log: use fdatasync instead of fsync where possible

Using years-old benchmark attached to PostreSQL ML[1], I've observed
tiny bit more than double boost in speed when using fdatasync instead of
traditional fsync, on two Linux machines, each equipped with an SSD.
While the observation may be disputable (there are various
interpretations to what "synchronized I/O" actually means), by logical
extension of what the two are supposed to do, one can expect fdatasync
will perform no worse than fsync.  Having the timestamps correct is
really not a priority, compared to timely processing of the message
stream.  So let's use it whenever possible with QB_LOG_CONF_FILE_SYNC
requested.

As an aside, PostreSQL seems to be be using "fdatasync" method of
updating out to disk by default on Linux till today[2].

[1] https://www.postgresql.org/message-id/[email protected]
    https://www.postgresql.org/message-id/attachment/20659/syncbench.c
[2] https://www.postgresql.org/docs/current/static/runtime-config-wal.html#GUC-WAL-SYNC-METHOD

Signed-off-by: Jan Pokorný <[email protected]>

configure.ac

@@ -201,8 +201,10 @@ AC_FUNC_CHOWN
 AC_FUNC_FORK
 AC_FUNC_MMAP
 AC_FUNC_STRERROR_R
-AC_CHECK_FUNCS([alarm clock_gettime ftruncate gettimeofday \
-		localtime localtime_r memset munmap socket \
+AC_CHECK_FUNCS([alarm clock_gettime \
+		fsync fdatasync ftruncate \
+		gettimeofday localtime localtime_r \
+		memset munmap socket \
 		strchr strrchr strdup strstr strcasecmp \
 		poll epoll_create epoll_create1 kqueue \
 		random rand getrlimit sysconf \
@@ -270,6 +272,7 @@ fi
 
 nongcc_memory_barrier_needed=no
 arch_force_shmlba=no
+file_sync=fdatasync
 AC_MSG_CHECKING([for architecture in ${host_cpu}])
 case $host_cpu in
 	sparc*)
@@ -388,6 +391,30 @@ case "$host_os" in
 	;;
 esac
 
+dnl iterated fallback scheme, break on first hit, fallthrough otherwise, until
+dnl empty or unsupported string reached; output QB_FILE_SYNC macro supposed
+dnl to operate on a file descriptor passed as its only argument
+while : ; do
+	case ${file_sync} in
+	fdatasync)
+		test "x${ac_cv_func_fdatasync}" = x || break
+		file_sync=fsync;;
+	fsync)
+		test "x${ac_cv_func_fsync}" = x || break
+		file_sync=;;
+	"")
+		break;;
+	*)
+		AC_MSG_ERROR([Cannot select file sync method]);;
+	esac
+done
+if test "x${file_sync}" != x; then
+	AC_DEFINE_UNQUOTED([QB_FILE_SYNC(fd)], [${file_sync}(fd)], [File sync method])
+else
+	AC_MSG_WARN([No file sync method applicable!])
+	AC_DEFINE([QB_FILE_SYNC(x)], [], [File sync method])
+fi
+
 AC_MSG_CHECKING([whether GCC supports builtin sync intrinsics])
 if test -z "$gcc_has_builtin_sync_operations"; then
 	gcc_has_builtin_sync_operations=no
@@ -709,7 +736,7 @@ AC_MSG_RESULT([  Arch-independent files   = ${datadir}])
 AC_MSG_RESULT([  State information        = ${localstatedir}])
 AC_MSG_RESULT([  System configuration     = ${sysconfdir}])
 AC_MSG_RESULT([  SOCKETDIR                = ${SOCKETDIR}])
-AC_MSG_RESULT([  Features                 =${PACKAGE_FEATURES}])
+AC_MSG_RESULT([  Features                 = ${PACKAGE_FEATURES}])
 AC_MSG_RESULT([])
 AC_MSG_RESULT([$PACKAGE build info:])
 AC_MSG_RESULT([  Optimization             = ${OPT_CFLAGS}])

lib/log_file.c

@@ -40,7 +40,7 @@ _file_logger(int32_t t,
 
 	fflush(f);
 	if (target->file_sync) {
-		fsync(fileno(f));
+		QB_FILE_SYNC(fileno(f));
 	}
 }