Introduce pg_pwrite_zeros() in fileutils.c

This routine is designed to write zeros to a file using vectored I/O,
for a size given by its caller, being useful when it comes to
initializing a file with a final size already known.

XLogFileInitInternal() in xlog.c is changed to use this new routine when
initializing WAL segments with zeros (wal_init_zero enabled).  Note that
the aligned buffers used for the vectored I/O writes have a size of
XLOG_BLCKSZ, and not BLCKSZ anymore, as pg_pwrite_zeros() relies on
PGAlignedBlock while xlog.c originally used PGAlignedXLogBlock.

This routine will be used in a follow-up patch to do the pre-padding of
WAL segments for pg_receivewal and pg_basebackup when these are not
compressed.

Author: Bharath Rupireddy
Reviewed-by: Nathan Bossart, Andres Freund, Thomas Munro, Michael
Paquier
Discussion: https://www.postgresql.org/message-id/CALj2ACUq7nAb7%3DbJNbK3yYmp-SZhJcXFR_pLk8un6XgDzDF3OA%40mail.gmail.com

src/backend/access/transam/xlog.c

@@ -2921,7 +2921,6 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 					 bool *added, char *path)
 {
 	char		tmppath[MAXPGPATH];
-	PGAlignedXLogBlock zbuffer;
 	XLogSegNo	installed_segno;
 	XLogSegNo	max_segno;
 	int			fd;
@@ -2965,14 +2964,11 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 				(errcode_for_file_access(),
 				 errmsg("could not create file \"%s\": %m", tmppath)));
 
-	memset(zbuffer.data, 0, XLOG_BLCKSZ);
-
 	pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
 	save_errno = 0;
 	if (wal_init_zero)
 	{
-		struct iovec iov[PG_IOV_MAX];
-		int			blocks;
+		ssize_t		rc;
 
 		/*
 		 * Zero-fill the file.  With this setting, we do this the hard way to
@@ -2983,29 +2979,10 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 		 * indirect blocks are down on disk.  Therefore, fdatasync(2) or
 		 * O_DSYNC will be sufficient to sync future writes to the log file.
 		 */
+		rc = pg_pwrite_zeros(fd, wal_segment_size);
 
-		/* Prepare to write out a lot of copies of our zero buffer at once. */
-		for (int i = 0; i < lengthof(iov); ++i)
-		{
-			iov[i].iov_base = zbuffer.data;
-			iov[i].iov_len = XLOG_BLCKSZ;
-		}
-
-		/* Loop, writing as many blocks as we can for each system call. */
-		blocks = wal_segment_size / XLOG_BLCKSZ;
-		for (int i = 0; i < blocks;)
-		{
-			int			iovcnt = Min(blocks - i, lengthof(iov));
-			off_t		offset = i * XLOG_BLCKSZ;
-
-			if (pg_pwritev_with_retry(fd, iov, iovcnt, offset) < 0)
-			{
-				save_errno = errno;
-				break;
-			}
-
-			i += iovcnt;
-		}
+		if (rc < 0)
+			save_errno = errno;
 	}
 	else
 	{
@@ -3014,7 +2991,7 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 		 * enough.
 		 */
 		errno = 0;
-		if (pg_pwrite(fd, zbuffer.data, 1, wal_segment_size - 1) != 1)
+		if (pg_pwrite(fd, "\0", 1, wal_segment_size - 1) != 1)
 		{
 			/* if write didn't set errno, assume no disk space */
 			save_errno = errno ? errno : ENOSPC;

src/common/file_utils.c

@@ -527,3 +527,76 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 
 	return sum;
 }
+
+/*
+ * pg_pwrite_zeros
+ *
+ * Writes zeros to file worth "size" bytes, using vectored I/O.
+ *
+ * Returns the total amount of data written.  On failure, a negative value
+ * is returned with errno set.
+ */
+ssize_t
+pg_pwrite_zeros(int fd, size_t size)
+{
+	PGAlignedBlock zbuffer;		/* worth BLCKSZ */
+	size_t		zbuffer_sz;
+	struct iovec iov[PG_IOV_MAX];
+	int			blocks;
+	size_t		remaining_size = 0;
+	int			i;
+	ssize_t		written;
+	ssize_t		total_written = 0;
+
+	zbuffer_sz = sizeof(zbuffer.data);
+
+	/* Zero-fill the buffer. */
+	memset(zbuffer.data, 0, zbuffer_sz);
+
+	/* Prepare to write out a lot of copies of our zero buffer at once. */
+	for (i = 0; i < lengthof(iov); ++i)
+	{
+		iov[i].iov_base = zbuffer.data;
+		iov[i].iov_len = zbuffer_sz;
+	}
+
+	/* Loop, writing as many blocks as we can for each system call. */
+	blocks = size / zbuffer_sz;
+	remaining_size = size % zbuffer_sz;
+	for (i = 0; i < blocks;)
+	{
+		int			iovcnt = Min(blocks - i, lengthof(iov));
+		off_t		offset = i * zbuffer_sz;
+
+		written = pg_pwritev_with_retry(fd, iov, iovcnt, offset);
+
+		if (written < 0)
+			return written;
+
+		i += iovcnt;
+		total_written += written;
+	}
+
+	/* Now, write the remaining size, if any, of the file with zeros. */
+	if (remaining_size > 0)
+	{
+		/* We'll never write more than one block here */
+		int			iovcnt = 1;
+
+		/* Jump on to the end of previously written blocks */
+		off_t		offset = i * zbuffer_sz;
+
+		iov[0].iov_len = remaining_size;
+
+		written = pg_pwritev_with_retry(fd, iov, iovcnt, offset);
+
+		if (written < 0)
+			return written;
+
+		total_written += written;
+	}
+
+	Assert(total_written == size);
+
+	return total_written;
+}

src/include/common/file_utils.h

@@ -44,4 +44,6 @@ extern ssize_t pg_pwritev_with_retry(int fd,
 									 int iovcnt,
 									 off_t offset);
 
+extern ssize_t pg_pwrite_zeros(int fd, size_t size);
+
 #endif							/* FILE_UTILS_H */