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

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index be54c23187456edc3ca7f554a3750d1230c0d8da..a31fbbff78dac988198d08d3e6cda9b01eb1a8d7 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/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;

diff --git a/src/common/file_utils.c b/src/common/file_utils.c
index eac05a13ed524e125c459f95df45c7cfe60732d0..d8507d88a52a8214e0ed8673d4714123fa5bb505 100644 (file)
--- a/src/common/file_utils.c
+++ b/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;
+}

diff --git a/src/include/common/file_utils.h b/src/include/common/file_utils.h
index 2c5dbcb0b1eea59ec2d2f75aa87e6bcd396c48e1..d2c64c2196ce780f08d992f5a6fadb0a0b243b36 100644 (file)
--- a/src/include/common/file_utils.h
+++ b/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 */