += XLOG_BLCKSZ)
+		{
+			errno = 0;
+			pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
+			fail = (int) write(fd, zbuffer.data, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ;
+			pgstat_report_wait_end();
+			if (fail)
+				break;
+		}
+	}
+	else
+	{
+		/*
+		 * In CoW filesystems, seeking to the end and writing a solitary byte
+		 * is enough.
+		 */
 		errno = 0;
 		pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
-		if ((int) write(fd, zbuffer.data, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
-		{
-			int			save_errno = errno;
 
-			/*
-			 * If we fail to make the file, delete it to release disk space
-			 */
-			unlink(tmppath);
+		fail = lseek(fd, (off_t) (wal_segment_size - 1), SEEK_SET) < (off_t) 0 ||
+			(int) write(fd, zbuffer.data, 1) != (int) 1;
 
-			close(fd);
-
-			/* if write didn't set errno, assume problem is no disk space */
-			errno = save_errno ? save_errno : ENOSPC;
-
-			ereport(ERROR,
-					(errcode_for_file_access(),
-					 errmsg("could not write to file \"%s\": %m", tmppath)));
-		}
 		pgstat_report_wait_end();
 	}
 
+	if (fail)
+	{
+		int			save_errno = errno;
+
+		/* If we fail to make the file, delete it to release disk space */
+		unlink(tmppath);
+
+		close(fd);
+
+		/* if write didn't set errno, assume problem is no disk space */
+		errno = save_errno ? save_errno : ENOSPC;
+
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write to file \"%s\": %m", tmppath)));
+	}
+
 	pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_SYNC);
 	if (pg_fsync(fd) != 0)
 	{
@@ -4053,14 +4079,17 @@ RemoveXlogFile(const char *segname, XLogRecPtr RedoRecPtr, XLogRecPtr endptr)
 	XLogSegNo	endlogSegNo;
 	XLogSegNo	recycleSegNo;
 
-	/*
-	 * Initialize info about where to try to recycle to.
-	 */
-	XLByteToSeg(endptr, endlogSegNo, wal_segment_size);
-	if (RedoRecPtr == InvalidXLogRecPtr)
-		recycleSegNo = endlogSegNo + 10;
+	/* Initialize info about where to try to recycle to, if needed. */
+	if (!wal_cow_filesystem)
+	{
+		XLByteToSeg(endptr, endlogSegNo, wal_segment_size);
+		if (RedoRecPtr == InvalidXLogRecPtr)
+			recycleSegNo = endlogSegNo + 10;
+		else
+			recycleSegNo = XLOGfileslop(RedoRecPtr);
+	}
 	else
-		recycleSegNo = XLOGfileslop(RedoRecPtr);
+		recycleSegNo = (XLogSegNo) 0;	/* keep compiler quiet */
 
 	snprintf(path, MAXPGPATH, XLOGDIR "/%s", segname);
 
@@ -4068,8 +4097,12 @@ RemoveXlogFile(const char *segname, XLogRecPtr RedoRecPtr, XLogRecPtr endptr)
 	 * Before deleting the file, see if it can be recycled as a future log
 	 * segment. Only recycle normal files, pg_standby for example can create
 	 * symbolic links pointing to a separate archive directory.
+	 *
+	 * Skip recycling on COW filesystems, though.  It's better to create
+	 * new files each time.
 	 */
-	if (endlogSegNo <= recycleSegNo &&
+	if (!wal_cow_filesystem &&
+		endlogSegNo <= recycleSegNo &&
 		lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
 		InstallXLogFileSegment(&endlogSegNo, path,
 							   true, recycleSegNo, true))
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 156d147c85b..cbcf475885e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1177,6 +1177,19 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"wal_cow_filesystem", PGC_SUSET, WAL_SETTINGS,
+			gettext_noop("WAL is stored on Copy-On-Write file system."),
+			gettext_noop("This option adjusts behavior to take advantage of "
+						 "filesystem characteristics specific to CoW filesystems, "
+						 "improving performance. "
+						 "It should be enabled on ZFS and other similar filesystems.")
+		},
+		&wal_cow_filesystem,
+		false,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"log_checkpoints", PGC_SIGHUP, LOGGING_WHAT,
 			gettext_noop("Logs each checkpoint."),
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 194f3120964..c9254c04bab 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -206,6 +206,7 @@
 #wal_compression = off			# enable compression of full-page writes
 #wal_log_hints = off			# also do full page writes of non-critical updates
 					# (change requires restart)
+#wal_cow_filesystem = off	# is pg_wal on a Copy-on-Write filesystem?
 #wal_buffers = -1			# min 32kB, -1 sets based on shared_buffers
 					# (change requires restart)
 #wal_writer_delay = 200ms		# 1-10000 milliseconds
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index f90a6a91391..fa9d69affe4 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -116,6 +116,7 @@ extern bool EnableHotStandby;
 extern bool fullPageWrites;
 extern bool wal_log_hints;
 extern bool wal_compression;
+extern bool wal_cow_filesystem;
 extern bool *wal_consistency_checking;
 extern char *wal_consistency_checking_string;
 extern bool log_checkpoints;
-- 
2.17.1

--BOKacYhQ+x31HxR3--